CA1161151A - Swept energy source acoustic logging system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An acoustic energy mode propagation speed or travel time measurement system for use in well logging is dis-closed. A downhole sonde is provided with an acoustic tansmitter and at least one acoustic receiver. The trans-mitter is repetitively driven with a unique or charac-teristic swept frequency signal. Propagated acoustic energy detected at the receiver is cross-correlated with the characteristic transmitter swept frequency signal to provide indications of the arrival of various modes of acoustic energy propagation at the receiver. Logs as a function of borehole depth of the speed of propagation of the various modes of propagation may be derived.

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Description

BACKGROUND OF T~-IE INVENTION
This invention relate~ to methods and systems ~r measuring acoustic wave travel times in earth formations in the vicinity of the well borehole. More particularly, the present invention relates to techniques for mea~urin~
multiple acoustic wave componen~ (or wave propagation mode) travel times in earth formations in ~he vicinity of a well borehole. The measurement method~ use swept frequency transmitting techniques and cross correlation CompariSOn techniques between the transmitted signal and received signal.
Sonic or acoustic well logging has become an important method or dete~mining the physical characteristics of ear~h ormations in the vicinity o a well borehole. Mea~uremen~
lS of the acoustic compressional wave velocity or travel kime between a transmitter and a receiver in a well bor~hole can define physical characteristics of the ~arth formations which are indicative of the capability of these formations to produce oil or gas. For example, a mea~urement of the compressional wave travel time or velocity gives a direct indication of the porosity o~ the formation in the vicinity o the well borehole. guch acoustic veloci~y or acou3~ic travel time measurements have therefore become practically standard for all new wells which are drilled.
In the prior art, acoustic pulse or pulsed 50nic ~logging techniques have been used to measure the travel time or velocity o~ acoustic waves in the earth formations in the vicinity of a borehole. Such me~hods of the prior art havP
typically used impulse driven acoustic transmitters. An acoustic transmitter is fired impulsively or pulsed and the ~3~

length of time necessary for the acoustic wave pulse generated by the transmitter to propagate from the tran~mj.tter through the ear~h formations in the vicini~y of the borehole and back to an acoustic receiver located a ~paced di~tance away from the txansmitter is measured. By appropriately combining the measuremen~s of acoustic wave ~ravel time at several - acousti.c receivers, spaced different di~tance3 from either a single tor multiple) acoustic transmitter, then the acoustic wave travel time or sonic compressional wave velocity of propagation of the earth formation may be determined. Quite elaborate schemes and geome~rical considerations for eliminating the effect on the travel time measuremen~ of the borehole and borehole fluids have also been developed.
In more recent years, it has become desired to measure other acoustic wave mode travel time~ than merely compres-~ional wave velocity. For example, in U.S. Patent 4,131,875 issued December 26, 1978, techniques are described for measuring the so called l'late arrival" waves or Stonely waves. Similarly, other prior art techniques such a~ tha~
shown in U.S. Patent 3,354,983 is~ued November 28, 1967, describes techniques for measuring acoustic shear wave velocities. In all of these techniques, an acoustic pulse i~
generated by the transmitter and the waveform of the acoustic signal at one or more receivers i~ analyzed in order to determine the velocity of compressional, ~hear, or Stonely ~waves in the vicinity of the borehole.
Pulsed acoustic techniques depend upon the amplitude detection of ~he arrival of acoustic waves at a receiver.
Such techniques are prone to errors generated by random noise which occurs as a well logging instrument is moved 5~
through the borehole. Acoustic noise maybe generated by the lnstrument body, or cen-tralizers on the instrument body, scraping along the sides of the borehole as the tool is moved therekhrough.
Similarly, pulsed acous~ic techniques involving pulsed acoustic transmitters for mea~uring sheax waves or 5tonely waves depend upon an elaborate interpre~a~ion of the wave-form of the arriving wave at the receiver. Such interpre-tations are generally based on theoretical calculations made with simplified mathematic~l models of the earth form~tions in the vicinity of the borehole. If the simpli~ied mathe-matical model proves ~o be in error, then the interpretation of the arriving waveform at the receiver may be in error and its relationship to more complicated real life geometries and conditions than taken into account in the model can lead to false interpretations of the waveorm of the arriving acoustic signal.
It would be highly desirable to provide a method for measuring the travel time of various components of acousti~
energy (compressional or primary wave r shear wave, Rayleigh or pseudo Rayleigh, direct (fluid) wave, extentional, and Stonely wave) in earth formation~ in the vicinity of a well borehole which was not dependent upon a theoretical inter-pretation of an arriving acou~tic pulse waveform in terms of a model. The system of the present invention provides a ~direct measurement of the travel time of several components of acoustic energy from a transmitter to a receiver in earth formations in the vicinity of a well borehole.
BRIEF DESCRIPTION OF THE INV~NT:I:ON
In the present invention, a downhole well logging instrument is provided with an acoustic transmitter and $~
at least one acoustic receiver ~hat i~ spaced a longitudinal dis~ance from the transmitter. I~ desired, multiple trans-mitters and receivers could be used. ~rhe output signal from the acoustic transmitter in the present invention is repeti~ively swept over a predetermined frequency range. The ~requency swept output of the transmitter i~ propagated in all the various modes of propagakion of acous~ic energy through the earth formations and borehole and i~ detected at the spaced receiver. A synchronization signal i8 also generated at the-commencement of each repetitive sweep of the transmitterthrough its predetermined frequency xange. The ~ynchroniza-tion signal and the received signal from the receiver are transmitted to the surface of the earth via conductors of the well logging cabl~. At the surface, the received signal is converted from analog to digital form and 3tored in a memory. ~he transmitter sweep signal i5 stored in a surace located sweep signal memory storage in digital form. Upon completion of a sweep of the transmitter and after receiving digitizing and storing the received signal for a predeter-mined le~gth of time the sweep signal from the tran~mitteris cross-correlated with the received signal. Because of the characteristic swept frequency pattern applied to the tra~smitter signal, indications are derived from the cro~s-correlation of the arrivals of various modes of acou~tic energy propagation at the receiver. The timing differences between the synchronization pulse and the arrival of the various modes of acoustic propagation at the receiver may then be interpreted in terms of the travel time o~ the variou~ modes of acoustic propagation at the receiver.
These signals may then be recorded as a function of borehole depth as the well logging instrument is moved through the borehole. The entire sweep, transmit, and receive process is repetitively performed during such movement of the borehole instrument.
In one aspect of the present invention, there is provided a well logging system for measuring and recording the acoustic energy propagation characteristics of earth formations penetrated by a well borehole comprising a fluid tight hollow body member sized and adapted for passage through a well borehole, means in the body member for repetitively generating swept frequency acoustic energy outputs ha~ing a linearly varying range of frequencies from a lowes-t frequency of approximately two kilohertz to a highest frequency of approximately twelve kilohertz in a characteristic pattern, the pattern having a duration of approximately four milli-seconds, means for digitizing the characteristic swept fre-quency signal and for providing a digital signal representative of the characteristic signal at the generating means' receiver means longitudinally spaced from the generating means by a distance of from eigh~ to twelve feet in the body member, for detecting acoustic energy propagated from the generating means through the borehole and earth formations~in the vicinity of the borehole and for generating digital signals representative of the detected acoustic energy, means for cross correlating the digital signal representative of the characteristic signal at the generating means and the digital signal representative of the detected acoustic energy and for providing a correlator output signal representative of the arrival at the receiver means of different modes of propagation of acoustic energy in the borehole and earth formations in the vicinity of the borehole, computer means responsi.ve to said correlator output signal for deriving therefrorn measurements of the speed of propagation of the different modes of pro~agation of acou~tic energy in the ~ 5 -earth formations in the vicinity of the well borehole, and means for recording the measurements of the speed o~ propagation of different modes of acoustic energy as a function of borehole depth, thereby providing on a record medium a well log of speed of propagation of different modes of acoustic energy.
The invention may be best understood by the following detailed description thereof, when taken in conjunction with the appended drawings in which:
B~IEF DESCRIPTION OF THE DRAr~INGS
Figure 1 is an overall block diagram illustrating schematically a well logging system in accordance with the conce~ts of the present invention.
Figure 2 is a schematic diagram illustrating an acoustic waveform received at a spaced receiver from a pulsed acoustic trans-mitter as utilized in the prior art.
Figure 3 is a graphical representation illustrating a typical swept frequency waveform applied to the acoustic transmitting transducer in the present invention.
Figure 4 is a graphical representation illustrating a swept frequency signal applied to an acoustic transmitter in accordance with the concepts of the invention, a composite or mixed mode arrival signal which arrives at the acoustic receiver of the pres~nt invention, and the output of a cross--correlation ~etween the sweep an~ the composite arrival signal in accordance with the present invention, and - 5a -Figure 5 is an illustration schematically showing a well log as a func-tion o~ depth of the acoustic compressional wave velocity and the correlator output showing compressional and shear wave arrivals in accordance with the concepts of the present lnvention 5b -D~SCRIPTION OF THE PREFERRED EMBO~I~ENT
Referring initially to Fig. 1, a sys~em or generating and receiving acoustic signals and for logging a well bore-hole in accordance with the concepts of the present in~en-tion is illustrated schema~ically. A well ~orehole 10penetrates earth formations 15 and is filled with a borehol~
fluid 12. A downhole well loggi~g sonde 11 is suspended, via a well logging cable 13, which pas3es over a heave wheel 14, in the borehole 10. The sheave whe~1 14 i~ electrically or mechanically coupled to a well logging recorder 2~ of conventional design as illustrated by dotted line 16 so that measurements made by the down hole sonde 11 may be recorded as a function of borehole depth.
The downhole sonde 11 comprises a ~luid tight, hollow, body member sized and adapted for pas~age through a well borehole. Housed inside the fluid tight sonde 11 i8 an acoustic transmitter 32 and an acoustic receiver 33. Cir-cuitry for drivin~ the acoustic transmitter 32 comprise~ a ~weep signal storage memory 29, which may comprise a read only memory (ROM) or the like, a digital to analog convertex 30, and a ~ilter 31.
The acoustic re~eiving transducer 33 iB shown longi-tudinally spaced from the transmitting ~ransducer 32.
~ypical spacing dis~ances of from 3 to 10 feet may be used as desired. It will be appreciated that acoustic trans-'`mitting transducer 32 and acoustic receiving transducer 33 are acoustically coupled to the borehole by acoustic impe-dance matching material such as oil or oil-filled bellows or the like (not shown) in a manner known in the art. The transmitting and receiving transducer3 may comprise plezo-electric transducers. The transmitting and receiving transducers are sized and arranged to have a linear or "flat" response over the swept frequency range used in the technique o~ the present invention.
While only one acoustic transmitter and one acoustic receiver are illus~rated in the system of Fig. 1~ It will be apprec1ated by those skilled in the art that the number of acoustic receivers could be varied and the number o~
acoustic ~ransmitters could be varied, if desixed. In such an instance different weep patterns could be u~ed for each acoustic ~ransmitter to characterize it~ output acoustic energy from that of any other acou~tic transmitter which is utilized in the logging instrument.
The sweep signal storage memory 29 contains digital numbers representative of the amplitude of sweep pattern to be applied to the transmitting transducer 32 as a function of time at a preselected sampling inter~al time or rate.
For example, a typical sweep frequency pattern could be that given by Equation 1.

I t=T2 f(t) = sin [~1 + (~2 ~l)t]t (1) t=T~ 2L

In Equation 1 a sine wave whose frequency changes in a linear fashion from ~1 at Tl to ~2 at T2 i8 described. Such sweep function amplitudes can be generated by computer as a ~function of time and the results then stored in a read only memory or ROM device for subsequent use in the subsurface tool and surface equipment as desired.
Digital signals from the sweep signal storage ROM 29 are read out sequentially and converted to analog signals ~y a digital to analog converter 30. The output o the digital 5~
to analog converter 30 is filtered by low pass filter 31 to remove ~he small sample -to sample skep introduced by the digital to analog converter (i.e. to remo~e high frequency components) and the output voltage signals from the filter 31 dxive ~he transmi~ter transducer.
A typical sweep pattexn ~uch a3 that described by Equation l is illustrated in Fig. 3. A synchronization pulse i8 generated at the beginning of a sweep cycle and is labelled as "sync pulse" in Fig. 4. A ~wept ~requency acoustic signal having a linearly increa~ing ~requency and starting at a ~ime approximately 0.1 millisecond after the synchronization pulse is illustrated. The frequency of the tran~mitter drive signal increases un~il a tim~ approxi-mately 5 milliseconds following the sync pulse, thus gene rating a swept frequency acoustic signal having approxi mately constant amplitude and linearly varying frequency of from, for example, 2 to 12 kilohertz and having a duration of approximately 4 milliseconds. It will be appreciated that other durations or ther swept frequency ranges could be used if desired.
The acoustic signals detected by receiving transducer 33 are filtered by a band pass filter 34 to remove any noise signals which are far removed from the pass band of the original swept frequency signal. After filtering, the signals are amplified by an amplifier 35 and applied to '`a telemetry system 36 which transmit~ the received acoustic signal waveform to the surface via conductors of well logging cable 13.

Timing for the transmitter sweep event and the synchroni-zation pulse is controlled by the telemetry sys~em 36 whichcontains a precise frequency clock ~uch as crystal controlled oscillator therein. The synchronization signal illustrated in Fig. 3 is transmitted ~o ~he ~ur~ace so thak the 5urface electronics may be exactly synchronized or each time o~
starting o the transmitter ~weep cycle. For a 4 milli~
second sweep rate and an approximately 10 millisecond receiver recording time, such as that illu~trated in Fig. ~, the entire cycle of transmitter sweep and receiver reception transmisto ~he surface may be repeated at a repetition rate of from lO to 20 cycles per second. It will be appre-ciated by those skilled in the art that the duration of recep~ion by the receiver and the transmission of recei~ed signals is a function o~ the spacing between the transmitter and receiver. For typical pacings on the ordex of four to six feet, the 10 millisecond receive Yignal transmis~ion cycle illustrated in Fig. 4 is appropriateO

lS At the surfaceS a synchronization detector and timing circuit 18 detects the synchronization signal and generates outputs to an analog to digital converter 21, a signal memory 22, a correlator memory 24 and a sweep ~ignal memory storage l9. The receiver Yignal from the downhole telemetry 3ystem is amplified in an amplifier 20 and con~erted to digital ~ormat by analog to digital converter 21, which is tim2d by the signal from the sync detector and timing circuit 18.
The digitized form of the received signal i~ th~n ~tored in a signal memory 22. At an appropriate time which allows for the complete receiver signal waveform to be digitized and ` stored in signal memory 22, the synchronization detector in ~iming circuit 18 supplies a strobe or outpu~ signal pulse : to the sweep signal torage memory 19 and to the ~ignal memory 22 which cause these two ~ignals to be supplied a~

input in digital form to a correlator 23.

The corxelator 23 per~orms a cross coxrelation ~u~ction , on the two input siynal~ which is defined by Equation 2.

K=N
~xy(~ ) ~ N (Xk)( ~ (2~

In Equation 2, Xk and Yk ara discreet functions o~ timeO
Hence the cross-correlation function ~xy i8 al30 a di~creet function of time. If Xk and Yk each contain N points and the shift amount T iS equal to the ~ampling interval of Xk and Y~ then the total number of points produced by the cross-correlator 23 will be 2N-l. The number of products formed by the cross-correlations for an example o N points is N~.
The digital output of the correlator 23 is supplied to a ~orrelator memory 24 which is al~o supplied wi~h ~iming pulses from the synchronization detectox and timing circuit 18 as previously described. The digital output from the correlator memory, upon receipt of an appropriate timing pulse rom circuit 18, is supplied to a digital to analog converter 25 where it is reconverted to analog fonm for display as illustrated in Fig. 5. The output from the digital to analog correlator 25 is then filter~d via band-pass filter 27 and supplied to the recorder 28 for recording as a variable density display as illustrated in the right-hand half of the well log as a function of depth illustrated in Fig. 5.
The output from the coxrelator memory 24 is also supplied to a travel time computer 26 which computes the travel time from the transmitter to the receiver for s~
selected arrivals at the receiver such as the compres~ional wave travel time and the ~hear wave traveL time. The compressional wave travel time or shear wave travel time is then supplied to the recorder 28 for recording a~ a function of depth as illus~rated i~ the left-hand half of ~he well log of Fig. 5.
Referring now ~o Fig. 4 ~he sweep ~ignal, the composlte receiver signal and the cross-correlation of the ~weep signal and compo~ite receiver signals ar~ illustrated as a function of time. It will be noted that the cro~s~correla-tion output formed illustrates peaks which may be inter-preted in terms of the compressional wave arrival, the shear wave arrival, the direct wave arrival, a~d the Stonely wave arrival. Travel times for these various acoustic modes may thus be computed by the travel time comput r 26 by comparing these arrivals with the sync pul~e and deriving the time diference from it to the~e arrivals.
It will be appreciated by those skilled in the art that power for the operation of the downhole electronics as well as the surface electronic~ may be supplied from a ~urface located supply 17 via conductors of the well logging cable 13. Appropriate downhole power converters (not shown) may be housed in the downhole ~onde 11 in ordex to provide operational voltages for the downhole electronic sy~tems in a manner well known in the art.
` Referring now to Fig. 2, an acoustic waveform from a pulsed transducer such as that used in the prior art is illustrated. The typical acou~tic waveform may be inter-preted according to propagation velocities of variou~ modes of acoustic energy propagation in the borehole. Thus the ~6~L~5~
initial arrival is generally lnterpreted as that from the compressional wave which is usually propagated faster through the earth formations in the vicinity o~ a well borehole. Appearing later on in arriving waveform are energy peaks which may be interpreted a~ the shear wave, the fluid wave and the Stonely wave portions of tha acoustic wave form. Depending upon the tran~mitter to receiver spacing and the amount of reflection o curring within the borehole, interference between the different modes of propagation can occur in prior art pulsed acoustic travel time measurements for the different modes of acoustic propagation. The present inver.tion by utili~ing a unique or characteristic variable frequency swept siynal and correla-tion of this signal with the entire acoustic w~ve train lS arriving at the receiving tran~ducer can produce more readily identifiable output pulses on the cross-correlator output as illustrated in Fig~ 4 to separate the various arrivals of acoustic mode propagation in a manner superior to that known in ~he prior art. Thus improved acoustic travel time measurements of compressional, shear, Stonely and other modes of acoustic propagation are pro~ided by the present invention which were subject to ambigious inter-pretation in the prior art.
It will be recognized by those skilled in the art that the acoustic transmitting transducer and asoustic ~receiving transducer of the present invention may be mounted on pad arms (not shown) and urged against the wali of the borehole if desired, rathex than being housed in the body of the sonde as illustrated in Fig. 1. Similarly, a back-up arm (not shown) could be used if desired, to urge the body of the sonde of Fig, 1 against one wall o~ ~he borehole.
Because of the statis~ical nature o the cro~s-correlation in detecting the arriving signal~ at the receiving trans-ducers in the present inven~ion so called "road noise~ or S noise generated by the motion of ~he logginy tool through the borehole is minimized.
Other changes and modification~ which fall within the true spirit and scope of the present invention may be suggested by the foregoing descriptions to those skilled in the art. Accordingly, it is the aim of the appended claims to cover all such changes and modifications as may be made apparent to those skilled in the art~

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A well logging system for measuring and recording the acoustic energy propagation characteristics of earth formations penetrated by a well borehole comprising:
a fluid tight hollow body member sized and adapted for passage through a well borehole;
means in said body member for repetitively generat-ing swept frequency acoustic energy outputs having a linearly varying range of frequencies from a lowest frequency of approximately two kilohertz to a highest frequency of approximately twelve kilohertz in a characteristic pattern, said pattern having a duration of approximately four milliseconds;
means for digitizing said characteristic swept frequency signal and for providing a digital signal representa-tive of said characteristic signal at said generating means;
receiver means longitudinally spaced from said generating means by a distance of from eight to twelve feet in said body member, for detecting acoustic energy propagated from said generating means through the borehole and earth formations in the vicinity of the borehole and for generating digital signals representative of said detected acoustic energy;
means for cross correlating said digital signal representative of said characteristic signal at said generating means and said digital signal representative of said detected acoustic energy and for providing a correlator output signal representative of the arrival at said receiver means of different modes of propagation of acoustic energy in the borehole and earth formations in the vicinity of the borehole;

computer means responsive to said correlator output signal for deriving therefrom measurements of the speed of propagation of said different modes of propagation of acoustic energy in the earth formations in the vicinity of the well borehole; and means for recording said measurements of the speed of propagation of said different modes of acoustic energy as a function of borehole depth, thereby providing on a record medium a well log of speed of propagation of different modes of acoustic energy.

2. The system of Claim 1 and further including means for recording as a function of borehole depth said correlator output signal, thereby providing a well log indicative of the time of arrival at said receiver means of different modes of propagation of acoustic energy.

3. The system of Claim 2 wherein said correlator output signals are recorded as a function of borehole depth in a variable density display pattern.

4. The well logging system of Claim 1 wherein separate well logs are recorded as a function of borehole depth for compressional wave propagation speed and shear wave propagation speed.

5. The well logging system of Claim 1 wherein separate well logs are recorded as a function of borehole depth for compressional wave propagation speed, shear wave propaga-tion speed and Stoneley wave propagation speed.