CA1129066A - Method and apparatus for acoustically investigating a casing and casing cement bond in borehole penetrating an earth formation - Google Patents

Abstract

ABSTRACT
Methods and apparatuses for acoustically investigating a casing in a borehole to derive the quality of a cement bond behind the casing and casing thickness are described. The techniques employ an acoustic pulse source having a frequency spectrum selected to stimulate selected radial segment of the casing into a thickness resource. The selected frequency spectrum enhances the reverberations between the inner and outer with significant amplitudes for a duration depending upon the amount of acoustic energy leaked into adjacent media.

The acoustic pulse causes acoustic returns which are formed by the reflections from interfaces between media of different acoustic impendence and acoustic energy leaked into the bore of the casing from the acoustic thickness reverberations stimulated within the casing walls. The acoustic returns are detected to generate a reflection signal which is processed to determine casing thickness or to evaluate the cement bond.
The acoustic pulse has a frequency spectrum which is particu-larly effective in discriminating different cement bond condi-tions caused by small cement separations known as micro-annuli, around the casing. Several signal processing techniques and tools are described to provide accurate and high resolution cement bond evaluation and casing thickness determination by processing a portion of the reflection signal representative of the thickness reverberations.

Description

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Field of the Invention This invention relates to methods and apparatuses ~or aco~lstically inve,tigating a borehole. ~Sore specifically, this invention relates to a method and apparatus using an acoustic pulse echo technique for investigating the quality of the cement to a casing and the thickness of the casing located in a borehole.

~ackground of the Invention In a well completion, a string of casing or pipe is set in a well hore and cement is forced into the annulus between the casing and the well bore, primarily to separate oil and gas producing horizons from each other and from water-bearing strata.

If the cement fails to provide a separation of one zone ~ !
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¦ from ancther, then fluids under pressure from or.e zone ~ay be able to migrate and contaminate an atherwise productive near~y zone. Migration of water in particular produces undesirable water cutting of a producing zone and possibly can make a well noncommercial.
Cement failures can occur in a variet~ of manners. For ¦ example, there may, for one reason or another, ~e a complete absence of cement behind the casing segment where the cement should be. This would be a gross cement bonding railure leading to rapid contamina~ion between zones intended to be separated.
Another type of cement ~ailure arises ~Jhen the cemant is present behind the casing, but a small cement-free annulus exists between the cement and casing. This annulus may be so thick as to enable hydraulic comm~nication between zones leading to undesirable contamination.
Such annulus, however, may also be so ~hin 25 to effectively preserve the hydraulic security functior of the cement. Such acceptable small annulus may arise from the tech-nique employed to introduce the cement in the irst ~lace. For example, the cement typically is introduced undar very high pressure such as produced by using a heavy mud to c~ase the cement plug down and into the annulus around the casing. The resulting pressure i~side the casing Caus2s a slight ex2ansion of the casing and subse~uent contraction when the hea w mud is ¦ removed. The magnitude o~ the contraction depends upon the pressure and caslng thic.~ness and tsnds to result in a slic,ht , I `

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separation, an annulus, between the cement and casing. It ls Lmportant to ~ncw wnether the cement ls performing its function, ¦
i.e. whether the cement bond is hydraulically secure.
Techniques have been proposed to asc~rtain the ~uality of the cement bond. In this sense the term "bond" as used herein, is to be understood to include both thoce cases where the cement actually ad~eres to the casing as well as when there is no adhesion but instead a small ~icro-annulus which is so small as to prevent fluid commu~icatlon between cement se?arated zones.

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In other words, the term "so~d ~ond" ~eans that separation of ¦ zones by the cement is ade~uate to prevent Lluid migration ¦ between the zones even in the presence of a micro-annulus. It is, therefore, desirable that cement evaluation technL~ues identi~y such micro-annuli as good cement ~on~s while recognizing annuli incapzble of separating zones as hydraulically insecure or bad bonds.
The pro~lem OL investigating the cement behind a thic.
casing ~all with a tooL located inside the casing haâ led to various cemen~ evaluating tecnnicues usinS acoustic energy.
For ex~mple, in the U. S. Pate~t 3,401,773 to Synnott III
a cemen~ logging technigu2 is described using ~ tool ~playing a conventional longitudinally spaced sonic trans~itter znd sonic re~eiver. The casing signal traveling through the casing ls lS processed wkereby 2 later portion, t~hich is afLected by the presence or absence o- cemen., is extracted. The extrzcted seg~ent is intègrated to provide a measu-ement of its energy as an indica~ion or the presence or absence o~ ce~ent behind the I casing. Althoush such techni~ue provides userul info~mation ¦ a~out cement defects behind the casing, the evaluation of the ¦ quality of the cement bond may not be sufriciently precise since the measurement averages cement conditions over a su~stantial I distance bet~;e3n the t-ans~itter and receiver and does not ¦ provide circumferential resolutioni.e.;n ormation as to the bo~.d condition a~ various points around the casing. Fur.hermore, _he Z9066 ( - I

techniqus may cilarac.erize a hydraulically sec~lre annulus as 2 ¦! defective cemont bond because o~ ina~equato energy transfer ~ro t~e casing sisnal to the cem2nt tnroush the annulus~
A more precise technique _or evalu2ting the cement con2ition is described in the ~. S. Pztent 3,697,937 to Ingræ~
and assisned to the sæme assignee as for this patent 2~Dlication.
Ingræm discloses a sonic transmitter-rec2iver with zero sp~cing to measure rerlection coeLficients from re~loctions produced by material discontinuities. Cement condirions in case~ borehol2s LO - are evaluated by comparing the relati-~2 amplitude and ohase or reLlected sor.ic energy impinsing upon paired acoustic transduce-, at a plurality o~ frequencies. ThQ sonic in~Jestigation is descri~ed as particu7arly useful zt sonic frequencies in the ~ range from about S ~z to 50 R~z. At such sonic rrecuencies th~
reflection coeCficients (the ratio of amplitudes of incoming ¦ waves to outgoing ~aves in tne mud inside the casing) ~ary as z ¦ func~ion of ~hethar there is a cemented ar unce~ented annulus, ~he wid~h of the annulus and hardness o~ the ~or~ation.
In the U.S. Patent 3,732,g47 to ~Ioran et al an acoustic 23 pulse techni~ue or cement evaluation logg ng is desc~ibed w~erei~
the attenuatian OL acoustic signals ~e21ected from material dis-continuities LS ...Qasu ~d at radially resonznt fre~uencies eCrec-ti~ely ~ithau~ circ~mfer-ntial resolution. The measured att-n~--tion constar..s a e tnen emploved to com~ute the unick.~ss or t:~.e ,~ annulus and ~he cement ~ith t;ne com~utation dependent upon thQ

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¦ type of forma~ion as well as upon measurements conducted at dif-ferent resonant frequencies. This technique employs lo~ rrsquen-cies'~ihere compens2tion for formation characteris.ics ~o be oDtained ~^rom ano'her well log are requlred. Further~ore, in ormation on the thic.';ness Oc the cement annulus is needed to derive an evaluation o~ the annulus bett~een the cement and casir~.
When acoustic ce~.er.. ev~luation techniques are carried ' ou~ at low f-e~uencies such as described in the patsnts to Ingram and Mor2n et al, so-callsd radial or hoop-mode resona~css are obser~ed. One resonance includes the casing-annulus system, a second higher resonance occurs ~or the cement annulus itselL.
The techni~u2 of employing such resonances to sense absence or presencr o~ cemen- in the annulus around the casinS does no~ ;
lend itsel easily to e~aluating the cem2nt bo2d quali.y in the '15 presence or smzll casing-cemen' annuli.
¦ In '~he U. 5. Patent 3,175,639 to Li~er., an ~cous~ic pulse echo technique is de5cribed to inves.isate the formation zon-~
alongsLa- a bor2hole. An acoustic pulse sener2tor op~ra'ins at I a rrequency o. the order or^ about 10 MHz is applied adjacer.' tn-! wall of a borehole and actuated to generate very short acoustic ¦ pulses towards the form2tion. The elapsed time betw-en ths transmitted sonic pulse generation and the re1ectad pulses ar2 ¦ measured as weli as the amplitude of the returned pulse. The I measurements are then used to derlve the acoustic imped2nce o-~ th: fo=nation ,1 . .I' ~ 5-i6 - I

In the Liben patent a processing apparatus is described with which the return pulse occurring aîter the transmitted ¦ pulse is rectified and integrated. The integrated signal is ¦ indicated as proportional to the average amplitude of the ¦ return pulse. The integrated signal is used to d~rive the acoustlc ¦l ~mpedance o~ the for=ation alongside the borehol- with the use !l f a measurement of the thickness of the mua cake, a knowledge of the amplitude of the transmitter pulse, the absorption character-~ istic of the mud and the acoustic impedance of the mud cake.
The acoustic pulse echo technique described in Liben does not lend itself well for evaluating the ~uality of the cement bond. ~he proposed frequency of operation by Liben is too high, there~y tending to characterize all micro-annuli as ~oor cement bonds. Furthermore, the acoustic transducer is mou~ted close to the borehole wall so that secondary transmission interference ¦ problems may occur such as when a returned echo is reflected from the transducer as a second transmission back to the forma-tion.
; In the U. S. Patent ~,3~0,953 to Zemanek, an acoustic t~rough-càsing formation bore~ole lcgging technique is described with acoustic frequencies determined by the casing thickness.
The apparatus functions by transmitting acoustic ener~ from a transmitter to a pair of remotely spaced receivers. ~he ¦¦ frequency of the acoustic energ~ is selectsd on the basis of a I particular relationship depending upon the velocity of the ll -7-.,, I

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l .' shear wav in the casing, an arbitrary dir.ens Gnless number and the casing thic.~ness. The-suggested .ransmitter frequencies are from 300 KHz to 460 KEz for a casing thickness of 1/4 inch thickness and correspondinsl~ lower fre~uencies for thic.~er S casings.
¦ The Zemanek system does not operate on a specific isolated casing segment but, because of the transmitter-receiver spacing along the borehole, provides an average evaluation over the spacing involved. Zemane.~ neither descri~es an apparatus nor a method for investigating the cement bond by analyzing the reflections from radially successi~e interfaces.
The U. S. Patent 3,883,841 to Norel et al describes ;
a similar acoustic pulse echo technique as in Liben for measuring the acoustic impedance of material alongside a wall in a borehole. The acoustic pulse transducer in Norel is provided with different acoustic coupling layers between the flush mounted transducer and the borehole. ~he Norel et al devica suggests employing a source pulse whose frequency spectrum occurs in the range from about 100 X~z to about 5 ~z. This is a frequency range or generally tne same ~andwidth as proposed in U. S. Patent 2,825,044 to Peterson who suggested an ultra-sonic device for e~ploration of a borehole wall with acoustlc waves at frequencies from 100 KHz to 10 rv~z.
~he acoustic echoes o~tained as oroposed by ~orel et 2g ~ al are s ted as userul ror checking the cement bond. ~lorel _~_ ' l~Z9~

: aches that to measure the acoustic impedance of the ~aterial in contact with the casins, t~o consecutive peaks of recelved impulses are to be extracted and their ratio generated ror use in a computation networ~ to derive the acoustic imped2nce.
Since a casing thic.~ness may vary ln practice as much as from 10~ to 20~, the Norel gatlng approacn to extract successive re~lection is difficult to implement. ~urth~rmore, the acoustic impedance coupling layers suggested by Norel introduce attentuation. As a result, the potential error in measuring individual r~flections is increased, ~hus reducing the effectiveness of Norel et al's analysis of the acoustic investigation.
In a simplified approach described with reference to ~ig.
15, Norel et al propose to check the cement bond by di-ectly integrating the entire received echo signal and recording the resulting integration 2S a function of depth. This technique includes the strong casing re~lection whose inclusion obscure~-_9_ Il - C ~ 9~
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. . ' the more significant l~ter reflec~ions and is lil-ely to include ~ormation echoes in well-bonded hard for~ations.
A frequency range such as proposed by Norel et al inclu~es at the low end fre~uencies tending to dri~e the casins-annulu5 into hoop-mode resonznce with the attendant sensitivitiQs ~Jhicn make cement bond e~alu~tions in the presPnce of small annuli difficu~t. ~t the high end o~ Norel et al's freque~cy r~nge, ~he casing-cemen~ anr~uli are li~ely to be consis~antly int-r~reted as bad cement bor.ds even t~ous:~ the cement might be hyd~ulically 1~ secure. Furthermo~e, ~h~e s~2cing ~e~;ean ~lorel's transducer Leele- and th2 casing tend to appear 2S a small annulus, thus obscuring the evaluation of the cemer.t bond.
When an acoustic pulse echo technique for inves.igating z borehole is employed, it is desirable to obtaln an adequ~t~
num~er of cycles in the reflected pulses beLore a second2ry inter.erence as herein described ~ith respect to Liben is observed When an acoustic pulse transducer 25 descri~e~ in Norel et al is mounted llush to the inner wall of a casing, the ~irst echo xeturn occurs v2ry soon and its rarlectio~ from.~h-2a transducer back to the casins c~us2s secor~d2ry re~lections which tend to inter~ere with the initial echo signals o. inter2st~
One can introduce sp-cizl acoustic couollns layers be-we~n ¦ the transducer and the casing as pro~osed ~r Norel et al. ~Yith I such l~yers, however, the echo sisn~ls t~rd to be also r-duced

2~ ~ in amplitude. Furtnen~ore, the oroximit-~r of t-e transducer 1~''' '. ' .

1~29~6 to the material interfaces reduces the number of echo signals with useful amplitudes berore secondary trans~ission interfer-I ence arises. Though use or high frequencies such as rrom one : ! to five ~Hz enaole sharper or shorter duration transmitter S I pulses, those same fre~uencies tend to be incompatible for evaluating small casing-cement annuli. Such hlgh rrequency sonic waves also tend to be affected by the casing surface whose roughness may cause destructive interference.
~Jhen an acoustic pulse producer such as described in Norel et al is employed in an ultrasonic echo testing device as ; described in Russian Patent SU 405095 or the U. S. Patent ; 3,974,476 to Cowles, the increased spacing suggested by the latter between the transducer and the casing en bles reception of a greater number of cycles. ~owever, in such case the intermediate layers proposed by Norel et al between the trans-¦ ducer and the casing tend to severely attenuate the echo sisnals which already arrive with reduced amplitude by virtue of the ! increased spacing, The U. S. Patent 3,339,666 to P~cDonald describes an acoustic pulse echo technique for a cased bore~ole using an acoustic frequency at which the casing appears transparent. ¦¦ The suggested acoustic pulse frs~uency range is about 100 X~z, wlth a particular range suggested bet~een 200 to gO0 X~z.
I The re~lections are transmitted rrom the borenola tool to ~he 1I surface where all of the reflections occurring after a gating 1' . I
li 1 . ~lZg~6 ¦l time of about 100 microseconds following the riring and I I
¦¦ before the next succeedins acoustic pulse from the transmitter ¦ i are rectified, integrated and recorded.
ll McDonald characterizes t~e reflection segment select-d i 1l for integration and recording as representative or the ¦! acoustic impedance of the formation. In practice, however, ¦i signiricant reflections from the formation at the casing ¦¦ thicXness resonance frequency occur in limited situations I such as whe~ the cement is well bonded to both the casing ard I the formation and when the for~ation itself can provide ¦ a strong reflection. ~ormation reflections tend to be cluttered by seconda~y transmission effects, such as when an initial acoustic reflection from the inner wall of the casing causes a secondary transmission when partially reflected ofr l; the face of the transducer.
When the borehole wall is roush or has craters or ¦ crevasses, as fre~uently occurs, ~he formation acoustic ¦ reflections te~d to be sca~tered and quite weak by the time ¦ they arri~e at the acoustic transducer. ~hen the cement I annulus is not properly bonded to the casing and formation, 1I further attenuation and scattering of the formation I reflection is liXely, resulting in further weaXenins or complete i loss of the for~ation reflection.
~ Mc~onald furtner proposes the transmission of the reflection throush suitable conductors in a cable. Technicues '' 1.

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I for the transmission o~ hig~ frequency signals of the order of ¦ 500 KHz such as occur in the rerlection signal are well known.
~ell logging cailes, however, are typically lL~ited to signals I whose frequencies occur below about 100 K~z. As a r2sult, a ¦ high ~requency reflection signal attributable to reverberations Il between the inner and outer casing walls would be highly attenu-¦l ated by the cable.
It is important in well logging operations to obtain in- ¦
¦ formation as to the current condition or the casing employed in ~I boreholes. The installed casing may be exposed to various cor-¦ rosions due to chemically active corrosive solutions, electro-lytic corrosion due to ground currents or contact between dis-similar metals. Corrosion of the outside casing wall may result in a highly undesirable hydraulic communication between formatio~
zones which must remain isolated from each other by tne cement.
Excessive wear may arise due to abrasion from fluid flows.
Hence, over a period of time, the borehole casing may deterior-ate with excessively thin and weakened resions. Such deterior-I ation can be harmful causing collapse of the protective casing and perhaps loss of the well or, if leaks develop in the casing, uncontrolled movement of fluids wlthin ~he well and adjacent formations. Unlike well tubing, once casing is installed in a well, it is difficult or impossible to remove the casing for inspection. It is, therefore, particularly useful to be able 1 to inspect the casing in situ to determine the ~resqnce and ll location o bad casing conditions.
¦l Ultrasonic pulse echo techniques for determining the ¦ thickness of materials ~ave been extensively proposed in the art. Commencing, for example, with the U. S. Patent 2,538,11 il I

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to ~. P. Mason, an apparatus is described ^or measur~ns the thic.~ess of a m2terial by r.stir.g its resonance f-esuency whe~
the material is i~radi2tQd with ultrason~c erergy. In ~he ¦ ~. S. Patent 2,848,3~1 to J. E. ~unter et al, a techni~ue is ¦ describec. w~ere~y the graln size ar materials is measured 3y obser~ing ~he ultrasonic ~recuency response of the mat9ria I~ the ~. S. Patent 3,5gS,069 to Fowler et al a sys.em is disclosed ~hereby an ultrasonic sensor is stimul~t-d i~to 2 reso~ance and the r~sonance rreauency measured to ~eter~ne ~he value of the ~arameter ror which tkQ sensor is used. In , the U. S. Patent g,003,244 to O'Bxien et al, t~e thic.~ess cf ; a material is measured by employing a pulse echo t~c~nique.
Various fre~uency domain technl~es h2~e been employed in acoustic in~estigatlons to deter~ine the thick~ess o~
materials For example, in an article entitled "Ultraso~c Signal Procsssing Concep~s for L~sasuring tne Thic~ness o~
Thin Layers", pu~lis~ed at page 249 of '.he Dac~mbe-t 1974 issu-or ~aterials Evaluati~n by J. L. Rose and P. ~. ~eyer, a frequency analysis is descri~ed for determinins t~ie t~ic~ess 2~ ¦ of a thin layer. As described in t~is article, ~n input acoustic pulse is applied t~ith su ricient bandwidth to cove-the fundamental or ha~monic resanance fresuency o^ 2 th~
layer placsd ~et~een t~.~o materials. A s~ectral profll~ of the echoes ~rom various layers is m2d~ as illustrated in F~ ss .
11 and 12 of this article. With par.icular re erencs .o r:~le I -1~1-broadband frequency spectra shown in Fig. 12, dips in the frequency spectrum occur at those f-e-luencies which bear a particular relationshi2 to the thic~ness of the material being measured. The center fre~uency of such dips, however, are not ¦ con~eniently measured, particularly when the frequency spectrum of an echo reveals several dips.
Acoustic techni~ues have been described with which a plate, whose thic~ness is to be measured, is driven into a thickness resonance by utilizing a feedbacX of resonating vibrations. One such technique is described in U. S. ~atent

3,741,334 which issued to W. Raule.
Kaule describes a particular ultrasonic technique for t determining the thic~ness of a plate by measuring its thickness resonance. Resonance is induced in the plate by first subjecting the plate to a noise source for a first inter~al and recording the decaying free resonance ultrasonic sound during a second subsequent interval. After the plate has ceased resonating, the pre~iously stored sound is played bac~
and used to induce resonant vibrations in the plate followed by a subsequent recoraing of the decaying sound after the second inducement. This process is repeated to achieve a high amplitude resonance in the plate and ena~le a measurement of the plate's resonance frequency and thus the plate's I¦ thickness. Frequency is measured by counting the amplitude peaks of the decaying resonatins vibratioAs over a particu1ar ~1 Il I
Il -15-i~
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lnterval or by d~t:rmLning ~he time needed to count a ~artic~lar number of pea~s.
An alleged improvement over the Kaule Patent 3,741,334 is described in U. S. Patent 3,914,987 to Bic~el et al. The improve-I ment appears to relate to use of a bidirectional counter and a delay, but determination of the resonance frequency still involve ~ie counting of individual peaks in ~he decaying vibratlons from the resonating plate.
¦ When an acoustic pulse echo techniqu~ is us2d to determine the , ¦ thickness of a casing cemented in a borehole penetrating an earth formation, the acoustic returns have a complex form. A wavefor~
representative of such acoustlc return is illustrated in Fig. 4 herein and shows that a reliable peak to peak frequency deter-mination is at best dlfficult and more likely impractical.
Purthermore, the casing bore is circular tending to produce acoustic interferences from reflections of surface irregularities and tne like; thus further cluttering acoustic returns.
In addition, the time available for theinvestiga~ionof the ¦ thickness of any one small casing segment is limited if an exten- I
¦ sive investigation of the casing is to be completed within a re~son-¦
; able time. ~ence, the time needed to execute an acoustic feed-back investigation of the type described in the Raul and Bic.~el et al patents does not in practice appear tolerable.
In an article entitled "Broad-Band Transducers, Radiation I Pield and Selected Applications" by E.P. Papadakis and ~.A.
Il Fowler and published at page 729 of Vol. 50 Number 3 (Part 1) o~ i ¦ the 1971 issue o The Journal of the Acoustical Societv of America, I; -a pulse induced resonance technique is described for deter~ini3s 1, the thickness of a thin material. The technique describes a li selective time-domain gating of pulses reflectsd by ~he thin material and an analysis o their frequsncy CQntent witn a spe~-~' t-um ~nalyzer. ~n zutoma~ic tschnicue ror deriv~ns ~ie t;lic.~nes j o- the thin materi21 is not dsscribed.
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Sum~ary of the Invention I In an acoustic pulse echo technique for investigating the ¦ casing in a borehole in accordance with the invention, an acousti~
¦ pulse is directed at a selectsd radial segment of the casing. Th~
I acoustic pulse has a frequency spectrum selected to stimulate the ¦ selected segment into a thic~nQss resonance wher~by an enhanced ¦
entrapment of reverberations bet~een the inner and outer casing walls is obtained. Acoustic returns caused by acoustic reflections~
and ~eakage from the reverberztions are detected to produce a re-flection signal from which both an evaluation Of the quality of the casing-cement bond and the casing thic~ness can be derived.
The acoustic waves at the casing thickness resonance have been found to be effectively insensitive to hydrauliczlly secure micro-annuli provided the wavelengths employed exceed the thic.~nes ;
of such micro-annulus by a sufficient amount. The spacing between the receiver-transducer and the casing inner wall is so selected that an adequate nl1mher of cycles of acoustic returns are re-ceived before secondary transmission interferences arises.
As described with reference to a preferred signal process-2a ing embodime2t, the strong casing reflection signal from the inside , wall of the casing is separated from the reflection signal and a ¦subse~uent reverberation segment Of the rerlection signal selected las indicative of the energy of the echo produced by the casing- I
Icement inter~ace. The selected reverberation segment is rectiried;
land integrated to generate a bond signal indicative O~ the ¦, quality of the cement bond.
¦ In order to remove the efrect of tool-tilts and borehole mud anomalies, the previously sapara.ed casing rsrlection signaL is 2mployed to normallze the bond signal. One descrlbed met.~od ror l,thls in'~olves a measuremen. of the ?eak o~ the casing rs lection .

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- i signal and effectivel~ dividing the signal representative of the energy in the s21ected reverberation segment by the measure ampli~
j tude of the casing peak to arrive at a normalized bond signal.
¦ The derived bond signal ma~ be plotted as a function o~~
,I borehole depth or compared with a signal representative of ¦I desired bond quali'~y to identify thos~ borehole regions where ¦I the cement bond is hydraulically inade~uate.
A cement bond signal derived in accordance with the inven-¦
I tion varies with casing thicXness changes. As described with ref-er2nce to one technique i~. accordance with the invention, the cement bond signal is normalized with a signal reprssentative of the casing thickness to substantially remove the effect of casing thic~ness variations.
With a technique for deriving the cement bond evaluation behind a radial segment of the casing, the casing thickness can ¦ also be advantageously obtained so that a precise estimate of ¦ the casing-cement interface can be mad2 at such location, while I also being sensitive to such local casing deteriorations caused ¦ by corrosion or wear.
I With the pulse echo technique for evaluating the cement bond in accordance with the invention, the ability to discri~inat between good and bad cement bonds is significantly enhanced. A
relatively sha~p discrimination between casing-cement annuli which are hydraulically secure and insecure is obtained indeDend- ¦
ZS ent of formation type.
It is, therefor2, an object of the inv2ntion to provi~e an acoUstic pulse echo bor2hole investisation method and apparatus for evaLuating ~he quality of the cement bond. I~
is a further object or the invention to enhance the sensitivity I, ~
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¦ of an acoustic pulse echo technique for the evaluation Of the ! quality of the bonZ between the casing and cement. It is still ¦ further an object or the invention to evaluate ~he guality of I the cement bond to the casing with good circumferential I resolution.
~ With an acoustic pulse echo technique in accordance with ¦ the invention, the selected reverberationcseqmentmay be further advantageously used to provide an indication of the casing thic.~-ness. As described with reference to one technique, the salected reverberation segment is analyzed to determine the fre~uency Of components which contribute to a desired peak in the ~requency domain of the reverberation seg~ent. The frequency Of this peak is used to determine the thic.~ness of the casing, With a technique for determining the casing thic~ness in accordance with the invention, a r21iable casing thicXness determination is obtained substantially free from into-ference due to casing surface irregularities, signal noise and borehole conditions and is particularly useful to determine large variations in casing thickness. Advantageously, the tochnique can determine variations in thickness along the circumrarence of the casing such that a thin section at one circumferential ¦ point will ~ot be overlookod or cancelled out by integration with offsatting thiCk sections as ~ith some prior art devices.
¦ One technique for deriving the frequency components in the reverberation segment of ~he reflection signal may employ a ~-19- 1 , . - . . . . . . . . . . . _ . .. ..
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spectrum analyzer. ~he outp~t rro~ the spectrum ana1~zer is recorded i~ a memory such as a magnetic disc or a solid state l, device. The recorded spectrum analysis of the reverberation ¦ segment is thereupon replayed and scanned by a peak detector ¦ to detect when a desired peak in the spectrum occ-~s.
¦ Detection o~ the desired peak causes activation o~ a sample and hold networ.~ which stores a signal representative of the frequency at which the peak occurs as an indication of the thickness o~ the casing. With a casing thickness detection 1~ techni~ue in accoraance with tne invention, a reliable determination of casing thickness can be obtained ef-ectively free from interference due to borehole environmental factors and casins conditions such as surrace irregularities.
It is, therefore, an object of the invention to provide lS an acoustic pulse echo technique ,or deriving an indication o~ the thickness of a casing installed in a boreholP.
As described with reference to several e~bodiments ~or investigating the casing, a tool is used having eitner an acoustic source, which is rotated as the tool is moved along a cased borehole, or which has a plurality of circumferentially distributed acoustic s~urces. ~ith such tool discrete radial casing seçments can be inspected with good circum~erential resolution. If desired, a precise location or flaws in the l c~sing thic~ness or cement bond can be obtained by providing 1l azimuth tool orientation inrormation.
jI The ter~ radial segment as used herein means the segment of the casing extending ~etween its w~lls and surrounding a given radius which extends generally nor~al to the casing wall f-om ths centex o~ the casing.
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In accordance with still another acoustic pulse echo technique for investigating a casing in accordance with the invention, the reflection signal is digitized downhole by a high speed analog to digital converter.
~le digitized reflection signal may then be processed with a tool mounted processor, but preferably the digitized reflection signal is transmitted at a suitable pulse rate to a surface located signal processor. The signal processor is programmed to derive an evaluation of the quality of the cement bond and the thickness of the casing from the digitized re-flection signal.
With acoustic pulse echo investigation and signal processing techniques in accordance with the invention, a reliable evaluation of the quality of the cement bond and the casing thickness is obtained with a single pass of the same investigating tool.
It is, therefore, a further object of the invention to provide an acoustic pulse echo investigation technique with which both the quality of the cement bond and the casing thickness can be determined.
According to one broad aspect of the invention there is pro-vided an apparatus for determining the quality of the cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; and means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing.
According to another broad aspect of the invention there is provided a method for determining the quality of the cement of a casing cemented in a borehole penetrating an earth formation from a reflection llZ9~

signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the step of measuring the energy in a reverberation segment of the reflection signal, wherein the reverberation segment is substantially representative of acoustic reverberations between the casing walls at said radial segment, and provide a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing.
According to another broad aspect of the invention there is provided an apparatus for determining the thickness of a casing cemented in a borehole penetrating an earth formation from a reflection signal de- , rived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverbera-tions between the casing walls; means for generating a spectrum signal representative of the frequency spectrum of said reverberation segment;
and means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.
According to another broad aspect of the invention there is provided a method for determining the thickness of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to st-imulate a thickness resonance inside the casing walls comprising the steps of generating a spectrum signal re-presentative of the frequency spectrum of a reverberation segm~ent of the reflection signal wherein said reverberation segment is substantially -21a-~ .

. . , .

~lZ9~j representative of acoustic reverberations between the casing walls at said radial segment; and measuring the frequency of components in said spectrum signal contributing to a peak value thereof and provide a thick-ness signal representative of said measured frequency as indicative of the casing thickness at said radial segment.
According to another broad aspect of the invention there is provided an acoustic pulse echo apparatus for investigating a casing cemented in a borehole penetrating an earth formation comprising means for generating an acoustic pulse from inside the casing in a generally radial direction towards a selected radial segment of the casing wherein said acoustic pulse has a frequency spectrum selected to enhance entrap-ment of acoustic energy between the inner and outer casing walls at the radial segment for stimulation of reverberations therein and generating a reflection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse with acoustic leakage from reverberations trapped inside said casing walls; means for selecting a reverberation segment of the reflection signal wherein said selected segment is substantially representative of said reverberation leakage in the acoustic returns; means for determining the energy in the selected ~0 reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing; and means for determining from said reverberation segment the frequency of components contributing to a peak value in the frequency domain of said reverberation segment and generate a casing thickness signal representative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement evaluation at said radial segment.
According to another broad aspect of the invention there is provided an acoustic pulse echo apparatus for investigating the quality of the cement and a casing located in a borehole penetrating an earth for-mation comprising means for generating from inside the casing an acoustic -21b-r~ 1~ 29066 pulse towards a radial segment of the casing and the formation and produc-ing a reflection signal representative of acoustic returns from the inter-action of the acoustic pulse with different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing and with the acoustic wave frequencies further being selected to render micro-annuli representative of good quality cement effectively transparent while enhancing reflections from annuli representative of poor quality cement; means for selecting a reverberation segment of the reflection signal following an initial casing reflection wherein said reverberation segment is substantially represent-ative of acoustic leakage from reveberations introduced in between the walls of the casing by said acoustic pulse, and means for measuring the energy in the reverberation segment of the reflection signal and produce a quality signal indicative thereof to characterize the quality of the cement.
According to another broad aspect of the invention there is provided an acoustic pulse echo apparatus for investigating the quality of the cement of a casing located in a borehole penetrating an earth form-ation comprising means for generating from inside the casing a highly damped acoustic pulse towards the formation wherein said acoustic pulse has waves at frequencies selected to stimulate a thickness resonance inside the casing walls, said acoustic wave frequencies further being selected to render micro-annuli representative of good quality cement effectively transparent while enhancing reflections from annuli representative of poor quality cement said acoustic pulse generating means being further respon-sive to acoustic returns produced by said acoustic pulse for producing a reflection signal representative thereof; said acoustic pulse producing means being at a predetermined minimum spacing from the casing to enable the detection of acoustic reverberations substantially free from second-ary transmission interference; means responsive to said reflection signal -21c-,-. ~ ~ : . ; : ' :, . . . .

-` llZ9~66 for detecting an initial casing reflection from the casing; means actu-ated upon the detection of the initial casing reflection signal for select-ing a reverberation segment following said initial casing reflection; and means for producing a quality signal indicative of the energy in the selected reverberation segment to characterize the quality of the cement.
According to another broad aspect of the invention there is provided an acoustic pulse echo apparatus for investigating the quality of the cement of a casing located in a borehole penetrating an earth formation comprising means for generating from inside the casing an acoustic pulse towards a radial segment of the casing and produce a re-flection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing with substantially reduced reflections from hydraulically secure micro-annuli representative of good quality cement and with a significantly longer duration reverberations in the casing in the presence of annuli representative of poor quality cement;
means responsive to the reflection signal for generating a casing reflect-ion signal indicative of the duration of an acoustic reflection from the :
casing; means responsive to the casing reflection signal for producing a reverberation segment selection signal to identify a reverberation segment of the reflection signal following the casing reflection; means enabled by the reverberation segment selection signal for measuring the energy in the reflection signal for the duration of the reverberation segment selection signal and produce a quality signal indicative of the quality of the cement located in the path of the acoustic pulse; means for producing a normalizing signal representative of a predetermined characteristic in the acoustic reflection from the casing; and means for combining said quality signal with the normalizing signal to produce a normalized signal represent-ative of the quality of the cement.
According to another broad aspect of the invention there is -21d-. ~

:: . ,......... : . ~ ~

--- 1129~66 provided an acoustic pulse echo method of investigating the quality of the cement of a casing located in a borehole penetrating an earth for-mation comprising the steps of generating a pulse of acoustic energy towards the formation from inside the casing with the acoustic energy having a frequency spectrum which is selected to stimulate the casing into a thickness resonance to trap reverberations in the casing and having a frequency bandwidth selected to generate acoustic waves at frequencies whose water wavelengths exceed the thickness of hydraulically secure micro-annuli by a factor sufficient to render said micro-annuli effectively transparent to said acoustic pulse; deriving a reflection signal represent-ative of acoustic returns from different layers of material in the path of the acoustic pulse; and determining the energy in a reverberation seg-ment of the derived reflection signal attributable to acoustic leakage from reverberations inside the casing as an indication of the quality of the cement located in the path of the acoustie pulse.
According to another broad aspect of the invention there is provided a method for acoustically investigating the quality of the cement of a casing located in a borehole penetrating an earth formation with a pulse echo technique comprising the steps of generating an acoustic pulse inside the casing towards a selected radial segment of the casing and the formation to cause acoustic returns attributable to the acoustic inter-action of the acoustic pulse with different layers of materlal in the path of the acoustic pulse, wherein said acoustic pulse has acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thick-ness resonance to trap acoustic reverberations inside the casing;walls, with the acoustic wave frequencies further being selected to reduce re-flections from micro-annuli representative of good quality cement while enhancing reflections from annuli representative of poor quality cement;
detecting the acoustic returns to produce a reflection signal indicative thereof; selecting a casing segment from the reflection signal represent-ative of a reflection from the casing; selecting a reverberation segment -21e-.. . . .

~12g~66 from the reflection signal representative of reflections occurring sub~
sequent to said casing reflection and substantially representative of leakage returns from reverberations introduced in the casing by the acoustic pulse; and processing said selected segments to cooperatively produce a quality signal indicative of the quality of the cement.
According to another broad aspect of the invention there is provided an apparatus for acoustically investigating the quality of the cement of a casing located in a borehole penetrating an earth formation with an acoustic pulse echo technique comprising means for producing an acoustic pulse having acoustic wave frequencies selected to stimulate the casing into a thickness resonance with enhanced entrapment of reverbera-tions inside the casing and provide a reflection signal representative of acoustic returns caused by the acoustic pulse; means for extracting from the reflection signal a frequency segment selected to include casing thickness resonance frequencies and generate a quality signal representa-tive thereof as indicative of the quality of the cement; means for ex-tracting from the reflection signal a reference frequency segment and produce a reference signal indicative thereof; and means for combining the reference signal with the quality signal to provide a normalized signal indicative of the quality of the cement.
According to another broad aspect of the invention there is provided a method for acoustically evaluating the quality of the cement of a casing in a borehole penetrating an earth formation comprising generating an acoustic pulse from inside the casing towards a radial seg-ment of the casing wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the radial segment of the casing; detecting acoustic returns attributable to the interaction of the acoustic pulse with materials in the path of the acoustic pulse and produce a reflection signal indicative thereof; selecting a predetermined frequency band from the reflection signal wherein the selected frequency band includes casing thickness -~lf-.. . . .
- :.

llZ9Qii~j resonance frequencies and produce a quality signal representative thereof to indicate the quality of the cement selecting a reference frequency band from the reflection signal and produce a reference signal indicative thereof; and combining the reference signal with the quality signal for normalization thereof.
According to another broad aspect of the invention there is provided an apparatus for investigating with an acoustic pulse a casing located in a borehole penetrating an earth formation comprising means for directing an acoustic pulse from inside the casing in a radial direction at a radial segment of the inner wall of the casing, wherei-a the acoustic pulse has acoustic wave frequencies selected to stimulate a thickness resonance inside the radial segment with enhanced entrapment of rever-berations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse; means for selecting from the re-flection signal a portion which includes acoustic returns attributable to the acoustic reverberations inside the casing walls; means for generating a spectrum signal representative of the frequency spectrum of the selected portion; and means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.
According to another broad aspect of the invention there is provided a method for acoustically investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of generating an acoustic pulse from inside the casing in a radial direction towards the formation wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the walls of a radial segment of the casing; detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal in-dicative thereof; selecting from the reflection signal a portion which includes acoustic returns produced by said acoustic reverberations inside -21g-~;`, llZ9~66 the walls of the casing; forming a frequency spectrum of the selection portion; and determining the frequency of components which contribute to a maximum peak in the frequency spectrum of the selected portion and pro-ducing a signal representative thereof as an indication of the casing thickness.
According to another broad aspect of the invention there is provided an apparatus for investigating with an acoustic pulse a casing cemented in a borehole penetrating an earth formation comprising means for generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate an acoustic resonance between the walls of the casing with acoustic reverberations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse;
means for generating digital samples of the reflection signal; means for selecting samples representative of said casing reverberations and occurr-ing subsequent to samples representative of an initial casing reflection;
means for generating a spectrum of the selected reverberation samples and form amplitude samples with associated frequency values; and means for determining a maximum amplitude sample and its associated frequency value as an indication of the thickness of the casing.
According to another broad aspect of the invention there is provided a method for investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thickness resonance with acoustic reverbera-tions between the walls of the casing; detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal indicative thereof;
-21h-llZ9066 converting the reflection signal to digital samples; forming a frequency spectrum of samples representative of casing reverberations occurring subsequent to samples representative of an in~tial acoustic reflection off the inner wall of the casing with the frequency spectrum composed of amplitude samples with associated frequency values; determining a peak amplitude sample in the frequency spectrum; and recording a thickness signal representative of the associated frequency value of the peak ampli-tude sample as an indication of casing thickness.
According to another broad aspect of the invention there is provided an apparatus for evaluating the quality of cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection slgnal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the ~0 cement behind the casing; mean: for determining from a reverberation seg-ment a casing thickness signal representative of the thickness of the casing at said radial segment; and means for normalizing said quality signal with said casing thickness signal to substantially remove the effect of casing thickness variations from the characteriæation of the quality of the cement at said radial segment.
According to another broad aspect of the invention there is provided a method for evaluating the quality cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside -21i-~.
~..' .

llZ9~g~6 the casing walls comprising the steps of deriving from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; measuring t~le energy in the selected reverberation segment and provide a quality signal indicative thereof to characterize the quality of the cement behind said radial segment; measuring the thickness of the casing effectively at said radial segment and provide a thickness signal indicative thereof;
and effectively removing from said quality signal with said thickness signal, variations, which are substantially attributable to casing thick-ness changes.
The above and other advantages and objects of the inventioncan be understood from the following description of several embodiments described in detail in conjunction with the drawings.

-21j-- - .. . .. -liZ9~66 . '.
, I Brief Description o~ Drawings I . .
Fig. 1 is a schematic representation or one apparatus for evaluating the quality of the cement bond and/or the thickness of the casing in accordance with the invention;
Fig. 2 is a wa~eform representation of a preferred acoustic pulse generated in the apparatus shown in Fig. l;
¦Fig. 3 is a plot of the fre~uency spectrum of the ¦ acoustic pulse shown in Fig. 2;
Figs. 4A, 4B and 4C are illustrative wave~orms representa-tive of acoustic reflections obtained in a pulse-echo investiga-tion technique conducted in accordance with the invention;
: Fig. 5 is an amplitude response curve userul in specify-ing the performance requirement of a trznsducer preferred for ,~ use in an acoustic borehole investigation in accordance with the invention;
Figs. 6A-6C are illustrative spectra of acous~ic re-flections observed with an acoustic investigation apparatus in accordance with the invention;
Fig. 7 is a bloc~ diagram of a signal processins apparatus for evaluating the cement bond in accordance with the invention;
¦ Fig. 8 is a bloc~ diagram o- another form for a signal processing apparatus for evaluating the cement bond in accord-ance with ~he invention;

,. Il 1129~6~i Fig. 9, on the fifth sheet of drawings, is a schematic re-presentation of another cement bond evaluation tool in accordance with the invention;
Fig. 10 is a block diagram of a signal processor for use with a cement bond evaluation tool of a type such as shown in Fig. 9;
Fig. 11 is a timing diagram of signals generated in the signal processor shown in Fig. 10;
Figs. 12 and 13 are top views in partial section of transducers for use in a tool such as shown in Fig. 9;
Fig. 14 is a partial side view in elevation of an acoustic investigation tool employing transducers as shown in Figs. 12 and 13;
Fig. 15 is a schematic representation of an apparatus for determining the thickness of a casing in accordance with the invention;
Fig. 16 is an amplitude frequency plot of several spectra obtained with the apparatus of Fig. 15;
Fig. 17 is a block diagram of a signal processing apparatus for deter~ining the quality of the cement bond and casing thickness in accordance with the invention;
Fig. 18 is a block diagram of part of an apparatus for detecting casing thickness in accordance with the invention; and Fig. 19 is a sectional view of an acoustlc borehole investi-gating tool employing a rotating reflector for scanning of tbe borehole. -.. ... .
, .:,. ~ . ;
., Z~o66 ~
I

I .
¦ Detailed Description OL Embodiments I I;
Figs. 1, 2, 3, 4 and 5 ~ith reference to Fiss. 1 through 3, a system 10 is illustrated for acoustically investigating the quality of the 5 ¦ cement bond between a casing 12 and an annulus of cement 14 in a borehole 16 formed in an earth fonmation 18. An acoustic pulse producing tool 20 is suspended inside the casing 12 wi~h a cable (not shown) having signal paths along which signals for control of tool 20 and ror its observations ara transmitted la between a signal processor 21 in tool 20 and sur~ace located controls and signal processing equipment such as shown at 22.
A depth signal, repre~entative of the depth in borehole 14 or , tool 20, is derived on a line 24 with a conventional depth monitor (not shown) coupled to the cable with which the tool 20 is moved aLong casing 12.
The cylindrical casing 12 is shown in partial section as well as the surrounding cement annulus 14. The shape of the borehole 16 is shown as uni~orm and the casing correspondingly illustrated as equidistantly spaced from the borenole wall.
In practice, however, the borehole wall is liXely to be irregular with crevasses and crac.~s. Hence, the cemen' annulus 14 may vary in thic.kness and the spacing bet~een the casing 12 and the formation 18 may vary.
¦~ The cement 14 is shown with various bond states !l Il -24-'I

Il ~iz9~66 ¦ frequently encountered. At region 26 the cement is shown as ad-¦ hering to the casing 12 while at 28 a micro-annulus, ~a, 30, which ¦¦ is hydraulically secure, occurs. In the region 32 the annulus 30 is shown enlarsed to a thickness T~ith which vertical zone separa-tion is no longer obtainable while at region 34 the cement is en- I
I tirely absent. The c2ment-free regions at 28, 32 and 34 normally !;
I axe filled with water or a combination of wzter and mud. Thesa I! cement conditions do not necessarily occur as illustrated and a-e' ¦ shown here ,or purposes of i~lustrating tne invention. Su~fice it to note that the cement conditions at regions 26 and 30 are to be evaluated as good bonds while those at regions 32 and 34 must be detected as bad.
Casing 12 is further shown wlth externally corroded seg-ments 33.1, 33.2 and an internally corroded segment 33.3 where the casing wall has been reduced in thickness. Such corrosions may occur at other regions and can be particularly harm~ul when one occurs in a region leading to hydraulic communication between zones which must remain isolated from each other. The illustratec corroded segments 33.1-33.3 may appear as actual gaps or occur as scaly segments which present a rough surface appearance and may even partially separate from the good parent metal. The scaly segments become saturated by the borehole rluid segments so that acoustic investigation of the good parent metal beneath the scaly seSments can still be made.
i The tool 20 rits ~Jithin ,he casing 12 which normally is !I filled with water or a mixture or water and mud. The tool 20 is kept central in the casing 12 with appropriate centralize~s (not shown) as are well known in the art. In the practice or ¦I the invention the tool 20 preferably is kept parallel to the 1ll casing wall, thougn the tool may be displaced relative to ,~e 1, 1 ll central axis of the casing 12. As will be further explained with reference to Fig. l, some compensation for tilt condition5, i.e. when the tool 20 forms an angle with tl~e casing axis, is obtained with the system lO.
Tool 20 is further provided with a transducer 36 ~unction-ing as a pulse transmitter and receiver. In some instances the transmitter and receiver functlons can be produced by separate devices. The transducer 36 is oriented to direct an acoustic pulse onto an acoustic reflector 38 and then through a window 40 onto a selected radial segment of the casing 12. The acoustic pulse is partially passed through casinq 12 and ~artially trapped in casing 12 with reverberations occurring in the radial segment at the thic.~ness resonance of the casing.
¦ The nature of the window 40 may vary and preferably is formed of such material and so inclined relative to the direction of travel of the acoustic pulses ~rom trans~itter 36 that the acoustic returns can pass through with a minimum of attenuation and source of reflections. Window 40 can be made o. polyurethane such as sold by the Emerson-Cummings Company as CPC-41 having an acoustic velocity of about 1,700 meters/second and a density o~ about l.l grams/cm3. Such material exhibits a similar acoustic impedance as a fluid placed in tne space between ! source 36, re1ector 38 and window 40 to equalize pressure across window 40.
Il The fluid with which the space lnside the tool between the transducer 36, 2nd window 40 ~s ~illed is preferably !l -25-il29~66 r--~
.. ~.
.

selected for low or minimum atter.tuation and an acoustic Lmpedance which will not contrast too widely from that or the borehole fluid in the frequency range of interest. An acceptable fluid may, for example, be ethylene glycol.
Window 40 is inclined at an angle 9 which is defined as .
the angle between the direction of propagation of the initial acoustic pulse from transducer 36 and the normal 41 to the window surface area upon which this acoustic pulse is incident.
Such inclination serves to deflect secondary transmissions such as 43.1 in a direction which avoids window produced interference. Suitable annular acoustic absorbing sur~aces such as baf~les 45 may be used inside the tool to trap and absorb acoustic reflections 43.2 from the inslde wall of window 40. The size of the angle Q may be of the order of 20 to 30" as suggested ln the U.S. Patent 3,504,758 to ~ueker.
Although the inclination o~ window 40 could be in a dlrection measured relative to the incident beam travel patn, ¦as shown in the U.S. Patents 3,504,758 to DueXer, or 3,504,759 ` to Cubberly, the preferred orientation is as illustrated in ~IG. l herein to ena~le use of a larger refl2ctor 38.
The size or reflector 38 i3 significant in that the reflector surface area influences rocusing of the acoustic energy onto the casing 12 and the capture of a surficient acoustic !
return for improved signal to noise ratio.

il I

i l Z9~6~

I f the rerlectors of Dueker or Cubberly are enlarged, the internal reflections from their windows are likely to be intercepted by the reflectors and redirected onto the receiver transducer in interference with the desired acoustic returns from the casing. When a window inclination as illustrated in FIG. 1 herein is employed, however, a large rerlector 38 can be used, with effective dLmensions su~flcient to either focus or preserve the beam shape of the acoustic energy directed onto casing 12 and pro~ide a significant acoustic return to receiver transducer ~ Ihe incLination of window 40 carl be clearly distLnsuishe2 ~rom that employed in Dueker or Cubberly with reference to the ~rientatlon OL tne internal window normal 41' relative to the paint of incidence of the acoustic beam alons its travel path D2 from reflector 38. When as shown in FIG. 1, the normal 41' lies between the beam travel path D2 and the acoustic receiver function oI transducer 36, the inclination angle and also the angle of incldence, c2n be considered as positive . This angle would also be positive when the internal normal lies between the beam travel path and a separate acoustic receiver such as em-ployed in the acoustic borehole apparatus illustrated in the previously identified Russian ~atent SU 405,095.
In case of a window oriention as shown in the Dueker ~lor Cubberly patants, the inclination angle or angle or incidence llcan be corstrued as negative because the internal window rormal ¦~is on the other side or t~e acoustic be&m travel path and points ¦laway from the receiver transduc3r.

1.i , ~l - 27~ - I

- llZ9~i6 :.
,', With the window inclination as illustrated in FIG. 1, care should be taken to avoid directing reflections such as .~1 43.2 onto the transducer 36; the inclination angla, therefore, ,I should be positive and sufriciently large, but not so large as S ~j to cause significant diffraction effects. The lnclination ¦¦ angle should also not be so large that reflections such as ¦ 43.2 fail to be either absorbed or intercepted by baffles 45.
! A portion of the acoustic pulse is passed through I, casing 12 and, in turn, is partially re1ected by the next ' interface, which in region 26 would be cement material, whiLe at the regions 28, 32 would be the annulus 30 and water-mud at 'l region 34.
!~ In the embodiment of ~IG. 1 the acoustic transducer 36 Il is selectively located so that its erfective spacing (the '1 travel time for an acoustic pulse) to the casing 12 is suffi-ciently long to permit isolation of interference from secondary transmission caused when the strong acoustic casing ~1 ll 'i~
~1 1 ., .

Z9~6 1, reflection is again partially reflected by either a window or the transducer 36 back to casing 12 to produce new reverberation and secondary acoustic returns. A desired total spacing D ~'s j I
obtained by Locating the transducer 36 generally at an axial I ' distanc~ Dl from reflsctor 38, which in turn is spaced a ! i distance D2 from the casing 12.
The total distance D = Dl + 32 between transduc~r 36 and casing 12 is further selected sufficiently lon~ so that the desired acoustic returns including those attributable to reverberations trapped between the casin~ inner and outer walls 13 and 13' respectively can be detected. The total distance D~
is thus su~ficiently long to include those acoustic returns prior to their decay to some small value as a result of leaXage into adjoining media. On the other hand, the total lS spacing D is kept sufficiently small to avoid undue attenuation by the mud external to tool 20 and the fluid inside tool 20.
In additlon to these spacing considerations, the distance Dl between transducer 36 and reflector 38 has been round to affect the sensitivity of the system to tool positions away from a concentric, relationship with the central a~is 47 of casins 12. It should be understood that tool 20 is provided with suitable centralizers, not shown, as are generally well ¦
known. Despite the presence of such centrali2srs some too}
~ displacement, shown as an eccsntricity distanc~ e between the casing axis 43 and tool axis 49, may arise from a n~mber of l . 'l ~ -28-.

11~906~i conditions inside casing 12. The distance Dl, ~or this reason is selected to tolerate a maximum amount of tool eccentricity e.
The optimum value for the spacing Dl depends f~rther upon such factors as the effective dimensions of surface 37 of transducer 36 such as its diameter in case of a dis~ transducer 36.
For a dis~ transducer having a diameter of the order or about one inch to produce a pulse such as 50 in Fig. 2 with a frequency spectrum such as 52 in Fig. 3, the total distance 31 is generally o~ the order between about 2 to about 3 inches.
A basis for selecting the total distance D is thus to assure sufficient time to receive all those acoustic returns which significantly contribute to an accurate judgment as to the quality of the cement ~ond in the presence of a small casing-cement annulus. The total distance D should be long enough to enable the ~ortion in the acoustic returns attributable to a ~ad cement bond to be received free from interrerencs.
The acoustic returns include acoustic reflections arising as a result of the interaction of the initial acoustic pulse with various media. A first acoustic casing reflection arises from the interface between the water or mud inside the casing 12 and the inside casing wall 13. Thls first re~lection tends to be consist~ntly the same, varying wi'h mud consistency, ¦
inside casing wall condition, and tilts of tool 20. Subsequent acous~:o returns arise as a f~mction or reflections ~rom - llZ9~6~ ` I
. 'I,, . I

successive media as well as the leakage of acoustic reverbera- ' tions entrapped inside the casing. ¦ ~, Thus, ater the first casing reflection, the acoustic portion transferred into casing 12 is now reverberating inside S the casing walls 13-13' and leaking energy at each re~lection.
The energy lost depends upon the coefficients of reflections rO
(the reflection coefficient between the fluid inside casing 12 and the casing) and rl (the reflection coefricient between casing 12 and the next layer which may be cement as in -egion 26 or water as in region 32). The duration over which signi~
cant reverberations last inside the casing walls 13-11' is a function of the casing thic~ness. Since casing of greater thickness tend to cause longer lasting reverberations, the total spacing D between the casing and receiver-transducer should be correspondingly increased.
. . i When a window, which is normal to the direction of travel of the acoustic pulse, as suggested in dotted line at 42 in Fig. 1 is employed, the casing reflection and other acoustic returns produce reflections at the interface between window 42 and the mud inside casing 12. Such reflections appear as secondary transmis3ions which are returned to tne casing to produce a second casing reflection with subse~uent l reverberations in the casing and thus also secondary acoustlc ¦ retu~ns. These secondary acoustic returns disturb t~e cement .1 . I

I - - 1129~6~ -~ .

evaluatior, particularly in case of a good cement bond when the fo~mation also has a smooth surface. In this latter situatlon reflections caused by secondary reverberations mi~Y
I ¦ with a significant reflection ~rom the formation, giving an overall erroneous impression of a bad b~nd.
~ence, another criterion for determining 2n acceptable casing to receiver distance may involve selecting a dista~ce D3, between 2 window 42 and casing 12, such that secondary acoustic returns decay below a preselectsd percenta~e of their initial value. Thus, it can be shown that the number Nr! f reverberations in the steel casing 12 in such ranse is given by the relationship Nr = ln (x) ~n (IrOrll) where x is the percentage fraction.
The distance D3 can then be shown as given by the relationship D3 ~ ~r L ~
where L is the thickness of the casing 12, CO the ~elocity of sound of the material inside the casing, mainly water, and C
the velocity o~ sound in the casing, namely ste~l.
As z numerical example to arrive at an acceptable total casing to receiver distance, one may assume the values îor the materials employed in the ~ollowing Table l.

1129~)66 . I' T~BLE 1 Acoustic Impedance enslty Velocity o~ Sound Z in g/cm2sec p in g/cm3 C in ft/sec water ZO - 1.5 x 105 p > 1 CO = 4920 steel Zl = 4.6 x 106 Pl = 7.8 Cl = 19,416 cement Z2 = 7 7 x 105 P2 ~ 1.96 C2 = 12,000 and Z2 = Z~ in case of a bad bond. I I
Using these constants the values for the reflection co-efficients can be deter~ined as rO = 0 937 rlG =-.731 (for a good bond rlg =-.937 (for a bad bond).
The casing to receiver distance or D3 can be deter~ined from the zbove constants and time setting constraints. For example, if the reverberatlons in the casins are to decay to akout five percent of their initial value, the distance D3 can be from about one and o~e-~uarter inch to about three inches for a normally occurring range o~ casing thicknesses L fro~
about .2" to about .65". By relaxing the final value o~ decay of the casing reverberations the source to casing distance can be decreased, though about one inch is likely to be a lowest possible limit for ~3. Since the largest casin~ ~hicXness is prefera~ly accommodated, the distance .rom the transducer 3Z .o , either window 40 or 42 is chosen such that there is no Z; secondacy transmission interierence over the time interval oi , ' I ~lZ9~36~

interest. The distance D3, r~hen applicable, is chosen such that secondary reflections attributable to the window do not present signal interference. When the tool 20 employs a window such as 40, secondary reflections rrom such window are no longer a consideration in selecting the transducer to l casing spacings.
In the selection of the transducer 36, a disk transducer having a diameter to wavelength ratio or greater .han unity is employed. In practice, a dis~ transducer having a diameter o~
about one inch has been found useful. The transmitter pulse is formed of such duration and frequency as to stimulate a selected radial segment of the casing upon which the pulse is incident into a thic~ness resonance. Acoustic energy is trans- a ferred into th~ casing and reverberates in an enhanced manner l; with the duration and magnitude o' reverberations highly sensitive to the layer of material adjacent the external surface of casing 12. Such sensitivity, however, snould not include hydraulically secure micro-annuli such as at region 28.
~ I = e selection of the ~re~uency spectrum or the scousti~

1l !

- llZ9066 . I' , . l pulse from transducer 36, a primary basis is determined by the fundamental thickness resonance frequency of casing 12. Sucn resonance enables a trap mode with which enhanced acoustic I energy is trapped in the casing. The subsequent reduction of S ¦ trapped energy in the casins may be considered the result of ,~
¦ leaXage attributable to the degree of acoustic coupling to I adjacent media. The frequency spectrum of the acoustic pulse should preferably include ei~her the fundamental or a higher , harmonic thereof. Expressed in mathematical terms, the st~mu-iating frequency in the acoustic pulse is given by fO = N Cl where Cl is the casing compressional velocity and L is the .
casing thickness measured normal to the casing wall and N is a whole int~-er.

~ I
~ ' .
, ~l -34- 1 11 ~ .

r - -!i C llZ9066 r . . ....... , .
. . . , , I , An upper limit o~~ the fre~uenCY spectrum OL the 2COUS.iC
pulse is set by practical considerations such as casing roushnsss,¦
grain size in the steel casing and mud attenuation. ~urthermore, ¦
¦ the hydraulically secur2 micro-annulus must appear transparent. "
l In practical cement ~ond applicatior.s a casing-cement annulus ¦ equal or smaller than . oas~ ( .127mm) represents 2 Sood cement I bond and .hus prevents hydraulic communications bet~een zones ¦ intended to be sepa-ated. ~hen annuli larger than this ¦ value occur, these shoula be construed as bad cement bonds.
Furthermore, as long as an~ annulus is less in thic~ness tha~ about 1/30 of a waveleng~h o an acoustic wave travelinq in water, such - ~nnulus is effectively transparent .o an acoustic wave of such wavelength. Hence, in t~rms of casing-cemen annuli, the frequQncy spectrum of the acoustic pulse should be selected such that : ~at) x 30 where CO is the velocity of sound in water and ~at is the thickness o~ the annulus.
In practical terms, casing thicl;nesses ~ normaily .
encountered are rom about .2" (5.08 mm) to about .65" (16.51mm~.
~ence, ~ith an effective frequency of from about 300 X~z to a~out 6Q0 R~2 or the acoustic pulse, the casing 12 can be stimulated into a trap mode which is insensitive to hydraulically secure micro-annuli. This frequency spectrum is selected so that the trap mode can be ~ stimulated with either the ~undamental fre~uency or its second 2; ha =onlc o= tle thic~r casings.

ll -35-''.

- llZ9~66 Within such frequency spectrum, the duration of the reverbera-tions inside the steel casings become sensitive to both good and bad micro-annuli. For an acceptable micro-annulus the casing reverberations (and their observed leakage) decay more rapidly than for an excessively large micro-annulus.
The acoustic transmitter pulse is thus formed with character-istics as illustrated in Figs. 2 and 3. The transmitter pulse 50 shown in Fig. 2 represents a highly damped acoustic pulse of a duration of the order of about eight microseconds. The frequency spectrum of such pulse 50 is shown in Fig. 3 with a frequency-amplitude curve 52 showing a 6 db (one-quarter power) bandwidth extending from about 275 KHz to about 625 KHz with a peak at about 425 KHz. The thick casings having a trap mode below 275 KHz are driven into resonance primarily with a higher harmonic such as the second which occurs with significant amplitude in the band-width of the spectrum 52.
The transmitter 36 can be formed of a variety of well known materials to produce pulse 50 with the frequency spectrum 52. For example, an electrical signal having these characteristics can be formed and ampli-fied to drive a suitable piezoelectric transducer 36 capable of operating as a transmitter and receiver.
Preferably transducer 36 is formed with a piezoelectric disk crystal which is backed with a critically matched impedance such that an acoustic pulse is formed at the resonant frequency of the disk. The back-ing material has an impedance selected to match that of the crystal while strongly attenuating the acoustic pulse to avoid reflections from the back.
In some applications a protective front layer may be employed integrally mounted on the front of the transducer 36. Such front layer is preferably made of a low attenuation material having an acoustic impedance which is approximately the geometric mean between the crystal impedance and the expected borehole fluid impedance. Such front layer has a quarter wave-length thickness as measured at the center resonant frequency of the crystal.

- -il29~6~

Since the disk is critically matched, the acoustic output pulse has a wide frequency bandwidth. Excitation of such transducer 36 may then be achieved with an electrical pulse of very short duration. For example, an inpulse having a rise time of from about 10 to about 100 nanoseconds and a fall time of 0.5 to about 5 microseconds can be used.
In the transmitter mode transducer 36 may be actuated in a repetitive manner at a pulse rate, say, of the order of a hundred pulses per second. At such rate a circumferential region around casing 12 can be scanned as tool 20 is moved upward along the casing by making reflector 10 38 and its associated window 40 a rotatable mounting as illustrated for rotation in the direction of arrow 53.
Fig. 5 defines the performance criteria for a suitable trans-ducer 36, The transducer has a center acoustic frequency at about 425 KHz with a 6 db bandwidth of 300 KHz. The Fig. 5 illustrates an acceptable received amplitude response curve 55 when transducer 36 is energized with a pulse drive signal of about two microsecond duration and directed at a water/air interface spaced from the transducer at a distance equivalent to about 100 microseconds of two-way acoustic wave travel time, Tl. The output signal from transducer 36 as a result of the echo from the interface 20 preferably should have an appearance as illustrated where the first echo, formed of the three main peaks 57.1, 57.2 and 57.3, should be of no greater total duration, T2, than approximately six microseconds. The level A2 of the noise immediately after the first echo should be about 50 db below the level Al of the peaks 57 and have a duration T3 of less than about 30 microseconds. The noise level A3 following interval T3 preferably should be at least 60 db below the level Al of peaks 57.
The controls and circuitry necessary for firing of the trans-ducer may originate from above ground equipment or from a suitable clock source located at tool 20. In either case, -recurring synch pu_ses are produced on a line 34 of Fig. 1 ~o ectivete a pulse network 56 which genera~es a suLtable pulse on line 58 to drive transducer 36 while simultaneously protectlng ¦ the input 60 to amplifier 62 with a signal line 6~.
S ~ The transducer 36 responds to the pulse from network 56 ¦ with an acoustic pulse of the type as shown in Figs. 2 and 3.
. The acoustic pulse is directed onto reflector 38 which acts to direct the acoustic energy at the wall or casing 12. The e~ect o~ reflector 38 aids in compensating for variatlons in alignments of the acoustic pulse out of the plane normal to the caslng wall. The rerlector 38 can be a flat surface at an angle a of abcut 45 to the acoustic e~ergy ~rom transducer 36 or a slishtly concave or convex surface.
When the acoustic pulse S0 impinges upon casing 12, some of the energy is reflected and some transrerred into the casing 12. The reflected ener~y is returned to transducer 36 via re~lector 38 and is reproduced as an electrical signal and applied to input 6a of ampli~ier 62.
The energy t~ans~erred into casing 12 reverberates, causing in turn further acoustic returns to transducer 36. The resulting received output from transducer 36 may have the appear-ance as illustrated with reflection signal waveforms 64, 66 and 68 in ~igs. 4A, 4B and 4C.
¦ The initial segment 70 or each re~lection signal wave~orm represents the strong iritlal ceslng rerlection whosa duration is Il -38-i j, ' ' , i f~ - -- ~129~66 ` I
1~
. , . . , of the order of about five microseconds. The remainder 72 i5 characterized as a reverberation segment in that it represents a large number of cycles of pulses representative or acoustic reverberations whose magnitudes decay over a period of time.
The decay period varies as a function of the type of c~ment bond, as can be observed for waveforms 64, 66, 68 obtained with respectively differently sized annuli 30 around casing 1l2. .
Except for the initial casing reflection se~ment 70, the reflection signals 64, 66, 68 do not have a highly predictive pattern wherein the peaks are precisely defined and extractable.
Accordingly a prior art technique such as shown in the pre~iously identified U. S. Patent to Norel et al for comparing adjacent peaks to ascertain decay time constants for the waveforms is ; 15 difficult to implement.
Instead, the signal processing segment 21 of the apparatus lO operates on each reflection signal by separating the reverberation segment 72 from the initial strong acoustic , casing reflection segment 70 and subsequently integrating the reverberation segment 72 over a particular time span to determine the energy therein.
In the embodiment of Fig. 1, the reflection signals from transducer 36 are amplified in amplifier 62 whose output is applied to a full wave rectifier 76 to produce on llne 78 a DC
signa1 r~ esentative of the amp litIde oe the received acoustic-"

Il llZ9~66 - ' wave. The DC signals are filter-d i~ a filt-r 80 to provide on line 82 a signal representative of the envelope of tne waveforms from transducer 36.
The envelope signal on line 82 is applied to a threshold detector 84 which initiates subsequent signal processing by detecting the start of the initial casing reflection segment 70 (see Fig. 4). The amplitude at which the threshold detector 84 operates can be varied with a selector control-applied to line 86 and can be automatically set.
The output on line 88 of threshold detector 84 is applied to activate an enabling pulse on output 90 from a pulse producing nett~ork 92. The pulse from this network 92 is selected of such duration that the envelope segment on line 82 and attributable , to the initial casing reflection 70 is gated t~rough an amplifier ~4 as a casing reflection signal.
The duration of the enabling pulse on output gO ls selectable so that the entire casing reflection segment 70 can be selected in the event its duration varies. ~he DC form of the casing reflection signal is applied to an integrator networ~ or peak amplitude detector 96 to produce a signal rep-resentative o~ the amplitude of the casing reflection 70 on line 98. This casing amplitude signal is stored such 2S with a qample and hold networ~ 100 actuated ~y an appropriate pulse !¦ derived on line 102 rrom networ~ 92 at the end of the pulse on I line 90.

Il I
1l! 40 ~ LZ9~66 The output 88 from the threshold detector 84 is also applied to a reverberation segment selection networ~ 103 including a delay 104 which produces an enabling pulse to pulse producing networ~ 106 at a time a~ter the initial casing S reflection 70 has terminated. Networ.~ 106 generates a segment selection pulse on line 108 commencing at the beginning of the reverberation segment 72 and having a duration su~ficient to gate the entire envelope form of the reverberation segment 72 (see Fig. 4~ through gating amplifier 110 to integra~or 112.
The segment selection pulse on line 108 commences after the initial casing reflection and terminates after the desired number of acoustic returns of interest have been received ~ut before secondary transmission interference arises. A typical pulse would start about six microseconds after the initial lS casing reflection is detected and would last for a period of about 40 microseconds after an acous,tic pulse issued such as s~own in Figs. 2 and 3 and with a spacing D of the order of about three inches.
The integrator 112 integrates the e~velope form for a time period determined by the pulse on line 108. At the end of this latter pulse a signal on line 114 from a pulse producer 106 activates a sample and hola networ~ 116 to store a signal representative of the energy in the reverberation segment 72.
The outputs from sample and hold networ.lcs 100, 116 are ~5 applied to a cambinin~ networ.~ in the form of a divider 118 Il llZ9~6~ 1 I ' . , , . . ll which forms a quotient by dividing the signal representative of the energy in the reverberation segment 72 by the normalizi~g signal indicative of the amplitude of the czsing reflection 70 ¦ to senerate a normalized energy bond signal on output line 120.
¦ The normalized energy signal on line 120 can be transmitt~d to above ground for recording reflection energy as a function of the depth on a plotter 122. The normalized energy signal may ¦ also be applied to a comparator 124 for comparison with a ref-I erence slgnal on line 126 derived from a network 12~ and rep-resentative of the threshold level between good and bad cement bonds. The output 130 from comparator 124 indicates the presence or a~sence of a good cement bond can also be recorded on plotter l22 as a function of depth.
With the signal processing embodiments~ the bond signal on line 120 is made less sensitive to tool tilts and - attenuation in the fluid wnereby the acoustic energy is directed at casing 12 along a plane which is skewed relative to the axis of the casing 12. When such condition occurs, the received acoustic returns are reduced in amplitude and may ba ¦ interpreted as good cement bonds when, in fact, the cement ¦bond may be bad. By employing the amplitude o~ the initial casing reflection as a gauge of tool tilt and mud conditions, ¦Ithe bond signal on line 120 provldes a reliable indication of ¦¦the cement bond quality.
There may in certlin cases ari.e a nead to obtain a 1, .

Il -42-! ~
.. I I . I

z9066 ` -l ........ .............................................................

bond sig~al which has not been normalized or which may be normalized at a later time. In such case the output 117 of the sample and hold networ.~ 116 is the bond signal w~ich may ~e ¦ transmitted to abcve ground equipment for recording such as on I a tape recorder or on plotter 122 or in the memory o~ a signal processor 130 after conversion to a digital form.
After a bond signal has been gene`ated and a new synch pulse occurs on line 54, the synch pulse is applied to several ¦ reset inputs of sample and hold networ~ 100, 116 and integrators 96, 112. The reset of the sample and hold networks 110, 116 can be delayed for a smoother output until such time as the outputs from integrators 96, 112 are ready for sampling.
The selection of a signzl representative of the acoustic reverberation return 72 is obtained with a pulse produced on line 108 as can be determined with a segment selection ne~worlc 132. This network controls the length of the delay 104 and the width o the enabling pu1s2 from pulser 106. As previously described with reference to Figs. 4A, 4B and 4C, the reverbera-tion segment 72 is selected in such manner that ~he casing reflection 70 is effectively excluded.
This exclusion c~n be advantageously achieved by the signal processor 21 since it is activated by the detection of i the strong casing reflection 70 as sensed by threshold detector 84. The resulting integration of the remaining envelope provl(es a sharp discrimiAation between a sood bor.d signal ~ d . ~

i I .

llZ9~

. ~, a bad bond signal. For example, the integration of the rever-I beration segment 72.1 of the waveforms 64 in Fig. 4A will be ¦ greater than the integration of ~he reverberation segment 72.3 ¦ of waveform 68 in ~lg. 4C by a factor of about 3. When the ¦ area of the envelopes are compared for an example as set fortn in Table l, with the resulting reflection coef.icients for and rl for good and bad cement bonds, an integration ratio of about 3.8-to-1 between bad and good signals occurs. ~ence, an extremely sharp good-to-bad bond contrast is obtained which is likely to be obtained even in the presence of a dense mud inside the casing 12.
With certain types of cement one may wish to construe a micro-annulus of a thickness of the order of about .010 inches (.25mm) as a good cement bond. In such case, the frequency spectrum 52 of the acoustic pulse 50 may be adjusted to investigate the cement. One may, for example, employ two types of acoustic pulses of different frequency spectrum, one having the fundamental frequency and the other acoustic pulse having a harmonic. If the results from these pulses do not ¦ give the same reading, a hydraulically secure micro-annulus can ¦ be concluded to be present.
¦ Theoretically a bond will appear as good for a micro-annulus having a thickness of the order of half wavelengtn (about 0.08 inches). Eowever, in practice such large ~ annulus i unlikely to arise and other conventional cement llZ9066 quality investigation techniques can be employed to identify such unliXely large annulus as a poor cement bond.

Fi~s. 6A-6C and 7 Figs. 6A through 6C illustrate tne effectiveness of S tool 10 when a frequency spectrum is made on the obser~ed entire acoustic returns such as illustrated in Figs. 4A-4C.
The spectra 140 of Figs. 6A-6C represent respectively a bad bond with a large annulus, an i~termediate bonding situation such as with an annulus of .OOS" and a good cement bond. The spectra 140 when originally obtained may have varied i~ absolute masnitude because the reflection changes in the taol eccentricit~
e and the coùpling of acoustic energy to the cement 14 behind the casing 12 varies. Thus for a good cement bond, the ¦ absolute amplitude of the acoustic returns is lower ~han for ¦ a bad cement bond. The relative size of dips 142, however, varies with a larger dip for a bad cement bond and a smaller dip 142 ~or a good cement bond. Por convenience, the spectra 140 are shown in Pigs. 6A-6C with generally equal amplitudes so that their dips 142 can be evaluated by a visual comparison with each other. The significance of dips 142 should be determined in light of the overall energy spectrum or ~e reflection signal.
The sharp dips 142 in spectra 140 are centered at the trap mode or thickness resonance of the casing from which the I
I

2906~

reflections came. In the spectra 140 the dips 142 occur at .S MXz (500 K~z) for a .23 inch thick casing and resemble the effect of a narrow bandwidth energy trapping filtar. In the case of a bad bond, such as for spectrum 14~.1 in Fig. 6A, j the dip 142.1 is is deep, indicative that a relatively sub-stantial amount of energy at the thickness resonance has been I trapped inside the casing walls 13-13'.
The improvement of the cement bond is evidenced in spectrum 140.2 by a correspondingly smaller amount of energy being trapped inside the casing walls 13-13'. ~ence, dip 142.2 in Fig. 6B is smaller in comparison with dip 142.1 in Fig. 6A
while dip 142.3 in Fig. 6C is the smallest for a good cement t bond.
~ig. 7 illustrates an embodiment 150 for evaluating the cement bond utilizing the sharpness of the dips 142 in spectra 14~ of ~igs. 6A-6C. The output 63 o' amplifier 62 in networ~
21 is applied to two passband filters 152 and 154. Filter 152 is a passband filter tuned to the casing thickness resonance frequency of the casing 12 under acoustic investiga-tion. The passband for filter 152 preferably is narrow witn ¦ sharp rising and falling slopes. The filter 152, however, should be sufficiently wide in its frequency band to overlap the frequency range of dips 142 for the expected tolerancs I variations in casing thickness. Generally, a filter 152 with a pass and or aoout 10% to lS~ o the center requency would -~6-llZ3066 .
Il . .

¦I suffice, though a smaller passband of about 5% may provida a ¦¦ dip amplitude indication on line 1;6. A digitaL as well as ¦¦ analos filter 152 may be used.
Filter 154 prerera~ly is tuned to a separatP non-overlap-¦¦ ping segment of tne spectrum of the signal on line 63 to provide a refPrence signal on line 158 indicative of the ¦¦ amplitude of the spectrum of the signal on line 63. Other ¦ devices can ~e employed to derive such r2ference slgnal such as I the peak detection technique described with reference to the embodiment in ~ig. 1. The dip amplitude signal on line 156 is thereupon normalized by dividing this signal ~y the reference on line 158 with a divider networ~ 160. A normalized dip value signal is then available on the output 162 of divider ~ 160 to provide an indication or the quality of the cement bond I for recording or plotting as the case may be.
" " . .
Fig. 8 Pig. 8 illustrates another . embodiment , ror _~
determining the cement bond. The output rrom transducer 36 on line 63 from amplirier 62 (see Fig. 1) is applied to a high 2a speed analog to digital (A/D) converter 172 whicn is actuated ¦a specified time after an acoustic pulse. This produces a ¦! digiti~ed reflection signal îormed or sequential numerlcal ¦'samples representative of the amplitude of the rerlection sisnal.
¦IThe A/D converter may be deactivated a certain tima period ,, .

' 1, ~
!

I1 1 l Z9~)6~ .

I . . I

following generation of an acoustic pulse.
A/D converter 172 is located downhole in tool 10 and ls capable of operating at a very high speed and is provided with ¦ sufficient storage capacity to initially store and subsequently I transmit the samples at a slower rate to a surface located ¦ signal processor 174. The latter could, if desired be also ¦i located in tool 10, but this would depend upon the scope of operations the signal processor 174 must perform.
¦ The sampled digital re~lection signal is stored in a I memory 176 which may be a solid state memory or a magnetic memory. The memory 175 can be an integral part of pracessor 174 for immediate processing of the samples or be a periph~ral device which is accessed at a later time after logging of the borehole 16.
lS Signal processor 174 may be programmed to select, at 178, those reflection samples, Ac, representative of the casing ¦ reflection 70 (see Fig. 4). The procedure can ~e similar to that illustrated in analog form in Fig. 1. Thus the rerlsction samples are scanned to detect the first sample which exceeds a predetermined threshold and this first sample becomes the arrival time of the casing reflection. A certain number of ¦ samples foliowing this first sample are then selected as representative of the casing reîlection 70 (see Fis. 4).
I A certain number of reflection samples, ~r~ following 1~ the c. ~s ing ref lection sa:np le s Ac ~ are 5 s lect2c at 18 0 as Il . I
Il . I
Il -48-i! i ll l llZ9(~66 I
.
representative of the reverberation se~ment 72 in the reflection signal (see Fig. 4~.
Integration of ~e reverberation samples is done by summing the absolute values of the samples at 182. This S summing step could be carried out as the reverberation szmples are sele~ted at 180. However, for purposes of clarity, the summing operation is shown as a separate step. The integrated sum ER is stored.
Integration of the caslng reflection samples Ac is obtained at 184 by summing the absolute sample values and storing the result, Ec.
A normalized bond value, CB, representative of the qùality of the cement bond may then be obtained at 186 by dividing the integral ER by the integral EC at 186. The bond value CB may be recorded in memor~ or plotted 25 desired ¦ ;
at 188.
. ' . ~, Figs. 9, 10 and 11 . Fig. 9 illustrates still another embodiment for inves-. tigating the quality of the cement bond. A tool 210 suspended 2~ from a cable 211 is provided with a plurality cr transducers, such as 36, but arranged circumferentially around the tool 210 to provide sufficie~t circumferential cement band evaluation resolution. The transducers 36 are a.~ially spaced to accommodat-the la~e number. A practical n~mber or transducer 36 ma~ be I ' I

i ~ 9066 Il i eight which ar~ circumfer2ntL~lly spaced at 45 interval..
The axial spacing is selected commensurate wit~ the size o~ the transducer 36.
¦ Figs. 10 and 11 relate to a signal processor 21i ~or S ¦ operating a tool such as 210 shown in Fig. ~. The signal processor 215 is described useful for a tool 210 employing eight transducers 36; however, a greater num~er of transducers ¦ can be accommodated. The signal processor 215 has ar. adjusta~
¦ clocX 212 on whose output 21g are pulses 216 (see Fig. 11) at ¦ a rate selec~ed to determine the resolution of the cement bond in~estigation. The clocX source may be derived from above ground de~ices or from a suitable oscillator located in the tool 210.
The clocX pulses 216 are applied throush a delay networX
218 to a transducer selector 220 and a transmitter pulse multi-plexer 222. The transduce~ selector 220 provides a discrete output enabling signal on line 224 to identify each different transducer 36 in succession. Hence, multiple~er 2~2 is enabled to sequentially fire pulsers 226 coupl~d to transducers 1, 1 .

- " ~
''' "'- , ~ , 11~ ~ ,,, , , I
~ ~ :

, 36~
The transduce-s 36 also actas recei~ers and produce sisnals on output 1ines 228 for a.mpli,~_cation in 2re-ampl~fiers 230 opera-tively associa~ed with each tr2nsduc2r 36. The output OL- amplifiers 230 are connected to a receiv^r multi~le~er 232 which iscontrollsd by the tra~sducer identifylng sig~als on llne 224 fro~ transducer selector 220. In addition, a segm^nt selection networ~ 234 is activated wlth each tra~sducer fir~ng to ge~erato enabl~ng sis-nals 236, see Fig. 1l, on an outpu. 238 to er^rectively ena~le multi-plexer 232 to select the desir~d seçment from the trarsducer outPutl while rejec ;ng or bla~ing out the initial .ransm~tter segments.
~he output 240 from multiplexe~ 232 wlll have an app-arznce as . illustrated at 244 in ~lg. ll. A small noise signal 242 preced-s ; the reflec~lon signal 2~4 t1hich has zn appearance gererally 2s , lS illustrated in Figs. 4A-4C.
Returning to Fig. 10, the reflec.ions on ou.put lLne 240 ¦are amplified ~y ttJo variable s2in a~plifiers (VGAj 246, 2~5.
IAmplifier 2~6 has its sain controll2d by 2 signal on line 249 and i! ¦derived from e~ther above srour.d e~uipment to adjust for mud ~20 lattenuation efrects or from a down hol9 automatic gain control. Th~
s2cond amp1ifier 2~8 has its g~in autom2tically contro1led in tool 21~ to adjust the eccen.erins OL^ tool 210 as will be further , Ie~plained.
, 1, The output 250 from æmplifier 248 is recti'ied in ~
'25 ~ne~70r,t; 76.l and applied to a casing reflection sensing net~;or.c ¦'ror~.ed or gated am~lifier 94, integra~or 96 and sample and hold Il , , -51- .
.' 1~ - . 'I

. ;~ llZ9~6 ., ,. -,' . ., ., . ' ' , ' ' '. . ' .
ne~70rk 100 as describad wi.h reference to Fig. 1.
, O
The output on line 250 ~rom amplifier 248 is further ampli-fied in an ampli'ier 252 by a su,^ficient amoun. to compens2te ~r the approximate dif~erence in signal am~plitude between the casing ¦ reflection and ~he acoustic returns indicati-~e of subsequent reverberations. An acceptable compensation ~ay be a gain ~acto~
of about 20 db for amplifier 252. The re^lections of interest are then applied to a full w2ve rectifier 76.2 for subse~uent integration with devLces as d~scribad wLth reference-to Fig. 1.
Control over the gating am.pliCiers 9~ is derived gen-erally as described ~7ith referenco to Flg. 1 with a threshold de-i~ tector 84 responsive to the output on line 78 from full wave d~-tector 70.1. A reerence threshold val~e is derived on line 80 as a resu}t of a s~milar previous cement bord investiga.ion made wit~
the particular transducer as shall be further explained.
,,' . I . . , , The output 8~ from threshold networ.~ 8~. is applied to the set input o. a latch networ~ 256. Networ.~ 256 has a reset input 258 responsive to the cloc~ pulses on line 214 (befora the delay from networ~ 218). When th~ thr~shold det_ctor senses a signa}
! on line 78 greater than the rererence value on line 8~, a pulse is applied to net~70r~ 256 whicn therezfter is inhibited l^ro~ respond-~ng to further in~uts from the thresnold d2tector until netwo_.c 256 is res2t by a pulsP on line 21~. The output on linP 260 ¦¦ will have the appearance as shoT.~n witn puls~s 252 (Fig. 11) 1, having an active state upon the occurrence OL the large casing Il .. . .
1'1 ' .- , 2 -1~29~6~

I
.' 1. . I

I reflection.
¦ The integration times, Tl and T2 (see also ~ig. 11), ror ¦ signals representative of the casing reflection and the reverbera I tions are derived with pulse networ~s 92 and 106 respectively, whose outputs 90, 103 are applied to enable gating amplifiers 94, 110. The duration and occurrence of the integration periods T
and T2 are respectively about 8 microseconds for the casing -e-. flection and a~out 30 microseconds for the reverberations.
Subsequent integration of the casing reflection signal by integrator 96 and the rever~exation seqment by integrator 112 are terminated at the end of pulses Tl and T2 when the output ' from amplifiers 94, 110 go back to zero. The integrator outputs are sampled at the end of pulse T2 and the samples made available for further processing with a suitable multiplexer 266 for trans-mitting the samples to above ground equipment. Transmission of information may employ an analog to digital converter 267 and suitable telemetry equipment 26~ for transmission up cable 24.
The integrators 96, 112 are resat by pulses on line 219 and the sample and hold networX by pulses on line 214 from transrer logic 271 at the time or clock pulsas 214.
As previously mentioned, the gain control for ampli~ier ¦ 248 is automated by sensing the peak value of the casing reflection on line 78 with a peaX detector 270. The peak value is then converted to a digital value with A/D convsrter 272 ¦ and this value placed in a storage nat~Nor.~ 274 in a location associatsd with the transducer from which the reflection was . l l .. l I . I
'i -;3-..: ` ~ ~129`~366 ( ,: 1. ' ' . .
1 obtained. The nex~ time tnis transducer.is energized,-the ¦! transducer selector 2Z0 provides an aopropriate address signal ¦ to a read-in read-out logic networ~; 27S to apply the prevlouslY ..
¦ stored peaX value to a gair. con.rol networ.~ 27~ and 2 threshold ¦~ reference signal producing retwo-.~ 278. . ..
. . ¦ .. For s2in co~trol the disital pea.~ -value is converted to ¦ an an210g sisn21 and an appropriate bias.applied to control the ~, . I gain of am~ ie_ 2'8. In a similar manner, tho- threshold . I reference value on lir.e 86 is maint2ined at the appro~riate level .
; 10 ¦ for each trarsducer 36 . . . .
.. ¦ :: The techniques employed in evaluating the cement bond as de-. , , , , : , .
¦ scribed herein adv2ntaseously enable accurate measuring of the eccentricity of the tool as it moves alons the casing. This. .
techniqua as shawn 'n Fig 10 i~vol~es a timer 280 which is ener- .
1I sized each tlme a transducer 36 is initially fired The timer 280 .
is deactivated to s~ore-~ ~easu-ed ~ime interval when a casing re- . .
¦! .flection is detected by the th~eshold detector 84 as eviden_~d by .
¦I the si~nal on li~e 260. The mea~sured tima intervals for the . .
~ various transducers should be the sa~e and any difrarqnce may then .
1~, be attributed to an off-center position o' the tool. The outpu~
!~ of timer net~ork can be recorded or plotte~ and sultabole processed to m~asur~ and loca~e the eccent-icity of tool 210.
' . .
~, The vertical resolutlon of the tool 210 is a function of .
the repetition ra'e with which tha transducers 36 are enersize~ .
2S ,~ and produce detectable casing re~lections and rever~erations.
., , .
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~.
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.' . . ..
; A repetition rate as high as 100 per second can be accommoda~ed to yield a resolution as small as about every one-tenth or an ; inch when the tool is moved a~ a losging rate or about 10 inches ¦ per second along the casing. A signal on line 213, see Fig.
S I 9, representative of the depth of tool 210 is obt2ined to : I enable a signal processor 215 to adjust for the difference in ¦
~ I levels of trans~ucers 36.
.' I . I
Figs. 12, 13 and 14 I Figs. 12, 13 and 14 illustrate an acoustic energy source !j, 10 and detector 300 ~or multiple use on a tool such as shown and described with reference to Fig. 3. The detector/source 300 is radially mounted to a cylindrical housing 302 wi~h a mountins bracket 304 having a central aperture 306 to receive a cylindrical or disk transducer 36. The mounting bracket 30 4 lS extends past the emitting surface 37 of transducer with a lightly outwardly expanding aperture wall 308.
Bracket 304 may be directly mount~d to housing 302 suc~
as shown i~ ~ig. 12 or with an intermediate spacer 310 as shown in Fig. 13. In the mounting of Fig. 12, the transducer to 2a casing spacing D can accommodate smaller casings~ say from ¦ about 5 1~2 inch diameter. The arrangement or ~ig. 13 can 11 accommodate larger casings.
; 1¦ The radial orientation or transducers 36 prererably I involves no window ~r intermediate ma.~rials. Furthermore, Lche ., 1, .

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the spacing D bet~een the transducer race 3,7 and casing 17 is , kept as small as possible.
Since too small a spacing D e~ables secondary t_ans-5i missions to interfere witn the reflections or interest, the spacing D cannot be too small. On the other hand, if the spacing D is too large, mud attenuation effects can be too large as well. Xence, a compromise spacing D may be selected based upon expected attenuations.
The attenuations may vary dependins upon the type o~
" 10 ' ~ mud used. For exam~le, a heavy or dense mud may cause an un-desirably high attenuation. ~ence, in the selection of an acceptable spacing D, it may be necessary to also specify an upper mud density limit. With such upper limit, the maximum attenuation may be about 4 to 5 db per inch in contrast with a ~; }5 heavy mud attenuation of abaut 8 to 10 db per inch.
With these general constraints, an acceptable spacing D
may be of the order of about one to about two inches ror most ~;~, casings.
The described arrangement for tool 20 with a rotatable ` ZO reflector 38 may be varied in a number o ways. For example, in ;~ 'some instances it may be desirable to mount the reflector 38 in a pad near the wall of casing 12 to reduce the attenuation effect of a ,dense mud fluid. Care should be exercised to assure that the reflector 38 remains su~ficiently spaced from the wall of the casi~g 12.

.

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~ Figs.
lS and 16 and 6 : Casing thic.~ness is measured by analyzing the frequency spectrum of the reverberation segment 72 (see Fig. 4A) repre-. sentative of acoustic returns ~ttributa~le to reverberations ¦ between the casing walls 13-13'. When an acoustic pulse such : 5 ~ as 50 is directed at the casing 12, a substantial amount of . I acoustic energ~ at the resonance frequency is trapped inside the . ~ casing walls.
The reverberation. segment 72 has predomin~nt components ¦ in a frequency portion 320 (see Figs. 6A-6C) generally in frequency alignment with dips 142. The dips 142 increase in depth as the quality of the cement bond decreases, but the .
.~, amount of energy trapped in between the casing walls increases .
., with poorer bonding between the cement and the casing. ~ence, the actual amplitude of the acoustic returns in the frequency ¦ portion 32C will vary. Generally, the actual amplitude of ~: the acoustic reverberations within the frequency portion 320 .: . reduce as the acoustic coupling between casing 12 and cement 14 becomes more erricient; i.e. as the cement bond becomes ., better.
This is illustrated in the spectrum plot of Pig. 16 with curves 322 and 324 which respectively illustrate the ~requency.
. spectrum of a frequency portion 320 for a bad cement bond and : a good cement bond.
. . ~hen thin spots develop in casing 12 such as at 33.1 .

, -57-129~366 ,. 11 ", 11 .

Il and 33.2 in Fig. 15, they are likely to affect the cement bond evaluation. The ef~ect or such thi~ spots upon the ce.~ent ~ond is not easy to predict and appears ll.'~ely to be a function or ¦ such factors as size an~ cement conditions. For example, I there is no cement bonding behind the thin spot 33.1, but ¦ since the casing is substantially thinner here, less acoustic ¦ energy remains tra~ped inside the casing walls 13-13' than i~
case of a normal thic.kness so that the thin spot 33.1 may appear as a go~d bond. On the other hand, if an isolated I external thin spot such as 33.2 occurs at a well bonded area the casing 12 may appear as poorly bonded. Hence, it is advantageous to be able to correlate a casing thickness measure-ment with an evaluation of the cement bond to remove ambiguities.
The measurement of casing thickness is done in the apparatus 326 of Fig. lS by forming a frequency spectrum o,~
the reverberation segment as derived on line 63 of Fig. 1.
The frequency spectrum is characterized by one or more peaks of which the largest occurs at a fundamental frequency whosa 2a wavelength is twice the thickness of the casing. Other peaks occur at frequencies which bear a whole multiple relationshi~
to the fundamental frequency.
~ig. 16 illustrates several frequenc~ s?ectra 322, 32 i ¦ o~ several reverberation segments 72 selected from dif~erent .; I . , I

I -~8-- 11 llZ9~66 I -I

signals. It should be understood that in the presentation or the various spectra in Fig. 16, there is no inte~t ~o set forth an amplitude rslationship between the spectr-~m 52 of the acoustic pulse 50 (see Figs. 2 and 3) and the other spectra 322,324; rather, it is only intended to show a frequency relationship in that the spectra 322, 324 occur within the 'requency bandwidth of the incident acoustic pulse. In practice, the absolute amplitudes of the acoustic spectra would be quite small in comparison with that of the trans~itted pu1s2.
Of particular interest is the relative frequency shift between the spectra peaks 328,330. The frequency difference b~tween peaks 328,330 can be attributed to a change in the thickness, L, of casing 12. Hence, by aetermining the fre-quency of the peaks predominantly attributable to acoustic returns from the reverberations betwe~n the casing walls, an indication of the casing thickness can be obtained.
The casing thickness, L, can be derived from the rollowing relationship L = N C , where fp is the ~requancy of thq pea~
in the spectrum, C the compressional velocit~ in the casing 12 ; and N is a whole integer depending upon whether the measured peak is for the fundamental frequency (~=1) or a higher harmonic.
¦I Since the frequency spect_um 52 or the acoustic pulse 2i l 50 has a bandwidth of from about 300 to 600 XH2 for use with i' I
,.

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¦¦ casings 12 over a wide range of thicknesses, from about .2" to ," I about .75", the second har~.onic (N=2) is likely to produce the : ¦ largest peak in the reverberation spectra ror the ~hicker ~ ¦ casinss while N=l for the thinner casings. The value for N, ; ¦ therefore, can be determined prior to an acoustic investigation ¦ from a Xnowledge or the type of casing installed in the borehole.
¦ For example, an installed casing is known to have a ¦ nominal thic.~ness of .362 inches, so that its fundamental thickness resonance occurs at about 331 XHz ror a value of C
of 20,000 ft/sec. As actually measured, the spectrum 322 may present a peak 328 at a frequency of fp2 of about 348 K~z corresponding to an actual casing thickness of .34; inches in , one radial segment of the casing. Spectrum 32a presents a peak 330 at a frequency fpl of about 303 K~z corresponding to an actual casing thickness of .395 inches. These measurements illustrate the resolution of the technique by detecting a casing thlckness variation of about + 7% due to manufac.ure ' from the nominal value of .362 inches. ' , In apparatus 326 or Fig. 15 the casing thic.kness is " 20 measured by selecting the reverberation segment 72 on a line;~ 332 with a selection network 334 c'oupled to the reflecti'on signal on line 63. ~he selection net~orX 334 employs a caslng reflec--¦ tion detector 336 to provide on output 338 a p~lse whose leadinS edge'is representative or the start of the casing rerlection 70 (see Fig. 4). ~etector 336 may ~e -ormed of a I
;

1~ 1 :
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¦ threshold detector 84 for rapid response or as s~own in Fig. 1 of a full w2ve rectifier 76, filter 80 and threshold detector 84.¦
¦ The pulse on lire 333 is delayed by a dela~ 340 for a time period commensurate with the duration ~r the strong initial . 5 casing reflection 70 to then actuate a pulse networ~ 342.
,, The latter ~roduces a reverberation segment selection pulse on ¦ line 344 to enable an analog gate 346 for a duration corres?ond-ing to the time needed to select the portion of the reflection signal predominantly representative of reverberations inside ,' 10 the casing walls.
A spectrum analyzer 384 is responsive to the reverberatian segment on,line 332 to provide on line 350 a signal representa-, . tive o~ the amplitude, A, of the frequency components in thereverheration segment 72 while o~tput line ~52 carries a corres-ponding frequency slgnal, f, representative of the frequency ; of the amplitude components on line 350.
'; ~he amplitude and frequency signals on lines 350, 352 are individually applied to analos to digital converters 354, .~ 356 which produce and store in a memory 353 of a signal ~; 20 proces30r.360, the digital signals representative or the amplitude Ai, and frequency, fi, of the frequency spectrum of .` . ¦the reverberation segment 72.
,~. ¦ ~he operation of spectrum analyzer 348 and ~/D converters 13S4, 356 is initiated b~ ~he reverbera.ion segment selection 2; pulse- gener.ted on lLne 344 rom pulse networ.k 342. Durirg , !
ll -61-~ :~129~66 . I
'' the latter pulse, a local oscillator, internal to spectrum analyzer 348, is repeatedly swept tnroush a frequency range to produce the amplitude spectrum on line 350. Each time tne ¦ local oscillator is swept ~hrough its frequency range, spectrum I analyzer 348 generates a spectrIm field or amplitude, Ai, and ¦ frequency, fi, signals. Hence, during the selection of a single reverberation segment 72 a plurality of spectrum fields are generated and stored in memory 353.
I Por a non-recurring re~erberation segment 72, a dlscrete I multiple of sweeps of the local oscillator in ~he spect ~m analyzer 348 can be sufficient to derive an indication of the frequency spectrum. The A/D converters 354, 356 are of such type that an adequate nu~ber of conversions can be made during each sweep of the local osclllator.
lS Once the spectrum fields formed of frequency, fi, and amplitude, Ai, signals are stored in memory 358, signal processor 360 is actuated to search for a peak amplitude value, Ap, at 362. This is done by searching all of the stored amplitude values, Ai, and comparing each with the next amplitude value and retaining the largex amplitude value ror the next c~mparison. By preserving the frequ~ncy value, fi, associ2ted with each each retained amplitude ~alue, tne frequency, fp, or the peak Ap can be found and both are appropriately stored at 364.
In certain instances se~eral peaks may occur in the .

- llZ9~66 ;~ ¦ stored spectrum samples. Althouqh the largest peak is used to derive a thickness determinatlon, one may als~ employ both peaks for this and select the casing thic.~ness meas~rement which is closest to the nominal value as the proper measurement.
The detected peak vzlues, both amplitude, A~, and frequency, fp, may then be recorded such as on plotter 122.
The frequency, fp, may be recorded directly as an indication proportional to casing thickness, L, or the latter may be compute2 on the basis of t~e previously described relationship and then recorded. Other information may be simultaneously recorded on plotter 122 such as well depth on line 24, the cement bond signal on line 120, azimuth of a rotational scanning reflector on line 37 to identify the depth and circumrerential l, location of the radial casing segment whose thic~ness was measured. -. ' . , Fig. 17 i; In an alternate embodiment for determining casing - thic~ness as shown in Fig. 17, the entire rerlection signal on ; 20 line 63 is digitized as described with respect to Fig. 8 for the evaluation of the cement bond. ~he digitizing process is commenced upon the detection o .he arri~al of '~he casing rerl~ction by detector 336 which is described with refer-nce to ~ig. 15.
Th output pulse on lire 338 rrom dete~tor 336 is a `. I
"' ' 11 . I

llZ90~fi 1 pulse o~ sufficient duration to enable digitizing of an entire i reflection signal such as 64 ~see Fig. 4~. This pulse activiates a net~.vor.~ 370 wnich generates a pulse on line 372 with a duration generally about e~ual to that of the casins S reflection segment 70 shown in Figs. 4. The pulse on line 372 in turn closes an analog casing logic 374 for thls time period to pass '~he casing re~lection segment 70 on.o A/~ converter 172. The latter digitizes the casing reflectlon segment 70 and stores the samples in a suitable memory (not show~).
When the casing rerlection segment has passad, the pulse on line 372 goes inactive which, in turn, activates a network 342 to provide an enabling pulse on line 344 to permit analog reverberation gate 346 to pass a reverberation segment 72 ; through an amplifier 376,having a gain controlling input 374, lS to A/D converter 172.
The am~lifier 376 permits a~plification or the normallv weak reverberation segment 72 for more precise signal procsssing.
The digitized reflection signal may be processed downhol~ or transmitted up the cable with a suitable telemetry de~JIce 380.
A signal processor 382 is provided to operate on the digitized rerlection signal rrom A/D converter 172. The I processor 382 provides a casing thic'cness deter~inatio~ at 38g ; I and a cement bond evaluation signal, CB, at 386.
¦ The casing thicXness is determined by salectins t~e ¦ reverberation samples AR at step 388 and generate a spectrum ' ', 1 112g~66 ll I Ij , .
I thereof at 390 with a fourier transformation. The spectrum I is formed of amplitude values Ai and associated frequency valueS Fi The spectru~ is the~ scarned to select the maximum ~ea~
S value. This may be done by setting, at 392, a countor equal to the number, DN, o reverberation samples, a constant K = 1 and the values of PL~AX and F~ X equal to zero.
A test is made at 394 whether the amplitude value A or the sample K is greater than ~L~Y. If so, then ~he values for A~AX and F~A~X are made equal to A(~) and F (K) at 392. The next samples may ~hen be ex&mined by incrementing X and de-crementing the counter by one at 398 and testins for whether the counter is equal to zero at 400.
! If not all of the samp~es have been scannedt the counter lS is not equal to zero and the search for a maximu.~ sDectrum value is repeated at 394. Once all of the samples have been ¦¦ scanned, the maximum values, A~AX and FM~Y can be plotted at ¦¦ 384 or the casins thic~ness, L, deri~-ed from the formula 2 ~FMAX r A cement bond evaluation can be conveniently made by signal processor 382 utilizing the ste~s as described with ~ ¦ reference to Fig. 8.
; I The cement bond signal CB varies as a functior of casirg ~ thickness. This variation can be substantlally removed rro~
the cement bond signal at 402. This involves dividing the '. jl I
I' I

~ -65-. ,.

~ Z9~

cement bond signal CB by a casing thic~ness signal ~ as de-termined at 4Q4 from the frequency measurement ~.~X using the ¦ casing thic~ness relationship as previously explained.
¦ This ncrmalization of the c~ment bond signal removes variations due to directly proportional casing thickness changes,~
leaving lesser second order casing thickness efects. The cement bond ,or a particular radial segment can thus be advantageously evaluated in a manner which is substantially . insensiti~e to the casing thickness at ~he same r~dial segment.
;~ 10 C~ment bond normalization relative to casing thickness may also De carried out directly with a cement bond signal such as available at 182 in Fig. 17 or on line 117 in Fig. 1 before normalization by the casing reflection signal. The latter signal may then be employed to further normalize the cement ; bond evaluation as describ2d.
l ., ; Fig. 18 Fig. 18 shows an alternate embodiment for dariving the ; re~uency of a peak in the spectrum of a reverberation sesment 72.
~ Th- outputs 350, 352 .rom spectrum analyzer 348 (see Fig. 15~ are ; l 1' !
. I . I

,. 1i 1 I llZ9Q~

recorded on continuous tracks 410.1, 410.2 of a storage medium 412 such as a magnetic disc or drum. ~fter recordi~s the output rrom analyzer 348 ror a reverberation seqment 72, the information is played back ror analysis for an associatPd signal processing network 41g to detect, store and record the amplitude and frequency peaX values, Ap and fp.
The spectrum analyzer outputs 350 and 352 are shown coupled through logic amplifiers 416, 418 to record/playback heads 420, 422 operatively disposed with respect to magnetic storage disc 412. The amplifiers 416, 418 are enabled by the segment select pulse on line 344 (see Fig. lS). ~he amplitude, A, and freguency, f, signals are recorded on separate continuous tracks 410.1, 410.2 which have sufficlent recordins length to record an entire reverberation segment 72.
A~ter recording o~ the reverberation segment, losic play-back amplifiers 424, 426 are enabled, by ~irtue of the remo~al of the disabling effect of the pulse on line 344 through inverter 428. ~his then permits playback of the previously : recorded amplitude, A, and frequency, f, signals.
2~ A peak detector 430 is provided to scan ror the peak value in the amplitude signals played back through amplifier 424. The detected peak value is then applied to a compar2tor 432 together with another playback of tne previously recorded ; amplitude signals on tracX 410.1 to enable the detenmination of the ~re~uency, .p, at the time the ~ee~ oc~urs.

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, When comparator 432 recognizes e~uality bet~een its ,l inputs, a pulse is produced on output llne 434 to activate a Il sample and ~ola network 436 coupled to sample tne played bac~
.j i j frequency sisnal, L/ from ampli~ier 426. The frequency, fp, : 5 1' of the amplitude peak value is then stored and made a~ailable " on output line 438 for recording and use as an indication o~
the thic~ness or the casing 22 as previously described.
¦l The recording, pea~ scanning and peak frequency selection are carried out in se~uence under direction of control signals '~ on line 440 from a control logic networ.~ 4~2. This net~or~ is j~ initiated by the pulse on line 344 and su~sequently by the playbac.~ of a recording of like pulses derived from a control ! I
track 410.3 on magnetic storage medium 412.
Fig. 19 illustrates another form 460 for ar acoustic ~, cement bond and casing investigating tool, wherein as in Fis. 1, i a rotating rerlector 38 is employed. TAe tool 460 is provided witn a stationary transducer 36 and a longitudinal cylinder 462 centrally and rotatably ~ounted relative to tool 460 about a 1I rotational axis 464 which in this embodiment is pre~erabl~
~I coincident with the centraL tocl axis. I
¦l The tool 460 has an annular acoustically transparent window 466 mounted ~etween an upper tool section 468 and a lower tool section g70. The cyllnder 462 lnternally ~rlages the window 466 and rotationally engages the upper and lower sections 4S8, 470 through bearings 472.

' ¦, The cylindsr 462 has a tubular section 474 into ~vhich transducer 36 ~rojects through an open end at 476. The tu~ular section 474 terminates at reflector 38 from where the c~linder Il 462 preerably is solid down to its end 476. Cylinder 462 is I provided with a pair of annular radially e~tending flanges 478.1 . i; , i ¦l and 478.2. Bearings 472 are clamped against flanges 478 with i annular bushings 480 afrixed to tool sections 468, 470 with screws such as 482. Bearings 472 fit in axially open annular Il grooves 484, 486 in flanges 478 and bushin$s 480 respec~ively.
1I Bearings 464 provide both thrust and radial low friction sU2port.' ¦l Additional bearings and flanges can be employed if needed.
¦¦ Cylinder 462 is of rugged strong construction to reinforce ¦ the lower tool section 470 to which a load producing device, ¦ such as an externally mounted centralizer tnot shown), can be ¦ applied. The cylinder 462, thus ser~es as a strong reinforc-d . ,! bridge over acoustic window 466. The ability to employ a ¦, centralizer below the rotating reflector 38 enables a precise placement of the rotational axis 464 relative to the casing 12 and thus preserve an accurate spacin~ o~ reflector 38 f~om 1l casing 12.
I The acoustic reflector 38 has a r~flection angle a li of a magnitude necessar~ to enable acoustic communication tnrough a side-locatsd opening 490 in tubular sec.ion 474. In f_ont of opening 490 and contisuous with the outer wall of upper tool section 468 is the acous.ic wincow 466 formed of a material .~ .
,'', , -68.~-1129~3~6 . . .

,I .

having a predeter~ined acoustic i~pedance and provided with a , shape selected to minimize undesirable acous'ic reflection.
, The acoustic window 46O is ror~ed of a material whose Il acoustic impedance closely matches the acoustic impedance of a S fluid, such as descriDed with reference to Fig. 1, and which i. . i i~ is placed in the space be~ een source 36, r~flector 38 and 'I window 466. The acoustic temperature and pressure coefficients, i, i.e. the change in acoustic impedance as a .unction of li temperature and Qressure =or both the îluid and the window 466 10 l, are selected as close as practically possible. The acoustic window 466 can be made of a material as descri~ed with ¦I reference to window 40 in Fig. 1 or or polysulfone, a material ! sold by the Union Carbide Corporation under the trade name ~I RADEL and having an acoustic velocity of about 2200 meters~
second. Hence, as an acoustic pulse is ~enerated from source 36 towards reflector 38, the acoustic energ~ passes tllrough ~. i ;~ ~i the fluid/window 1ntsrface 492 with a minim~ of reflaction. In order to further reduce tne erfect of acoustic re-I' flections from a window interposed bet~een ~.ha source 36 and jl casing 12, the windqw is conically shaped w~th an inclination angle ~ rela'ive to re~lector 38 as described with -eference to Fig. 1 to permit use of a large reflsctor 38 and aLso to .
deflect secondary transmissions awa~ from ~he casing 12.
; Transducer 36 in Fig. 19 is mounted to a ~rac~et 49~
2~ attac~.ed to the wall of tool section ~63. An electrical caDle ' ~

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, 496 connects transducer 36 to electronic circul~ry (~ot shown).
¦~ A rotational drive for cylinder 462 is provided by an ' electrical motor 493 mounted inside tool 460 and having an Il output shaft 500. A gear coupling ;02 interconnects the motor i~ shaft 500 to the cylinder 462.
The gear coupling 502 may take a variety of different forms and is, for illustrative purposes, shown composed or a I) pair of pinions 504, 506, with the latter mounted to a shaft ¦¦ 508 journaled in a bushing 510 on bracket d 94, A bevel drive, 'I formed of 45~ bevel gears, 512, 514, is used to interconnect ¦ the shaft 508 with cylinder 462.
¦~ Wi.h a tool 460 as shown in Fig. 19, the structural integrity of the tool is extended to below the annular window 1l 466. This provldes additional strength below the window and , lS " permits its centralization relative to cas~ng 12 with a `; ', centralizer. ~7indow 466 can be mzde sufficii-intly strong to ,' withstand such twistlng forces as may be coupled througn from ,i the rotating cylinder 462.
1~ Having thus explained techniques for investigating a cas-1' ing cemented in a borehole to evaluate the cement bond ænd casing thickness, the advantages of the invention can be appreclated.
¦, Variations from the described embodime~ts presen.ed herein ar-for illustration, with the scope of the invention to be deter~ined by the following claims.

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What is claimed is: ~
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Claims (65)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for determining characteristics of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing; and means for determining from said reverberation segment the frequency of components contributing to a peak value in the frequency domain of said reverberation segment and generate a casing thickness signal representative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement evaluation at said radial segment.

2. The apparatus as set forth in Claim 1 wherein said means for producing a quality signal further includes: means for determining the energy in a casing segment of the reflection signal representative of an acoustic reflection off an inner wall of the casing and provide a casing signal indicative thereof; and means for normalizing said quality signal with said casing signal to correct for borehole conditions.

3. The apparatus as set forth in Claim 2 wherein said casing thickness signal generating means further includes means for producing a spectrum signal representative of the frequency spectrum of the rever-beration segment; and means for scanning said spectrum signal for said peak value to derive the frequency of said components associated with said peak value.

4. A method for determining characteristics of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed from inside the casing at a radial segment thereof and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the steps of deriving from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment;
measuring the energy in the selected reverberation segment and provide a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing; and measuring the frequ-ency of components contributing to a peak value in the frequency domain of the reverberation segment and provide a thickness signal indicative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement evaluation at said radial segment.

5. The method for determining casing characteristics as set forth in Claim 4 and further including the step of recording said quality and said thickness signals as a function of borehole depth to provide a composite log for the correlation of the cement quality with casing thick-ness.

6. The method for determining casing characteristics as set forth in Claim 5 wherein said reflection signal is in a digitized form composed of samples and wherein said energy measuring step further comprises summing absolute values of the samples representative of the reverberation segment as said quality signal; and wherein said frequency measuring step further comprises forming a fourier transformation of samples representative of the reverberation segment and composed of amplitude samples with associated frequency values; scanning said ampli-tude samples for a peak thereof; and selecting an associated frequency value of the peak sample as a measurement of the thickness of the casing.

7. The method for determining casing characteristics as set forth in Claim 6 wherein said energy measuring step further comprises summing absolute values of samples representative of the casing reflection pro-duced by said acoustic pulse as an integral of said casing reflection;
and forming a quotient between the respectively summed reverberation segment samples and the casing reflection samples to produce a normalized signal.

8. An apparatus for determining the quality of the cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment;
and means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing.

9. A method for determining the quality of the cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the step of measuring the energy in a reverberation segment of the reflection signal, wherein the reverberation segment is substantially representative of acoustic reverberations between the casing walls at said radial segment, and provide a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing.

10. The method for determining the quality of the cement as set forth in Claim 9 wherein said reflection signal is in a digital form composed of samples and further including the step of summing absolute values of samples representative of the energy of the casing reflection;
and dividing said quality signal by said sum for normalization thereof.

11. An apparatus for determining the quality of the cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal obtained from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means responsive to said reflection signal for detecting an initial casing reflection from the casing and produce a signal indicative thereof; means actuated by the detected initial casing reflection signal for selecting from the reflect-ion signal a reverberation segment substantially representative of acoustic reverberations between the casing walls; and means for producing a quality signal indicative of the energy in the selected reverberation segment to characterize the quality of the cement bond behind said radial segment of the casing.

12. The apparatus for determining the quality of the cement as set forth in Claim 11 wherein said reflection signal is in the form of digital samples and wherein said means for producing said quality signal includes means for producing a sum of the absolute values of reflection signal samples representative of said reverberation segment as said quality signal.

13. The apparatus for determining the quality of the cement as set forth in Claim 12 wherein said quality signal producing means further includes means for producing a sum of the absolute values of reflection samples representative of the initial casing reflection as a measure of the magnitude thereof; and means for producing a quotient between said sums to normalize the signal.

14. The apparatus for determining the quality of the cement as set forth in Claim 13 wherein said means for producing said casing re-flection signal includes a threshold detector effectively responsive to the reflecting signal to determine when said reflection signal exceeds a level representative of the presence of said initial casing reflection.

15. The apparatus for determining the quality of the cement as set forth in Claim 14 wherein said threshold detector is formed of means for scanning samples of the reflection signal to determine the location of said initial casing reflection.

16. An apparatus for determining the thickness of a casing cemented in a borehole penetrating an earth formation from a reflection signal de-rived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverbera-tions between the casing walls; means for generating a spectrum signal representative of the frequency spectrum of said reverberation segment;
and means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.

17. The apparatus for determining the casing thickness as set forth in Claim 16 wherein the reflection signal is formed of digital samples and said spectrum generating means includes means for generating a fourier transform of samples representative of the reverberation segment as said spectrum signal.

18. A method for determining the thickness of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the steps of generating a spectrum signal representative of the frequency spectrum of a reverberation segment of the reflection signal wherein said reverberation segment is substantially representative of acoustic reverberations between the casing walls at said radial segment; and measuring the frequency of components in said spectrum signal contributing to a peak value thereof and provide a thick-ness signal representative of said measured frequency as indicative of the casing thickness at said radial segment.

19. The method for determining casing thickness as set forth in Claim 18 wherein the reflection signal is formed of digital samples where-in said generating step produces a spectrum signal formed of samples indicative of amplitudes and associated frequency values and wherein said measuring step further includes scanning said amplitude samples for said peak value and selecting the associated frequency of sample contributing to said peak as representative of the casing thickness.

20. An acoustic pulse echo apparatus for investigating a casing cemented in a borehole penetrating an earth formation comprising means for generating an acoustic pulse from inside the casing in a generally radial direction towards a selected radial segment of the casing wherein said acoustic pulse has a frequency spectrum selected to enhance entrap-ment of acoustic energy between the inner and outer casing walls at the radial segment for stimulation of reverberations therein and generating a reflection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse with acoustic leakage from reverberations trapped inside said casing walls; means for selecting a reverberation segment of the reflection signal wherein said selected segment is substantially representative of said reverberation leakage in the acoustic returns; means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind said radial segment of the casing; and means for determining from said reverberation segment the frequency of components contributing to a peak value in the frequency domain of said reverberation segment and generate a casing thickness signal representative thereof as the casing thickness at said radial segment for the evaluation of the cemented casing and the resolution of potential ambiguities in the cement evaluation at said radial segment.

21. The acoustic pulse echo apparatus for investigating a casing as set forth in Claim 20 wherein the selecting means further includes means for detecting a casing segment in the reflection signal representative of an initial casing reflection produced by the acoustic pulse and generate a casing signal indicative thereof; and means enabled by the casing signal for amplifying said reverberation segment following the initial casing reflection.

22. An acoustic pulse echo apparatus for investigating the quality of the cement and a casing located in a borehole penetrating an earth for-mation comprising means for generating from inside the casing an acoustic pulse towards a radial segment of the casing and the formation and produc-ing a reflection signal representative of acoustic returns from the inter-action of the acoustic pulse with different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing and with the acoustic wave frequencies further being selected to render micro-annuli representative of good quality cement effectively transparent while enhancing reflections from annuli representative of poor quality cement; means for selecting a reverberation segment of the reflection signal following an initial casing reflection wherein said reverberation segment is substantially represent-ative of acoustic leakage from reverberations introduced in between the walls of the casing by said acoustic pulse; and means for measuring the energy in the reverberation segment of the reflection signal and produce a quality signal indicative thereof to characterize the quality of the cement.

23. The cement quality investigating apparatus as claimed in Claim 22 wherein said quality signal producing means further includes means for producing a reverberation segment select signal commencing at a time commensurate with the arrival time of the reverberation segment of the reflection signal and continuing for a time commensurate with the duration of the portion of the reflection signal indicative of a poor quality cement; and means controlled by the reverberation segment select signal and coupled to the reflection signal for selecting said reverber-ation segment from the reflection signal.

24. The cement quality investigating apparatus as claimed in Claim 23 wherein said quality signal producing means still further includes means for rectifying said selected predetermined segment; and means for integrating said rectified predetermined segment effectively for the duration of said reverberation segment select signal.

25. The apparatus for investigating the quality of the cement in accordance with Claim 22 wherein said reflection signal producing means is selectively located within the apparatus to establish a predetermined minimum spacing between the casing and the reflection signal producing means to produce a reflection signal with said reverberation segment substantially free from secondary transmission interference.

26. The apparatus for investigating the quality of the cement in accordance with Claim 25 and further including means responsive to the reflection signal for generating a casing reflection signal representative of a predetermined characteristic of said initial acoustic casing re-flection; and means for normalizing said quality signal with the casing reflection signal.

27. The apparatus for investigating the quality of the cement in accordance with Claim 26 wherein said casing reflection signal gener-ating means further includes means for measuring the amplitude of the casing reflection signal.

28. The apparatus for investigating the quality of the cement in accordance with Claim 26 wherein said casing reflection signal gener-ating means further includes means for effectively measuring the energy of the casing reflection signal.

29. The apparatus for investigating the quality of the cement in accordance with Claim 26 wherein the means for generating the casing reflection signal further includes a threshold detector responsive to the reflection signal for sensing a predetermined magnitude indicative of the arrival of said initial acoustic casing reflection and to produce an enabling signal representative thereof; and means responsive to the reflection signal and enabling signal for selecting said casing reflection signal.

30. An acoustic pulse echo apparatus for investigating the quality of the cement of a casing located in a borehole penetrating an earth for-mation comprising means for generating from inside the casing a highly damped acoustic pulse towards the formation wherein said acoustic pulse has waves at frequencies selected to stimulate a thickness resonance inside the casing walls, said acoustic wave frequencies further being selected to render micro-annuli representative of good quality cement effectively transparent while enhancing reflections from annuli representative of poor quality cement said acoustic pulse generating means being further respon-sive to acoustic returns produced by said acoustic pulse for producing a reflection signal representative thereof; said acoustic pulse producing means being at a predetermined minimum spacing from the casing to enable the detection of acoustic reverberations substantially free from second-ary transmission interference; means responsive to said reflection signal for detecting an initial casing reflection from the casing; means actu-ated upon the detection of the initial casing reflection signal for select-ing a reverberation segment following said initial casing reflection; and means for producing a quality signal indicative of the energy in the selected reverberation segment to characterize the quality of the cement.

31. The apparatus for investigating the quality of the cement in accordance with Claim 30 wherein said predetermined spacing between the casing and the reflection signal producing means is selected sufficiently large to enable the detection of acoustic returns having a magnitude above a predetermined level and substantially attributable to leakage from said reverberations inside the casing walls as a result of said acoustic pulse.

32. The apparatus for investigating the quality of the cement in accordance with Claim 31 wherein said apparatus has a surface capable of generating secondary transmission interference by reflecting acoustic energy back towards the casing and wherein a predetermined minimum spacing, D, between the casing and said surface is determined in accordance with the relationship where L is the thickness of the casing, CO is the velocity of sound of the material enclosed by the casing, C1 is the velocity of sound inside the casing material and Nr represents a substantial number of reverbera-tions produced within the casing as a result of acoustic energy entrapment from the thickness resonance producing acoustic pulse and is determined by the relationship where ro and r1 are respectively reflection coefficients between the material enclosed by the casing and the casing itself and between the casing and the material adjacent outside of the casing, and where x represents the predetermined level expressed as a fraction of the initial level of the reverberations.

33. An acoustic pulse echo apparatus for investigating the quality of the cement of a casing located in a borehole penetrating an earth formation comprising means for generating from inside the casing an acoustic pulse towards a radial segment of the casing and produce a re-flection signal representative of acoustic returns from different layers of material in the path of the acoustic pulse, said acoustic pulse being generated with a bandwidth selected to stimulate a thickness resonance between the inner and outer walls of the casing with substantially reduced reflections from hydraulically secure micro-annuli representative of good quality cement and with a significantly longer duration reverberations in the casing in the presence of annuli representative of poor quality cement;
means responsive to the reflection signal for generating a casing reflect-ion signal indicative of the duration of an acoustic reflection from the casing; means responsive to the casing reflection signal for producing a reverberation segment selection signal to identify a reverberation segment of the reflection signal following the casing reflection; means enabled by the reverberation segment selection signal for measuring the energy in the reflection signal for the duration of the reverberation segment selection signal and produce a quality signal indicative of the quality of the cement located in the path of the acoustic pulse; means for producing a normalizing signal representative of a predetermined characteristic in the acoustic reflection from the casing; and means for combining said quality signal with the normalizing signal to produce a normalized signal represent-ative of the quality of the cement.

34,, An acoustic pulse echo method of investigating the quality of the cement of a casing located in a borehole penetrating an earth for-mation comprising the steps of generating a pulse of acoustic energy towards the formation from inside the casing with the acoustic energy having a frequency spectrum which is selected to stimulate the casing into a thickness resonance to trap reverberations in the casing and having a frequency bandwidth selected to generate acoustic waves at frequencies whose water wavelengths exceed the thickness of hydraulically secure micro annuli by a factor sufficient to render said micro-annuli effectively transparent to said acoustic pulse; deriving a reflection signal represent-ative of acoustic returns from different layers of material in the path of the acoustic pulse; and determining the energy in a reverberation seg-ment of the derived reflection signal attributable to acoustic leakage from reverberations inside the casing as an indication of the quality of the cement located in the path of the acoustic pulse.

35. The method of investigating the quality of the cement in accordance with Claim 34 wherein said processing step still further in-cludes the steps of selecting a casing segment of the reflection signal representative of the casing reflection; producing a casing signal in-dicative of a predetermined characteristic of the selected casing segment;
and applying the casing signal to normalize the determined energy in the reverberation segment relative to said predetermined characteristic of the casing reflection.

36. The method of investigating the quality of the cement in accordance with Claim 35 wherein said casing signal producing step pro-duces a casing signal effectively representative of the energy in the casing reflection.

37. The method of investigating the quality of the cement in accordance with Claim 35 wherein said casing signal producing step pro-duces a casing signal effectively representative of an amplitude of the casing reflection.

38. The method of investigating the quality of the cement in accordance with Claim 34 wherein said detecting step is carried out at a predetermined distance from the casing to provide said reflection signal substantially free from secondary transmission interference.

39. A method for acoustically investigating the quality of the cement of a casing located in a borehole penetrating an earth formation with a pulse echo technique comprising the steps of generating an acoustic pulse inside the casing towards a selected radial segment of the casing and the formation to cause acoustic returns attributable to the acoustic interaction of the acoustic pulse with different layers of material in the path of the acoustic pulse, wherein said acoustic pulse has acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thickness resonance to trap acoustic reverberations inside the casing walls, with the acoustic wave frequencies further being selected to reduce re-flections from micro-annuli representative of good quality cement while enhancing reflections from annuli representative of poor quality cement;
detecting the acoustic returns to produce a reflection signal indicative thereof; selecting a casing segment from the reflection signal represent-ative of a reflection from the casing; selecting a reverberation segment from the reflection signal representative of reflections occurring sub-sequent to said casing reflection and substantially representative of leakage returns from reverberations introduced in the casing by the acoustic pulse; and processing said selected segments to cooperatively produce a quality signal indicative of the quality of the cement.

40. The method of investigating the quality of the cement in accordance with Claim 39 wherein said processing step further includes the steps of measuring the energy in said segments; and normalizing the measured energy of the reverberation segment with the measured energy in the casing segment to produce said quality signal.

41. The method for investigating the cement in accordance with Claim 40 and further comprising the step of preferentially amplifying the selected reverberation segment relative to the casing segment for enhanced accurancy in obtaining a measurement of the quality of the cement.

42. An apparatus for acoustically investigating the quality of the cement of a casing located in a borehole penetrating an earth for-mation with an acoustic pulse echo technique comprising means for pro-ducing an acoustic pulse having acoustic wave frequencies selected to stimulate the casing into a thickness resonance with enhanced entrapment of reverberations inside the casing and provide a reflection signal re-presentative of acoustic returns caused by the acoustic pulse; means for extracting from the reflection signal a frequency segment selected to include casing thickness resonance frequencies and generate a quality signal representative thereof as indicative of the quality of the cement;
means for extracting from the reflection signal a reference frequency segment and produce a reference signal indicative thereof; and means for combining the reference signal with the quality signal to provide a normalized signal indicative of the quality of the cement.

43. The apparatus for investigating the quality of the cement as set forth in Claim 42 wherein the extracting means includes a pass band filter having its pass band aligned with the casing thickness resonance frequency.

44. The apparatus for investigating the quality of the cement as set forth in Claim 43 wherein the pass band of the filter has a bandwidth generally less than about 15 per cent of the casing thickness resonance frequency.

45. A method for acoustically evaluating the quality of the cement of a casing in a borehole penetrating an earth formation comprising generating an acoustic pulse from inside the casing towards a radial seg-ment of the casing wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the radial segment of the casing; detecting acoustic returns attributable to the interaction of the acoustic pulse with materials in the path of the acoustic pulse and produce a reflection signal indicative thereof; selecting a predetermined frequency band from the reflection signal wherein the selected frequency band includes casing thickness resonance frequencies and produce a quality signal representative thereof to indicate the quality of the cement selecting a reference frequency band from the reflection signal and produce a reference signal indicative thereof; and combining the reference signal with the quality signal for normalization thereof.

46. The method for evaluating the quality of the cement as claimed in Claim 45 wherein the step of selecting the predetermined frequency band selects a band of signals over a frequency range of generally less than about 15 percent of the casing thickness resonance frequency.

47. An apparatus for investigating with an acoustic pulse a casing located in a borehole penetrating an earth formation comprising means for directing an acoustic pulse from inside the casing in a radial direction at a radial segment of the inner wall of the casing, wherein the acoustic pulse has acoustic wave frequencies selected to stimulate a thickness resonance inside the radial segment with enhanced entrapment of rever-berations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse; means for selecting from the re-flection signal a portion which includes acoustic returns attributable to the acoustic reverberations inside the casing walls; means for generation a spectrum signal representative of the frequency spectrum of the selected portion; and means for determining the frequency of components in said spectrum signal contributing to a peak value thereof and producing a thickness signal representative thereof as the casing thickness.

48. The apparatus for investigating a casing as claimed in Claim 47 wherein said means for determining the peak value further includes means for producing samples of the spectrum signal with associated values of the frequency of the samples; means for scanning said spectrum samples for a peak value thereof and selecting the associated frequency value as an indication of the thickness of the casing.

49. The apparatus for investigating a casing as claimed in Claim 48 wherein said portion selection means further includes means responsive to the reflection signal for detecting a signal therein representative of an initial acoustic reflection signal from the casing; and means responsive to the detected casing reflection signal for selecting said portion fol-lowing the initial casing reflection.

50. A method for acoustically investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of generating an acoustic pulse from inside the casing in a radial direction towards the formation wherein the acoustic pulse has a frequency bandwidth selected to stimulate a thickness resonance with acoustic reverberations inside the walls of a radial segment of the casing; detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal in-dicative thereof; selecting from the reflection signal a portion which includes acoustic returns produced by said acoustic reverberations inside the walls of the casing; forming a frequency spectrum of the selection portion; and determining the frequency of components which contribute to a maximum peak in the frequency spectrum of the selected portion and pro-ducing a signal representative thereof as an indication of the casing thickness.

51. The method of acoustically investigating the casing in a borehole as claimed in Claim 50 wherein said frequency determining step further includes the steps of digitizing the frequency spectrum to form samples thereof with associated frequency values for the samples; scanning the samples to determine a peak value thereof; and recording the frequency value of the peak value of the samples as an indication of the thickness of the casing.

52. The method of acoustically investigating a borehole as claimed in Claim 50 wherein said frequency spectrum forming step further includes the steps of applying said selected portion to a spectrum analyzer to generate an amplitude signal representative of the amplitude of the frequency components in the selected portion and a frequency signal re-presentative of the frequency of the components in said amplitude signal;
storing said amplitude and frequency signals; scanning said stored ampli-tude and frequency signals to detect a peak value of the amplitude signal with its associated frequency signal as an indication of the thickness of the casing.

53. The method of acoustically investigating a borehole as claimed in Claim 50 wherein said frequency spectrum forming step further includes the steps of digitizing said selected portion to form digital samples thereof, and forming a fourier transform of the digital samples of the selected portion.

54. The method of acoustically investigating a borehole as claimed in Claim 53 and further including the step of increasing the amplitude of the selected portion of the reflection signal prior to said digitizing step.

55. An apparatus for investigating with an acoustic pulse a casing cemented in a borehole penetrating an earth formation comprising means for generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate an acoustic resonance between the walls of the casing with acoustic reverberations and providing a reflection signal representative of acoustic returns caused by the acoustic pulse;
means for generating digital samples of the reflection signal; means for selecting samples representative of said casing reverberations and occurr-ing subsequent to samples representative of an initial casing reflection;
means for generating a spectrum of the selected reverberation samples and form amplitude samples with associated frequency values; and means for determining a maximum amplitude sample and its associated frequency value as an indication of the thickness of the casing.

56. The apparatus for investigating a casing as claimed in Claim 55 and further including means for summing the absolute value of the selected samples representative of the reverberations in the casing as a measurement of the quality of the cement.

57. The apparatus for investigating a casing as in Claim 55 and further including means for selecting samples representative of an initial acoustic casing reflection of the inner wall of the casing; means for summing the absolute values of the samples representative of the initial acoustic casing reflection; means for summing the absolute values of the selected samples representative of the casing reverberations as a measure-ment of the quality of the cement; and means for forming a quotient between the respective sums generated by the summing means to normalize said measurement of the quality of the cement.

58. A method for investigating a casing cemented in a borehole penetrating an earth formation comprising the steps of generating a highly damped acoustic pulse from inside the casing in a radial direction towards a radial segment of the casing with said acoustic pulse being generated with acoustic wave frequencies in a bandwidth selected to stimulate the casing into a thickness resonance with acoustic reverbera-tions between the walls of the casing; detecting acoustic returns arising from the interaction of the acoustic pulse with materials in the path of the acoustic pulse and producing a reflection signal indicative thereof;
converting the reflection signal to digital samples; forming a frequency spectrum of samples representative of casing reverberations occurring subsequent to samples representative of an initial acoustic reflection off the inner wall of the casing with the frequency spectrum composed of amplitude samples with associated frequency values; determining a peak amplitude sample in the frequency spectrum; and recording a thickness signal representative of the associated frequency value of the peak ampli-tude sample as an indication of casing thickness.

59. The method for investigating a casing as claimed in Claim 58 and further including summing absolute values of samples representative of casing reverberations to provide a signal indicative of the quality of the cement.

60. The method for investigating a casing as claimed in Claim 59 and further including summing absolute values of samples representative of the inttial casing reflection; and forming a quotient between said respectively summed samples to provide a normalized signal.

61. An apparatus for evaluating the quality of cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising means for selecting from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; means for determining the energy in the selected reverberation segment and producing a quality signal indicative thereof to characterize the quality of the cement behind the casing; means for determining from a reverberation seg-ment a casing thickness signal representative of the thickness of the casing at said radial segment; and means for normalizing said quality signal with said casing thickness signal to substantially remove the effect of casing thickness variations from the characterization of the quality of the cement at said radial segment.

62. The apparatus for evaluating the cement as claimed in Claim 61 wherein said quality signal producing means further includes means for determining the energy in a casing segment of the reflection signal representative of an acoustic reflection off an inner wall of the casing and provide a casing signal indicative thereof; and means for normalizing said quality signal with said casing signal to provide a characterization of the quality of the cement at said radial segment and corrected for borehole conditions and casing thickness.

63. A method for evaluating the quality cement of a casing cemented in a borehole penetrating an earth formation from a reflection signal derived from an acoustic investigation of the casing with an acoustic pulse directed at a radial segment of the casing and formed of acoustic waves at frequencies selected to stimulate a thickness resonance inside the casing walls comprising the steps of deriving from the reflection signal a reverberation segment substantially representative of acoustic reverberations between the casing walls at said radial segment; measuring the energy in the selected reverberation segment and provide a quality signal indicative thereof to characterize the quality of the cement behind said radial segment; measuring the thickness of the casing effectively at said radial segment and provide a thickness signal indicative thereof;
and effectively removing from said quality signal with said thickness signal, variations, which are substantially attributable to casing thick-ness changes.

64. The method for evaluating the quality of cement of a casing cemented in a borehole as claimed in Claim 63 wherein said step for removing variations attributable to casing thickness changes comprises dividing the quality signal by the thickness signal.

65. The method for evaluating the quality of cement of a casing cemented in a borehole as claimed in Claim 64 wherein the quality signal producing step further includes the steps of measuring the magnitude of a casing reflection segment in the reflection signal; and normalizing the quality signal with the measured magnitude of the casing reflection to provide a signal which characterizes the cement quality substantially independent of borehole conditions and casing thickness.