CA1240026A - Method for interpretation of seismic records to yield indications of gaseous hydrocarbons - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
METHOD FOR INTERPRETATION OF SEISMIC RECORDS
TO YIELD INDICATIONS OF GASEOUS HYDROCARBONS
The present invention indicates that gas-containing strata of an earth formation have low Poisson's ratios and that the acoustic contrast with the overburden rock has a surprising effect as a function of the angle of incidence on a seismic wave associated with an array of sources and detectors: viz., a significant -- and progressive -- change in P-wave reflection coefficient as a function of the angle of incidence occurs.
Thus, differentiating between high-intensity amplitude anomalies of nongas- and gas-containing media is simplified: progressive change in amplitude intensity of resulting traces generated by the field array as a function of offset between each source-detector pair, is associated with the last-mentioned medium only.

Description

FI ELD OF THE lNVENTI021

2 The present invention pertai~s to the arl of seismic

3 prospecting ~or petroleum reservoirs Dy ~ultiple-point surveying

4 techniques~ ar.d more particularly ~o the art of converting high-S intenslty reflsction amplitude anom~lias associated with on~ or 6 more common centerpoints observed on seis~rlic record traces into 7 diagnostic indicators of the presence of gas in the underlying 3 subsurface strata.
9 B ACKROU ND_OF THE IN V2NTION
Por several decades, seismic prospecting for petroleum 11 has involved the creaticn of acoustic disturbar.ces above, upon, 12 or just below the surface of the earth, using explosives, air 13 gunsy or large mechanical vibrators. Resul~ing acoustic waves 14 propagate do~nwardly in the earth, apd are partially reflected back to~ard the surface when acous~ic im~edance changes ~ithin 16 the ear~h are encounter~d~ A changa from one rock type to 17 another, for example, may be accompanied by an acoustic impedance 1~ change, so that the reflectivity of a ~articular la~er depends on 19 the velocity and density content bet~een that layer and the layer ~hich overlies it, say according to the formula 21 C Reflect = AR/Ai = Y2d~ - Vld 22 Vzd2 * ~ld~
23 where AR is the amplitude from the reflected signal and Ai is the 24 amplitude of the incident signal; Y~ is the velocity of the wave in the overlying m~dium 1; Vz is th~ velocity in the medium layer 26 belo~ the contact line; d~ is the de~sity of the overlying medium 2J 1; and dz is the density of the underlying medium.
28 In early years, signal traces of the rerlected acoustic 29 waves were rscorded immediately in the field as visible, side-by-side~ dark/ ~iggly lines on white paper ("sélsmograms"). ~t 31 present, th2 initial reproductions -- in a digital format - are 2 r~ r ~ ~f~

1 on magnetic tape, and finally are reducea to visible side-by-side 2 traces on p~per or film in large central computing facilities.
3 At such centers, sophisticated processing makes 4 possible the distinction of signals ~rom noise in cases that -would have seemed hopeless in the e~rly aays of seismic 6 prospecting. Until 1965, almost all seismic surveys conducted 7 used an automatic gain control which continuously adjusted the 8 gain of amplifiers in the field to account ~or decreasing amounts 9 of energy from late reflection arrivals. As a result, reflection coefficients could not be accurately d~termined. However, with 11 the advent of the expander circuit and binary gain amplifiers, 12 gain of the amplifiers can now be controlled and amplitudes 13 recorded precisely; this makes it possibie to conserve not only 14 the special characteristics of the r~flections, but also tbelr absolute amplitudes.
16 ~here was als~ another problem in the prior art 17 equipment. Computers often precluded the use of a comparison 18 technigue because of their small word size and tiny core storage.
19 rroday, more powerful computers ~ith array processors and economical fLoating poin-t capabilities no~ enable modern 21 geophysicists to maintain control ol the amplitude of all 22 recorded signals. The "floating point" capability is especially 23 effective in expanding computer ~ork siz~ by a large factor and 24 in eliminating the naed for computer automatic gain control.
That is to say, in summary, as a result of the above 26 advances, reflections from many thousands of eet below the 27 earth's surface can now be confidently detected and follo~ed 28 through sometimes hundreds of side-by-side traces, the shortening 29 or lengthening of their corresponding times of arrival being i~dicative of the shallcwing or de~pening of actual sedi~entary 31 strata of interest~ Still, as a gene al rule, all that can be 1 hoped for the seis~ic reflection method lS to detect strati-2 graphic interfaces and the interfaces dS they deviate from hori-3 zontality of these interfaces, so that subsurface "structures"
4 could be defined in ~hich oil or gas mlg~t possibly be trapped.
Apropos of the above has baen use of ultra-high 6 amplitude anomalies in seismic traces to i~fer the presence of 7 natural gas in situ. Seismic in'er~setators have used so~call~d 8 "bright-spot" analysis to indicate severdl large gas reservoirs 9 in the world, especially in the Gulf Coast of the United States.
Such analysis is no~ rather common in the oil industry, but i~ is 11 not without its critics. Not only canno~ the persistence of such 12 increased amplituds anomalies be takell as confirmation of the 13 lateral e~te~t of the gas reser~oir, but also the anomaly itself 14 (in some cases) may not represent reflections of a discontinuity of a gas-bearing medium and its oYer- or underlying associated 16 rock strata. 2~g., experience has sho~n that in certain 17 situations~ similar phenomena occur ~hich can confuse the 18 interpreter. E~g~ the shape of the hori~on is such ~hat it 19 focuses the energy back to the surface, it may increase the amplitude of on~ or more of the records akin to reflections from 21 gas-saturated strata. Lithology of the horizon - singly and in 22 combination -- can also have a similar effect, producing high-23 amplitude reElsctions in the absenca of gas within the pore space 24 oE the strat~m of interest. ~xampl2s of the latter:
2S conglomeratic zones, hard streaXs of silt or lime and lignite 26 beds.
27 The present invention improves the ability of the 2~ seismologist to correctly differentiate high-intensity anomalies 29 of multiple-point-coverage seismic traces of gas-bearing strata from those of similarly patterned re~lections of other types of 31 stratlgraphic configurations containing no gas accumulations.

OBJECT QF ~HE l~VENl:ION
2 An ob ject of the inveiltion is Ihe proYision of a novel 3 me.hod of correctl~ differentiating high-intensity anomalies 4 provided by multiple-point seismic traces o~ gas-bearing S structures from those of a similarLy patterned intensity 6 associated ~ith strata containing ~o gas accumulation.
7 SUM-'IA~Y QF T~IE INVgNTION
8 In accordance ~ith the present invention, 9 lnterpretation of high-intensity seis~lc events from traces obtained from multiple-point covera~e of a subterranean earthi 11 formation using an array of source means a~d detectors adjacent 12 to the earth's surface is obtai~ed to indica~e gas-bearing strata 13 in a highly surprising and accurate mann_r. After the field data 14 haYe been obtained in ~hich the data of common centerpoints are associated ~ith ~ore than one source-de~ector pair, the da~a are 16 inde~ed ~"addressed") whereby all racorded traces are indicated 17 as bei~g a product of respecti~e source-detector pairs o~ known 18 horizontal o~fset and c~nterpoint location. Thereaftert high-19 intensity amplitude anomali~s in said traces are correctly asso-ciated ~ith gas-bearing strata on a surprisingly accurate 21 selection b~sis: amplitude intensity of said anomaly must change 22 - progressively in an increasing or decreasing manner -- as a 23 function of horizontal offset. A further refinement of the 24 method of the present invention may be in order under some circumstaLces. A single common-centerpoint trace, or ev2n single 26 common-cent~rpoi~t yathers, may have a major dradbac~ in such 27 cases ~ poor signa:L~to-noise r~tios. As a result, progressive 28 ampli~ude change as a function of offs2~ cannot be resolved. In 29 accordance ~ith this inYention and as a means of signal enhancement, trace summations can prove beneficial in improving 31 record résolution, say on a basis OL a stac~ing 'Iwindow" having a ~ ~ ~3~
two-dimensional index for addressing the traces: X
common offset dimension long by Y common centerpoints wide. For example, where 2400~ common-centerpoint stacked traces have been obtained (i.e~, 24 traces per

5 gathsr) by multiple-point-coverage field techniques, each gather can in turn he "de-stacked" to provide original but corrected locational traces. Then on the basis of a stackiny window four (4) common offset dimensions long hy five (5) common centerpoints wide, several such trases, say 10, can be stacked and the stacked trace displayed as a function of offset.
Result: progressive change in amplitude intensity as a function of similar intensity changes in offset can be more easily observed whereby in situ gas is, more likely than not, in the pore space of the structure of interest.
An aspect of the invention is as follows:
A m~thod for identifying gas-bearing strata in the earth associated with high-intensity amplitude events located by common depth point traces, comprising the steps of:
(a) generating seismic data includinq a record of signals from acoustic discontinuities associated with said strata of interest for a series of points located along a line of survey, by utilizing for each of said points multiple pairs of sources and detectors, each of said points being a centerpoint located hori20ntally midway between the respective source and detector of each pair, said sources and detectors of each pair being progressively horizontally offset from said centerpoint and thus being at progressively increasing angles with respect to a line between said centerpoint and a common depth point located below said line of survey, said recorded signals being the output of said detectors;
(b) by means of automated processiny means, statically and dynamically correcting said recorded signals to form corrected txaces whereby each of said corrected traces is associated with a centerpoint hori20ntally midway between a source-detector pair from which ~aid corrected trace was originally derived;
~.

(c) by means of automated processing means, indexing said correctPd traces in two dimensions whereby each of said corrected traces is identified in its relationship to neighbouring traces on the basis of progressive changes in horizontal offset value versus progressive changes in common centerpoint location;
(d) displaying a series of said traces of step (c~
on a side-by side basis as a function of progressively changing horizontal offset values, said displayed traces all being associated with at least the same general common group of centerpoints;
(e) identifying progressive change in a high-intensity amplitude event from trace to trace o~
said displayed traces as a function of progressive changes in horizontal offset value whereby more likely than not said event relates to reflections from acoustic impedances associated with strata containing gaseous hydrocarbons.
DESCRIP~ION OF THE DRAWINGS
Further features of the invention will become more apparent upon consideration of the following detailed description of the invention when taken in connection with the accompanying drawings, wherein:
FIG. 1 is a plan view of a grid of centerpoints produced in the field by the systematic positioning and energization of an array of seismic sources and detectors whereby a series of locational traces associated with individual centerpoints between respective source detector pairs are ultimately generated;
FIG. 2 is a model of typical reflecting horizons within an earth formation that can be associated with the characteristics of the locational traces of FIG. l;
FIGS. 3, 4 and 5 are plots of reflection coefficient as a function of angle of incidence of seismic waves associated with the reflecting horizons of FIG. 2 which aid in the determination of the presence of gas within an earth formation;

; -6a-~l.,,~, 1 EIGS. 6(aj, 6~b), 6(c~ and 6(d~ are plots of various 2 quantities of a mathematical nature, as a function of percentage 3 o~ gas saturation, illustrating the relationship of Poisson's 4 ratio to the determination of the prasence of gas ~ithin an earth formation;

6 ~IGS. 7 and 8 are plots of ca~terpoints produced by an

7 array of sources and detectors ~herein a geometrical

8 transformation has occurreA to ~etter illustrate processes asso-

9 ciated ~ith the method cf the present invantion;
PI~S. 9(a~ and 9~b) are f10~ dlagrams of processes akin ~1 to those shown in FIGS. 7 and 8 for carrying out the method o~
1Z the present invention, ucing a programmed digital computing 13 system;
14 FIGS. 10 and 11 are schema-tic diagrams of elements ~ithin the digital computing system of FIG~ 9; and 16 FIGSo 12-21 are true seismic r~cord sections and 17 portions of sections, illustrating t~e diagnostic capability of 18 the me~hod of the present invention in predicting the prese~ce of 1g gas s~rata in actual field examples.
. PREFERRED EM~ODIMENTS OF_~IE INVENTION
21 Before discussion o~ an em~odiment of the invention 22 within a~ actual field environment, a brief description of the 23 mathe~atical and theoretical concepts behind it may prove 24 beneficial and are presented below.
Firstly~ it may be of interest to indicate lithology 26 limitations associated ~ith the prss2~t invention. ~or example9 27 anomalies associated with gas sanAs over shale cap rock are one 28 example in which the method of the prese~t invention offers 29 surprising results; another relates to gas-saturated limestone over shale~ also of import is the relationship between Poissou's 31 ratio and resul~ing high intensity amplitude anomalies pro~idad 3~ on seismic traces~

DU2~6 1 While Poisson's ra~io (~) ~as the general formula 2 ~v~2 c = -- _ 2. ~VP) -l~

3 where Vp is compressional velocity and Vs is shear velocity of 4 the medium, the concept does haYe physical significance. For example, consider a slender cylindrical rod of an elastic 6 material and apply a compressional force to the ends. As the rod 7 changes shape, the length o~ the rod ~ill decrease by ~L, ~hile 8 the radius ~ill increase by ~R. Polsson's ratio is defined as 9 the ratio of the relative change i~ ~adius ~R/R) to the relative change in length (~L~L). Hence a co~pressi~le material has a lo~
11 Poisson's ratio, while an incompressible material (as a liquid) 12 has a high Poisson's ratio.
13 The equation above also indicates the relationship of 14 the comp~essional and shear wave velocities of the material, Vp ~nd Vs respectively; i.e., tha~ Poisson's ratlo may be de~ermined 16 dyr~amicall~ by measuring the P-~ava and S-wave velocities. Only 17 t~o of the thre~ variables are independent, however.
18 Recent published studies on reflection and transmission 19 seismic va7es useful in geop~ysical appllcations include:
(1) Koefoed, O., 1955, for "On the Effect of Poisson's 21 R~ios of Rock Strata in Ihe Reflection Coefficients of 22 Plane ~aves", Geophysical Pros~ecting, Yol. 3, No. 4.
23 (2) Koefo~d, O., 1962, for "R~flection and ~ransmission 24 Coefficients for Plane Longitudinal Incident '~aves", Geophysical Prospecting~ Vol~ 10, No. 3.

1 (3) ~uskatr ~. and Meres~ ~.W., 1940, for "Reflection and 2 Transmission Coefficients for ~lane r~a~es in Elastic 3 nediall, Geophysics, Vol. 5, No~ 2.
4 (4) Tooley, P~.D., Spencer, T.~. and Sagoci H~Fo~ or "Reflection and Transmission ol Plane Co~pressional 6 ,la7es", Geophysics, Vol. ~0, No. 4 (19653.
7 (S) Costain, J.K., Coo~ K.L. and ~lgermisshi, S.T., for 8 ''Amplitude, ~nergy and Ph-lse Angles of Plane sP ~aves 9 and ~heir Application to Ear~h Crustal Studies" t ~ull.
S2is. soc. Am~, Vol. 53, y. 1639 et~ seq~
11 All of the above have focused on the complex modeling 12 of variation in reflection and transmission coefficients as a 13 function of angle of incidence.
14 The problem is complicated, ho~ever. EOg., isotropic media with layer i~de~ of the s~ra~a, i=1 for the ir.cident medium 16 and i=2 for the underlying medium, have Deen modeled using 17 equations for P-~ave re~lec~ion coefficient Apr and ~or P-~ave 18 transmission displacement amplituda coefficient Apt. In such 19 e~uations~ the ~alue of Poisson's ratio (~) is required, since both the P-wave and S-wave ~elocities are utili2ed. For each of 21 the media, i.e.~ the incident or underlying medium, three 22 independent variables exist: P-~ave velocity~ ~ and bulk 23 density, or a total of six variables for both media. But for a 24 l single interface, only four independe~t variables were required:
P~ave velocity ratio, the density ratio, Poisson's ratio in the 26 incident medium and Poisson~s ratio in the underlyi~g medium.
27 But to provide for the many combinations of possible 28 ~ariations, the above-listed studies hava either:
29 ~a) generated many (literally thousands) plots of a mathematical nature for various parameters, values in 31 which there was little relatlonship with true g 1 gQophysical applications, since the latter ~ere 2 hopelessly ob~cured and unappreciated; or 3 (b) made simplistic assu~p-tions ~dt, al~hough using actual 4 calculations, nevertheless did not express the true nature of -transmission and reflection coefficients, in 6 particular lithological si~uations associated ~ith 'he 7 accumulation of gaseous hydrocarbons Nithin an actual 8 earth formation.
9 In summary, while reference (2) concluded that change in ~oisson's ratio in the two bounding m~dia can cause change in 11 the reflection coefficient as a function o~ angle of incidence, 12 (2) did not relate that occurrence to lithology associated with 13 the accumulation of gaseous hydrocarbo~s i~ the surprising manner 14 of the present invention.
The present invention teaches that gas-containing 16 strata have lo~ Poisson~s ratios and that ~he contrast ~ith the 17 overburden rock as a function of horizon~al offs2t produces a 18 surprisi~g res~lt: such contrast provldes for a signiflcant ~
19 and progressive - change in P-wave reflection coe~icien~ at the interface of interest as a function of angle of incidence o the 21 incident ~ave. Thus, diffqr6ntiating be~een high-intensity 22 amplitude anolnalies of nongas and gas medid is simplified by 23 relating progressive change in ampLitude intensity as a function 24 of offset bet~een source-detector pairs, i.e., angle of incidence b~ing directly related to offset.
26 Also, the behavior of P-~ave trav~l as a function o-E
27 lithology and horizontal offset between a respective source-28 detector pair associate~ with a given locational trace pro~ide 29 the foLlo~ing amplitude response signatures of interest:
(1) ~here thé gas-containing media are gas sands underlying 31 shale, such as found i~ the Gulf Coast~ amplitude 32 responses increase ~ith offset;

- 10 1 (2) where the gas-containing media include limestone 2 un~erlying shale, such as found in the ~orth Sea, the 3 amplitude anomalies of th2 ~tarface decrease ~ith 4 offs~t.
Now in more detail, attention should ~e directed to the ~ ~igures, particularly FIG. 1. Note that, lnter alia, FIG. 1 7 illustrates in some detail ho~ the ~erms of interest in this 8 application are derived: e.g., the term '7cen~erpoint" is a geo-9 graphical location located midway between a series of sources 10 S~,S2Sn of a geophysical ~ield system 9 and a set of detectors

11 Dl~DzDm at a datum horizon near ~he earth~s surrace. The

12 centerpoints are designated C~,Cz...Cp in the Yigure, and are

13 associated with a trace derived by place~ent of a source at that

14 centerpoint location folloYed immedLately by relocating a detector thereat.
16 I.e., if the sources Sl... Sn aré excited in sequence at 17 the source lmcations indicated, traces r~cei~ed at the different 18 detector locations shown can be rel3ted to com~on centerpoints 19 therebetween. If such traces are summedr a gather or group of traces is formed. I.e., if the reflecting interface is a flat 21 horizon, the depth point ~here refl~ctlon occurs will de~ine a 22 vertical line ~hich passes through the cbnterpoint of interest.
23 Applying static and dynamic corrections to -the fieLd traces is 24 e~uivalent tunder the above facts3 ko placing the individual sources S~,S2...Sn at the centerpoint in sequence foLlowed by 26 rsplacement with the de~ectors D~D~ OL interest at the same 27 locations. I~ the traces associated ~ith a common centerpoint 2a are summed~ a series of enhanced traces, sometimes called CDPS
29 (Com~on Depth Point St~ck3 traces, is provided.
~IG. 2 illustrates reflection phenomena of a three-31 layer modal typical of a young, shallo~ geoloyic section 10, such as found in the GUlf Coastt illustra~ing ho~ reflection phenomena 2 associated ~ith the traces associated wi~h the field system 9 of 3 PIG. 1 can be related tc the presence of gas.
4 Section 10 includes a gas sand 11 embedded in a shale stratum 12. Assume a Poisson's ratio of 0.1 for the gas sand and 6 of 0.4 for the shale, a 20~ velocity reduction at interface 13, 7 say from 10,000'/sec to 8000'~sec, and a 10~ density reduction 8 fro~ 2.~0 g~cc to 2.16 g/cc.
9 Th~ actual P-~ave reflection coefficient Apr can be rela~ed to section 10 by Equation (1J ~eiow; also, P-wave 11 transmission displacement amplitude coefIicient Apt can similarly 12 be related in accordance with Equation (2~ below.
~ z-~ (6) ~-1 (7) A f - r; (~) ~ Sz+2b2 ~a) ~ 2~b2 ¢9) _ h22a,k,2(c,~ ) (2) ~;~ k2-2b~ ~10) ~ - ~2/~l (Il) P~ h,(f ~ ;V~i (12) b:h,sln~ (13) f = k, k?~a,c2~c2c,j (~) a~=h2-b (14) C~=kz-b' (I;) f hi i/Vp~ ) k~ 5i (17) = b rl ~ a c y2 (4) Vpi, ~.WAVE VeLOClry Vsi ~ 5.WAvE `/EL~CITY
_ a2 c2 (~ ~ 4 ~,c,`~; b ) (5), EAYEl INOEl ~ ~NGlE Of INCl5eNCE
13 Equations (1) and (2) are, of course, the two basic 14 eguations of ~ave travel in an earth formation and are for

15 isotropic media with the layer index bei~g i=1 for the incident

16 medium and i=2 for the underlying ~edium. Equa-tions ~'~ through

17 ~17) simply de~ine intermediate varlablas.

18 As an example of calculations associa~ed therewith, if

19 ~=0 ~normal incidence), the e-wave refl2ction coefficient Apr is

20 equal to about -0.16 and ~0.16, respectively.

1 FIG~ 3 illust~ates ~hange i~ raflection coefficient as 2 a function of angle of incidence ~ lor the ~hree-layer model of 3 FIG. 2~
4 Note that solid lines 20, 21 illustrate the effects of 5 reflection (and transmission, by omission) on the top and base of 6 the gas sand. In line 20, at ~=0~, note that the A~r equals 7 ~0.16; ~.lhile a+ ~=~0, the Apr is about -0.28. That is, rather a 8 surprisingly large change in the reflection coe~ficient as a 9 function of angle of incidence occurs, ~ith ~he greatest change occurring bet~een ~=20 and ~=40.
11 For the bottom layer, line 21 changes at a~out the sama 12 rate, but in opposite sign. I.e., at ~=0, Apr is about ~0.16 13 and at ~=40, Apr is about +0.26. Again, the greatest change in 14 Apr occurs bet~een ~=20~ and 3=40. ~ As a result7 the amplituds of the seismic wave reflected from this model would increase about 70~ over the angle of incidence range shown~ i.e., over the 17 incremental 40 degrees shown~
18 While angles of incidence equal to 40 ~ay seem a 19 little large for reflection pro~iliny (heretoforet most da-~a arriving beyond 30 being thought useless and muted out),

21 experience has no~ nevertheless sho~n that reflsction data can

22 and do arri~e at reflection angles greater than 30. Hence, the

23 angles of incidence must be determinedJ and one of the more

24 important ~echnigues, the straight-ray a~proach to estimate such angles of incidence (using depth-to-reflector and shot-to-26 detector and-sho-t-to-group offset), is dS set forth below:
27 ~1 - arc tan (X/2Z) (18) 28 where ~1 is the angle of incidence; X is the shot-to-detector or 29 shot-~o-group offset and Z is the reflector depth. Velocity changes with depth can like~ise be ~ccommodated by assuminy 31 section velocity change is of the form V1=YO~ where K is a ~24~ %~

1 constant so that all ray paths are lrcs of circles having centers 2 ~o/K above the reference pla~e of in-~rest, say the earth's 3 surface. Thus~ the approach should De irl accordance with 4 ~l = arc tan ZZ~2vo(Z/K)-~z/~) (19) 6 ~aving no~ established a Lirm mathematical and 7 theoretical basis for the process or the present inYention, 8 perhaps a description of ho~ a geologlcal section containing no 9 gas therein ~ould affect impedance cont~ast is in order. FIG. 4 illustrates the changes in reîlection coefficiPnt as a function 11 of angle of incidence ~ in the manner of FIG. 3, but in which the 12 gas sand 10 of FIG. 2 contains no sas, simulating, e.g., a lo~-13 velocity, brine-saturated~ young sa~dstone embedded in shale.
14 Th~ solid lines 22, 23, represcnting reflection coefficients, are seen to be about horizontal between ~=0~ and 16 40, slightly decreasing in magnitude as the angle of incidence 17 increases, i.e., as ~ approaches 403~ In the above example, it 18 should be noted that the Poisson's ratio of the sandstone was 19 assumed to be 0.4.
FIG. 5 illustrates yet another plot associated ~ith a 21 three-layer model akin to that shown in FIG. 2, but in which the 22 sandstone contains gas but is buried deep below the earth's 23 surface. The valuss for the three-layer model of FIG. 2 are 24 again used except that the ~elocity ch-ange from shale to sand is only 10~, or Erom 10,000'/sec to 9000'~sec. As sho~n, curves 25 26 26 are even more significant: both cur~s are seen to increase 27 in magnitude from over ~he 40 of change in the angle of 28 incidence. Ho~ever, field results have ~o; ~erified these 29 results, since Poisson's ratio in such gas sands may be strongly affected by depth, and not be as lo~ as is no~ surprisingly 31 taught by ~he present invention.

~2~

1 FIGS 6(a), 6(b), 6(c) and 6(d) offer a possible 2 ~xplanation for lo~ eoiSson's ratio Ln gas-containing strata in 3 gsneral and in gas sands in particular. In the Figures, various 4 quantities are plotted as a function of percentage of gas saturation. -In Fig~ 6~a), P-wa~e velocity is so plotted; in FIGo 6 6tb), S-wave ~elocity is depicted; in FIG~ 6(c) 9 the ratio of 7 Vp/Ys is the valus of interest; and in PIG. 6(d), ~oisson's ratio 8 is shown as a function of perc~nt gas sa~urationc 9 Nots that FIGS. 5(a~ and 6 (b) are for sandstones buried at 6000 fePt ~ith 35Z poroslty. P~GS. 6(c) and 6(d) result from 11 PIGS. 6 (3) and 6(b) using appropriate equations. But in PIG.
12 6(d), ~oissonJs ratio drops from aboul ~.3 to 0O1 from 0% to 10 13 gas saturation; on the cther hand, tn~ same ratio changes Yery 14 Little from 10~ to 100~ gas saturati~n (average value is about 0.09).
16 Hence, from the above mathematical and theoretical 17 concepts, displays of reflection data can now be used to indicate 18 change in reflection coefficient as a fu~ction of angle of 19 incidence to indicate the presence or gaseous hydrocarbons/ SQch data are also ~ow conveniently available, say using today's 21 conventional field-gathering ~echniques lnvolvins mul~iple-area 22 coverage, since the former can he derivea rrom and is compatible 23 ~ith one oE today's conventional forms of recorded reflection 24 seismic data: co~mo~-depth-point ~D~) gathers. And, progressive changes in reflection amplitude vs. shot-to-detector 26 (group) offset can form the basis o~ such a determination, since 27 ~ offset of any particular source-detector pair is directly relat~d 28 to the angle of incidence in accorda~ce with Equation (19), 29 supra.
~ut corrected locational trace data, be~ore stacking in 31 accordance ~ith CD~S techniques, oftan have poor signal-to-noise ~ - 15 -1 ratios. Thus, changes in amplitude vs. offset may be difficult 2 to observe in such data.
3 ~IG. 7 is a diagram which illustrates a data 4 "addressingl' technigue ~hich improves am~litude versus offset resolution in such situations; in tha Figure, the traces ~ere 6 generated using an end-shooting array o 48 detectors with source 7 and detectors advancing one detector interval per shot point.
a Result: a 24-fold CDP-stacked record section was generated.
9 Note further: each centerpoint is assoclated with 24 separate traces of varying offset.
11 In order to geometrically associate each generated 12 locational trace ~ith its common centerpoint, address guidznce, 13 as pro~ided ~ FIG. 7~ is important. To understand the nature of 14 PIG~ 7, assume that the sources S1,S2~n are sequentially 15 located at shotpoints SP~,SP2SPn at the top of the Figure.
16 Assume also that the detectors are placed in line with the 17 sources, i~e.~ along the same line of survey A at ~he detector 18 locatio~s D~D2.~.Dm. After each source is activated, 19 reflections are received at the detectors, at the locations shown. Then by the 1'rollalong" tech~igue~ the source and 21 detector sprqads can be moved in the direction B of survey line A
22 and the process repeated to provida a series of traces. The 23 latter are associated ~ith centerpoin~s midway between the 24 respective detector pairsc In ~he Figur~, assume source Sl has been located at shotpoint SP~ and e~citea. Mid~ay between SPl 26 and each of the detectors, at Dl, D2...Dm, is a series of center-27 points C~; CzCn~ The latter are each associated with a trace.
28 In this regard and for a further description of such techniques, 29 see ~.S. Patent 3~597,727 for "Method o~ Attenuating Multiple Seismic signals in the Determination of Inline and C~oss-Dips 31 ~mploying Cross-Steer~d Seismic Data", Judson et 21, issued ~ \

1 August 3, 1971, and assigned to ~he assignee of the present 2 application. '~ith appropriat~ sta~lc ana dynamic corrections, 3 the data can be related to the common ce~terpointx midway bet~een 4 individual source points and detectors, as discussed in ~he 5 abovq-noted referenc~.
6 But by such a field ~ech~lque, data provided generate 7 24 separate traces assicat~d with ~h2 same centerpoint C1Cn.
8 In order to index l"address") these traces as a function of 9 several factors includincJ horizontal offset and centerpoint location, a stacking chart 44 as shown in FIG. 8 has been 11 developed~
12 Chart 44 is a diagram in ~hich a trace is located along 13 a plurality of oblique ccm~on profile lin2s PLl,PLz~ between a 14 series of common offset and centerpoint locations at 90 degrees to each other. For best illustration, focus on a single 16 shotpoint~ say SP~ and on a single detector spread having 17 detectors D~,D2~Dm of FIG. 8 along survey line A. Assume a 18 source is located at shotpoint SP1 and activated thereafter. The 19 detector spread and source are "rollad'' forward along survey line A in the direction ~ being advanced one station per activation~
21 Then aftar detection has occurred, and if the resulting 22 centarpoint pattern is rotated 45 a~out angle 46 to profile lin~
23 PLi and projected belo~ the spread as in FIG. 8 as a function of 24 common offset values and centerpoint positions~ the chart 44 o FIG. 8 results~ Of course, each centerpoint has an amplitude vs 26 time trace associated therewith, and for didactic purposes that 27 trace can be said to project along a line normal to the plane of 28 the ~igure.
29 It should ~e ~phasized that the centerpoints provided in FIGS. 7 ancl 8 are seccJraphically located along the line of 31 survey A in line with the source points Ses,SP2As the 1 locational ~races are generated, t~e chart 44 aids in ~eeping a 2 "tag1' on eaoh resulting trace. As the detector spread and 3 sources are rolled forward one station and the technique 4 repeated, another series of traces is generated associated with centerpoints on new profile line eL2~ That is, although the 6 centerpoints are geographically still associated within posltions 7 along the survey line A of FIG. 7t by rotation along the angle 8 46, the new centerpoint pattern C1~,C2'...Cn' can be horizontally 9 and Yertically aligned with centerpoints previously generated.
I.e., at co~mon offset values ~in horizontdl alignment) certain 11 centerpoints are aligned~ viz, centerpoint Cl aligns with C~' as 12 sho~n; further C2 is aligned ~ith C29, etc. Also, there are 13 traces that ha~e common centerpoints. I.e., at common 14 centerpoints ~in vertical alignment)~centerpoint C2 aligns ~ith 15 CenterPOint CL~ ~ and centerpoints C3~Cz~ and C1" are similarly 16 alig~ed. Thus, via chart 44, each trace associated with a 17 centerpoint can be easily l'addressed" as to:
18 (i) its actual geographical location (i.e~, along phantom 19 lines normal to diagonal profile lines PLI,P~2.... along common location lines LLl,LL2... ), so that its actual 21 ~ield location is like~ise easily kno~;
22 (ii) its association with other traces along common 23 horizontal offset lines COLlyCOLz... COLX; and 24 (iii) its association ~i-th still other traces along common vertical centerpoint location lines CPLl,CPLz 26 Also, "addressing" the traces ~y (ii) and ~ allo~s 27 such traces to be easily combined ~summed) ~y calling out 28 "windows" ~ithin the chart in which any traces withln the windo~
29 can be summed. E.g.~ it has been found convenient to establish a standard wlndo~ "width" equal to a~ lncreased group centerpoint 31 line (~CP~) vaLue of say 5, and a ~indo~ ightl' equal to an 32~
1 incremental common group ofrset line (~COL) Yalue of say 4; hence 2 by indexing the intersecting ~indo~ intervals on a seguential 3 basis, summation of traces therein can occur. The results are 4 summed traces ~hich a~e outputted to a dlsplay on a side-by-side basis, say as a function of amplitude intensity as a function of 6 increasing or decreasing offset between respective source-7 detector pairs. Actual offset values are not required, since 8 relative values are usually sufficient for most diagnostic 9 purposes.
In carrying out the above summation process on a hlgh-11 speed ~asis, a fully programmed digital computer can be useful.
12 sut electromechanical systems well known in the art can also ~e 13 used. In either case, the field traces must first undergo static 1~ a~d dynamic correction ~efore the traces can be displayed as a function of offset to determine thelr potential as a gas 16 reservoir. Such correction techniques are ~ell known in the art 17 - see, e.g., U.S. Patent 2,838,743, of O.A. ~redriksson et al, 18 for "~ormal ~Yoveout Correction uith Common Drive for Recording 19 Medium and Recorder and~or ~eproducing Means", assigned to the assignee of the present application, in which a mecnanical de~ice 21 and method are depicted. Modern process ng today uses properly 22 programmed digital computers ~or that lask in which the data 23 words ars indexed as a function o~, Lnter alia, amplitude, time, 24 datum height, geographical iocation, group offset, velocity, ana are manipulated to correct for the angular and horizontal offset;
26 in this latter environment, see U.S. 3,7~1,269, Judson et al, 27 issued May 1, 1973, for "Static Corcections for Seismic Traces by 28 Cross-Correlation ~ethod", a computer-implemented program of the 29 above type also asslgned to the assignee of the present invention. Electrocnechanical sorting and stacking eguipment is 31 also well kno~n in the art and is OI th- oldest ways of _ 19 cancelling noise~ See, for example, the follo~ing patents 2 assigned to the assig~ee of the present invention which co~tain 3 sorting and stacking techniques, including beam steering 4 technigues:
Patent Issued Inventor Titl~
6 3~597,727 12/30/68 Judsor. et al Metho~ of Attenuating Multiple 7 Seismic Signals in the Deter-3 mination of Inline and Cross 9 Dips Employing Cross-Steered Seismic Data 11 3,806,863 4/23/74 Tilley et al Methoà of Collecting Seis~ic 12 Data of Strata Underlying 13 8Odies of Uater 14 3"638,178 1/25/72 Stephenson ne~hoa for Processing Three~
Dimensional Seismic Data to 16 Selec~ and Plot Said Data 17 a T,ro-Dimensional Display 18 Surface 19 3,346,t840 10/10~7 Lara ~ouble Sonogramlnirlg for Seismic R~cord Improvement 21 3r766~519 10/16f13 Stephenson Method ~or Processing Surface 22 Detected Seismic Data to 23 Plotted Representa~ions of Sub-24 surIace Directional Seismic Data 3,784,967 1/8~74 Graul Seismic Re~ord Processing ~ethod 26 3,149,302 9/15/74 Klein et al Information Selection Programmer 27 Employing Relative Amplitude~
28 Absolute Amplitude and Time 29 Coherence 3,149,303 9/15/64 ~lein e-t al Seismic Cross-Section Plotter 31 FIGS. 9 (a) and 9(b~ are flow diagrams illustrative of a 32 computer-dominated proc ess in which the functions required by the 33 method of the present invention can ~e easily ascertained.
34 Preliminary to the steps shown in FI~. 9 ~a), assume that a 35 section of seismic data has been analyzed for "bright "spots";
36 such events are ~nown by geographical location and/or a 37 time~depth basis; a:~d the traces have been dynamically and 38 statically corrected.

. ". ,"._ _ ... . . _ . _ . . _ _ 1 The steps of FIG. 9la~ include generating addresses fo~
2 the data that include a common offset address in the manner of 3 ~IG. 8, common centerpoint address and an actual geographical 4 location address in the manner of EIG. 8. Finally, the corrected S traces are classified whereby the amplitude event of interest is 6 displayed as a Eunction of changing horizontal offset valves. If 7 the eva~t progressively changes as d function of offset, there is 8 a high likelihood that the event is indicative of strata 9 containing gaseous hydrccarbons.
After the addresses have ~een generated, trace 11 amplitude summation can also occur as suggasted in FIG. 9(b) on a 12 predetermined selection basis: adding tLaces within a sel~cted 13 'l~indo~", say windo~ 49 bounded by commo~ group centerpoint lines 14 (~CPL) and common group offset lines (~COL~ (see FIG. 8~, usually the "~idth" l~CPL) of ~indow 49 is held constant, and the ~indo~
16 'Iheight" (~COL) is increment~d~ frame-~y-frame, to change common 17 offset ~alues on a progressive basis, say from near offset values 18 to far offset values. ~.g. r in FIG. 8t holding the ~indow 19 "width" constant and beginning at tne lo~sr boundary of the chart 20 44 r the Yindow "hei~ht" (~COL) can be incremented until the upper 21 plot ~oundary is reached. Equipment-wise, the addresses of the 22 boundary lines in the line and colum~ directions are compared 23 within each window frame. T~hen the compariso~ is a match, the 24 address register is incremented9 and ~ne process repeated for the

25 next ~indou frame.

26 After the summed traces are tagged on the basis of

27 changing commo~ offset~ say near tArough far offset, summed

28 traces can be dis~layed~ If the amplitudes exhibit the required ~9 characteristics, as previously stated~ then a determination that the reflections are f~om (or not from~ a gas structure can be 31 made.

1 Of course~ if the edge ol a gas rield is to be 2 determined, the above-mentioned proc~ss ~ould be se~uentially 3 elongated toward a side of the chart 44 of FIG. B in a direction 4 along the survey line. I.e., to say, after the windo~ "height"
has been incre~ented to its f ar o~Lsa~ value and the results 6 . displayed, the ~indo~ "~idth" is incr~mented a common group 7 cer.terpoint interval (~CPL) and the process repeated and the 8 results displayed.
9 FIG. 10 illustrates particular elements of a computing system for carrying out the steps o ~IGS. 9(a) and 9(b). ~hile 11 many computing systems are available to carry ou~ the process of 12 the invention, perhaps to best illus~rate operations at the 13 lowest cost per instruction, a mic~ocomputing system ~0 is 14 didactically bes-t and is presented i~ detail belo~. The system 50 of FIG. 10 can be implemented on hardware provided by many 16 diferent manufacturersv and for this purpose~ elemen~s provided 17 by Intel Corporation, Santa Clara, California, may be preferred.
18 Such a system 50 ca~ include a CPU 51 controlled by a 19 control unit 52~ T~o memory units 53 and 54 connect to the CPU
51 through aus 55. ~rogram memory unit 53 stores instructions 21 Eor directing the activities of the CPU 51 whil~ data memory unit 22 54 contains data (as data words) reLated to the seismic data 23 provided by the field acquisition system. Since the seismic 24 traces contain large amcunts af bit data~ an auxiliary memory unit 55 can be provided. The CPU 51 can rapidly access data 26 stored through through addressing the ?articular input port, say 27 at 56 in the PigureO Additional input ports can also be provided 28 to receive additional i~formation as requi`red from usual external

29 equipment ~ell kno~n in the artt e.g. t floppy disks, paper-tape readers, etc., including such equipment interfaced through input 31 interface port 57 tied to a keyboara unit 58 or such devices.

- ~2 ~

1 Using clock i~puts, control circuity S2 maintains the proper 2 sequence of events required for any processing task. After an 3 instruction is ~etched and decoded, the control circui~ry issues 4 the appropriate signals (to units both internal and external) for 5 initiatiny the proper processi~g action. Often the control 6 circuitry ~ill be capable of respondlng to external signals, such 7 as an interrupt or ~ait request. An i~terrupt request will cause a the control circuitry 52 to temporarily interrupt main program 9 execution~ jump to a special routine ~o service the interrupting device, then automatically return to the main program. A wait 11 request is often issued by memory units 53 or 54 or an I/O
12 element that operates slo~er than the CPU.
13 For outputti~g information, the system 50 can include a 14 pri~ter unit 59 ~hereby the amplitude of the summed traces as a functlon of time is printable. Of more use as an o~tput unit, 16 howe~er, is disk unit 60, which ca~ temporarily store the data.
17 Therea~ter, an off-line digital plotter capable of generating a 18 side-by-side display is used in co~junction ~ith the data on the 19 disk unit 60. Such plotters are available in the art, and one proprietary model that I am familiar with uses a computer-21 controlled CRT for optically merging onto photographic paper, as 22 a display mechanism, the seismic data. ~riefly, in such a 23 plotter the seismic data, after summation~ are converted to CRT
24 de~lectian signals; the resulting beam is dra~n on the face of the CRT and the op~ically merged record oE the event indicated, 26 say via photographic film. After a predetermined number of side-27 by-side lines haYe been drawn, the ~ilm lS processed in a photog-28 raphy laboratory and hard copies returneu to the interpreters for 29 their re~ie~.
FIG. 'I 1 illustrates CP~ 51 and control uuit 52 in m~re 31 detail.

1 As sho~n~ the CPU 51 includes an array of registers 2 generally indicated at 62 tied to a~ ALU 63 through an in ernal 3 data bus 64 under contrcl of control unit 52. The registers 62 4 ~re temporary storage areas. Program countQr 65 and instruction register 66 have dedicated uses; t~e oth~r registers, such as 6 accumulator 67, have more general uses.
7 The accumulator 67 usually stores one of the seismic 8 operands to be manipulated by the ALU 63. ~.g., in the su~mation 9 oE traces, the instruction may direct the ALU 63 to not cnly add in sequence the contents of the temporary registers containing 11 predetermined trace amplitudes togethe~ ~ith an amplitude value 12 in the accumula~or, but also store tne result in the accumulator 13 itself. Hence, the accumulator 67 operates as ~oth a source 14 ~operand3 and a des~ination tresult) register. The additional registers or the array 62 are useful in manipulation of seismic 16 data, since they eliminate the need to shuffle results back and 17 forth betwaen the external memory u~its of FIG~ 10 and 18 accumulator 67. In practice most ALU's also provide other built~
19 in functions, including hardware subtraction, ~oolean logic operations, and shift capabilities~ The ~LU 63 also can utilize 21 flag bits generated by FP unit 73 vhich specify certain 22 conditions ~hat arise in the course of aLithmetical and logical 23 manipulations4 Flags ~ypically include carry, zero~ sign, and 24 parity~ It is possible to program jumps which are conditionally 2$ depeadent on the status o~ one or more flags. Thus, for example, 26 the program may be designed to jump to a special routine if the 27 carry bit is set follo~ing an addition i~structlon.
28 Instructions making up the proyra~ for operations 29 involving seismic data are stored in the program memory unit 53 of the CPU 51 o~ FIG. 11. The program is operated upon in a 31 sequential manner except when instructions in the memory units 1 53, 54 call for special commands such as "jump" (or "caLl") 2 instructions. While the program associated ~ith the present 3 invention is a relatively straightforward one, hence avoiding 4 most "jump" and "call" instructions, "call" instructions for S subroutines are common in the processing o~ seismic data and 6 could b3 utilized, if desired. In "call" instructions, the CPU
7 51 has a special way of handling subroutlnes in order to insure 8 an orderly rsturn to the main program. ~hen the processor 9 receives a call instruction, it incremen~s the program counter 65 and notes the counter's contents in a reser~ed memory area of the 11 memory u~it known as the l'stack".
12 cPn~s have different ~ays of mai~tainir.g stack 13 contents. Some have facilities for tha storage of return 14 addresses built into the CPU i~self. Other CPU's use a reserved area of external memory as the stack a~d si~ply maintain a 16 "pointer" registsr, such as pointe~ register 70, FIG. 11, ~hich 17 contains ~he address of the most rec2nt stack entry. The stack 18 thus saves tha address of the instructio~ ~o be ~xecuted after 19 the subroutine is completed. Then the C2U 51 loads the address specified in the call into its program counter 65. Th~ next 21 instruction fe~tched will therefore be the first step of the 22 subroutine. The last instructio~ ln any subroutine is a 23 "return"~ Such an instruction need specify no address.
24 Having no~ briefly described the operations of the C~
2S 51, Table I is presented below containing a full instruction set 26 f or its operations.

~2~

1 TA~L2 I
2 Summar~ 0~ Processor Instructions~ay Alphabe~ical Order 3 Instruction Code~ ClockZ
4 Mnemonic . _.=a~L_~n __ D? D~ D~ D~ D~ D? DL Dn Cycles S ACIAdd immediate to A ~ith ~ carry 1 1 0 0 1 1 1 0 7 7 ADC ~ Add memory to A with carry 1 0 0 0 1 1 1 0 7 8 ADC r Add registsr to A ~ith 9 carry 1 0 0 0 1 S S S 4 ~DD ~ ~dd memory ~o A 1 0 0 0 0 1 0 1 7 11 ADD r ~dd register to ~ 1 0 0 0 0 S S S 4 12 ADI Add immediate t~ A 1 1 0 0 Q 1 1 0 7 13 ANA M And memory ~ith A 1 0 1 0 0 1 1 0 7 14 ANA r And register with A 1 0 1 0 0 S S S 4 ANI And imm~diate with A 1 1 1 0 0 1 1 0 7 16 CAL~ Call unconditional 1 1 0 0 1 1 0 1 17 17 CC Call on carry 1 1 0 1 1 1 0 0 11~17 18 CM Call on minus 1 1 1 1 1 1 0 0 11~17 19 C~A Compliment A O 0 1 0 1 1 1 1 4 CMC Co mpliment carry O 0 1 1 1 1 1 1 4 21 CI~P M Compare memory with A 1 0 1 1 1 1 1 0 7 22 C~P r Compars register with A 1 0 1 1 1 S S 5 4 23 CNC Call on no carry 1 1 0 1 0 1 0 0 11~17 24 CNZ Call on no zero , 1 1 0 0 0 1 0 0 11/17 CP Call on positive 1 1 1 1 0 1 0 0 11/17 26 CPE Call on parity even 1 1 1 0 1 1 0 0 11~17 27 CPI Com pare immediate ~ith A 1 1 1 1 1 1 1 0 2~ CPO Call on parity odd 1 1 1 C 0 1 0 0 11j17 29 CZ Call on zero 1 1 0 0 1 1 0 0 11/17 DAA Decimal adjust A O 0 1 0 0 1 1 1 4 31 DAD B Add ~ ~ C to H ~ L O O O 0 1 0 0 1 10 32 DAD D Add D ~ E to H & L O 0 0 1 1 0 0 1 10 33 DAD H Add H ~ L to H ~ L O 0 1 0 1 0 0 1 10 34 DAD SP Add stack pointer to H ~ L O 0 1 1 1 0 0 1 10 DCR M Decrement memory O 0 1 1 0 1 0 1 10 36 DCR r Decrememt register O O D D D 1 0 1 S
37 DCX B Decrement ~ ~ C O O O 0 1 0 1 1 5 38 DCX D Decrement D ~ E 0 0 0 1 1 0 1 1 5 39 DCX ~ D~crement ~ ~ L O 0 1 0 1 0 1 1 5 DCX SP Decrement stack pointer O 0 1 1 1 0 1 1 5 41 DI Disable interrupt 1 1 1 1 0 0 1 1 4 42 EI Enable interrupts 1 1 1 1 1 0 1 1 4 43 HLT Halt 0 1 1 1 O 1 1 0 7 44 IN Input 1 1 0 1 1 0 1 1 10 INR M Increment memory O 0 1 1 0 1 0 0 10 46 IUR r Increment register O O D D D 1 0 0 5 47 INX ~ Increment B ~ C registers O O O O O 0 1 1 5 48 INX D Increment D ~ E registers O O 0 1 0 0 1 1 5 49 INX H Increment H ~ L registers O 0 1 0 0 0 1 1 5 50 . INX SP Increment stac~ pointer O 0 1 1 0 0 1 1 5 51 JC Ju~lp on carry 1 1 0 1 1 0 1 0 10 52 JM Jump on minus 1 1 1 1 1 0 1 0 10 S3 J~P Jump unconditional 1 1 0 0 0 0 1 1 10 54 J~C ~ump on no carry 1 1 0 1 0 0 1 0 10 JNZ Jump on no zero 1 1 0 0 0 0 1 0 10 56 JP Jump on posi'ive 1 1 1 1 0 0 1 0 10 57 JPE Jump on parity even 1 1 1 0 1 0 1 0 10 sa JPO Jump on parity odd 1 1 1 0 0 0 1 0 10 59 ,~JZ Jump on zero 1 1 Q 0 1 0 1 0 10 LDA Load A direct O 0 1 1 1 0 1 0 13 Instruction Codet _ Clock2 ,~ne~onic Descri~tion D7 D6 35 D~ D3 D2 Dl Dn C~cles . . . ~
1 L3AX B Load A indir~ct O O O 0 1 0 1 0 7 2 LDAX D Load A indirect O O 0 1 1 0 1 0 7 3 LHLD Load ~l ~, L direct O 0 1 0 1 0 1 0 16 4 LXI B Load immediate register Pair B ~ c O O O O O O 0 1 10 6 LXI D Load i~mediate register 7 Pair D ~ E O O O I O O 0 1 10 -8 LXI ~ Load immediate register 9 Pair H ~ L O 0 1 0 0 0 0 1 10 LXI SP Load immediate stack 11 pointer O 0 1 1 0 0 0 1 10 12 MVI ~ ~ove immediate memory O 0 1 1 0 1 1 0 10 13 ~VI r Move immediate register O O D D D 1 1 0 7 14 MOV M,r ~ove register to memory 0 1 1 1 0 S S S 7 UOV r,,~ ~oqe memory to regist~r 0 1 D D D 1 1 0 7 16 MOY r~,r2 Mo-re register to register 0 1 D D D S S S 5 17 NOP No operation O O O O O O O 0 4 18 ORA ~ Or memory ~ith A 1 0 1 1 0 1 1 0 7 19 ORA r Or regis~er ~ith A 1 0 1 1 0 5 S S 4 ORI Or immediate ~ith A 1 1 1 1 0 1 1 0 7 21 OUT Output 1 1 0 1 0 0 1 1 10 22 PCHL H ~ L to ~rogram counter 1 1 1 0 1 0 0 1 5 23 POP B Pop register pair B ~ C
24 off s~ack 1 1 0 0 0 0 0 1 10 POP D Pop register pair D ~ E
26 off stack 1 1 0 1 0 0 0 1 10 27 POP H Pop register pair H ~ L
2~ of f stack 1 1 1 0 0 0 G 1 10 29 POP PS~ Pop A and Plags off stack 1 1 1 1 0 0 0 1 10 PUSH B Push register Pair ~ ~ C
31 on s~ack 1 1 0 0 0 1 0 1 11 32 PUSH D Push register Pair D ~ E
33 on stack 1 1 0 1 0 1 0 1 11 34 PUSH H ~ush register Pair H ~ L
on stack 1 1 1 0 0 1 0 1 11 36 PUSH ~S~ Push A and ~lags on stack 1 1 1 1 0 1 0 1 11 37 RAL Rotate A left through 38 carry O O 0 1 0 1 1 1 4 3g RAR Rotate A right through carry O O 0 1 I 1 1 1 4 41 RC Return on carry 1 1 0 1 1 0 0 0 5/11 42 RET Return 1 1 0 0 1 0 0 1 10 43 RLC Rotate A le~t O O O O 0 1 1 1 4 44 RM Return on minus 1 1 1 1 1 0 0 0 5/1'1 RNC Return on no carry 1 1 0 1 0 0 0 0 5~11 46 R~IZ ~eturn on no zero 1 1 0 0 0 0 0 0 5/11 L~7 RP Return on positi~e 1 1 1 1 0 0 0 0 5/11 48 RPE Return on parity even 1 1 1 0 1 0 O 0 5~11 - 49 RPO Return on parity odd 1 1 1 0 0 0 0 0 5~11 RRC Rotate A right O O O 0 1 1 1 1 4 51 ~ST Restart 1 1 A A A 1 1 1 11 52 RZ Return on zero 1 1 0 0 1 0 0 0 5,~11 53 sas M Subtract memcry frcm A
54 with borrow 1 0 0 1 1 1 1 0 7 SBB r Subtract register from A
56 wi~h borrow 1 0 0 1 1 S S S 4 57 S3I Subtract immediate from A
58 ~ith borrow I 1 0 1 1 1 1 0 7 59 S~ILD Store H ~ L direct O 0 1 0 0 0 1 0 16 SPHL H ~ L to stack pointer 1 1 1 1 1 0 0 1 5 61 STA Store A direct O 0 1 1 0 0 1 0 13 62 ~STAX B Store A indirect O O O O O 0 1 0 ?
63 STAX D store A indirect O O 0 1 0 0 1 0 7 ~ 27 -.

~2~
lnstruction Codel Clock2 Mnemonic Descri~tionD7 D6 Ds_D~ D3 D2 D~ Do Cycles 1 STC set carry O 0 1 1 0 l 1 1 4 2 SUB M Subtract memory from A 1 0 O 1 0 1 1 0 3 SUB r Subtract register from A 1 0 0 1 0 s S s 4 4 S~I Subtract immediate from A 1 1 0 1 0 1 1 0 7 XC~G Exchange D ~ E, H ~ L
6 Registers 1 1 1 0 1 0 1 1 4 7 XRA M Exclusive Or memory 8 with A 1 0 1 0 1 1 1 0 7 9 XRA r Exclusive Or register with A 1 0 1 0 I S S S 4 11 XRI Exclusive Or immediate 12 with A 1 1 1 0 1 1 1 0 7 13 XTHL Exchange top of stack, 14 H ~ L 1 1 1 0 0 0 1 1 18 lDDD or sss-oooB-oo1c-o1oD-o1lE-1ooH-1o1L-11o Memory-111A.
16 2T~o possible cycle times (5/11~ indicate instruction cycles 17 dependent on condition flags.
18 EXAMP~ES
19 Diagnos~ic capability provided by thé method of the 2~ present invention is better illustrated in the Examples set rorth 21 ~elow.
~2 Ex_mple_I
23 Seismic data ~ere obtained ov~r a gas field near 24 Sacramento~ California. These data, i~ CDP-stacked forml are shown in FIG. 12. The ~ield, discovered i~ 1972, consists of a 26 100-foot sand which is almost fully gas-saturated~ The discovery 27 well is located at about SP-86 of FIG 12, ~ith the currently 28 developed portio~ of the field extending from a~out SP-75 to SP-29 115~ Gas occurs at a depth o~ abou~ 700~ feet, ~hich corresponds to a time of about 1.75 seconds on Lhe plot.
31 Common-depth-point gathers from 3 locations, A, ~ and C
32 of FIG. 12, are shown in FIGS. 13~a), 13~b) and 13(c). Both 33 single-fold and 10-fold summed gathers are sho~n for locations A
34 and B, ~hile only the summed gather is snown for location C.
Shot to-group offset for all gathers increases to the left~ as 36 indicated with the minimu~ and maximum trace offset distances 37 annotated~ These distances change on the sum~ed gathers because 38 the summing is done over 4 offsets.

1 Note the strong amplituds increase with increasing 2 offset at locations ~ and 3. The 10-foid summing ob~iously 3 improves signal-to-noise ratios and a~ amplitude increase by a 4 factor of about three is indicated from ~ear to ar offset.
Gathers at locatio~ C, however, show no Lndication of ampli~ude 6 increase ~itn offset, and in fact s~o~ a decrease. ~his possibly 7 indicates 2n absence of gas in the vlcinity of location C. This a possibility is also supported ~y the presence of a gas-~ater 9 contact in a ~ell structurally projected at about SP-1Z0.
Exa~le_II
11 Seismic data ~ere obtained in ~he Fallon Basin of 12 Nevada and are depicted in CDP-gathered format in FIG. 14. A
13 well was drilled at SP-127 in FIG~ seismic amplitude 14 anomaly is indicated at location A at about 1.6 seconds. Upon drilling, the amplitude anomaly ~as found to originate from two ~6 basaltic layers, 100 ~eet and 60 feet in thickness. As its 17 structural position indicates, this well was a stratigraphic test 18 in an undrilled basin~
19 The common~depth-point gath~rs at the ~ell location are shown in FIG. 15. Here, there is a strong indication of 21 reflection amplitude decrease with lncraasing offset. This 22 finding is consistent uith the abse~ce OI gas in the geologic 23 section and the expected Poisson's ratios for sediments and 24 basalt.
~a_Ple_III
26 Seismic data obtained from a4 area in the Sacramento 27 Valley, California~ are depicted i~ FIC. 16. ~ well was drilled 28 at SP 61~ Note the a~plitude anomaly exte~ding from about SP-45 29 to a~out SP-S0 at 1.5 seconds. Ho~ever, the amplitude anomaly was found to originate ~rom a high-YaLocity conglomerate layer.

- 2~ -1 Sho~n in FIGS. 17(a) and 17(b~ are the single-fold 2 common-depth~point gathers at t~o locations: lccation A at the 3 well and location B, about 1~2 mile to the west. The gathers at 4 location A do indeed indicate the absenc2 of sas, i.e.~ no noticeable increase in r fl~ctor am~ ude ~ith offset. HoweverO
6 the gathers at location B do show a sliyht increase in amplitude 7 ~ith offset~ i.e., possi~le gas.
8 Ex_m~le_IV
9 Seismic data were obtalned for a~other area and are depicted in FIG. 18~ The possible gas-related amplitude 71 a~omalies are loca~ed (i) bet~een SP~270 a~d -310 at about 1.3 12 seconds and (ii) between SP-250 and -300 at about 1.0 second.
13 The ten-fold CDP gathers at loca~ions A and s of FIG.
14 18 are sho~n in FIGS~ 19(a) and 19~b), r2specti~ely. Here~ there do ir.de~d appear to be indicatio~s of amplitude increase ~ith 16 offset. In FIG. 19a, the anomaly appears oYer a region where 17 amplitude increases with offse~. In ~IG. 19b7 the anomaly at 1.0 18 seconds is thought to be related to low-velocity shale 19 ExamPle V
Seismic data ~ere obtained for another area and are 21 depicted in FIG. 20. The geologic saction was limestone embedded 22 in shale. The gas-related anomaly is located over the indicated 23 rectangular area of the Figure.
24 Here, note that for this lithology, gas is indicated by decreases in amplitud~ ~ith offset~ as sho~n in FIG. 21 26 representing CD~ gathers at surface loca~ions 102 and 103 of FIG.
27 20, as ~ie~ed respectively from right ~o left in FIG. ?1~
28 The method of the present lnvention as described 29 provides a geophysicist ~ith a strong tool for differentiating gas-filled and non-gas reserYoirs ln a variety of structural 31 combinations~ e . g ., sar.d and limestone embedded in shale.

1 Ho~ever, the in~ention is not limite~ to the above structural 2 combinations alone, but is applicaDle to other anomalous circum-3 stances as kno~n to those skilled in th~ art. It should thus be 4 understood that the invention is not limited to any specific embodiments set forth herein, as variations are readily apFarent, 6 and thus the in~ention is to be giYen the broadest possible 7 interpretation ~ithin the terms of the followir.g claims.

Claims (7)

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

1. A method for identifying gas-bearing strata in the earth associated with high-intensity amplitude events located by common depth point traces, comprising the steps of:
(a) generating seismic data including a record of signals from acoustic discontinuities associated with said strata of interest for a series of points located along a line of survey, by utilizing for each of said points multiple pairs of sources and detectors, each of said points being a centerpoint located horizontally midway between the respective source and detector of each pair, said sources and detectors of each pair being progressively horizontally offset from said centerpoint and thus being at progressively increasing angles with respect to a line between said centerpoint and a common depth point located below said line of survey, said recorded signals being the output of said detectors;
(b) by means of automated processing means, statically and dynamically correcting said recorded signals to form corrected traces whereby each of said corrected traces is associated with a centerpoint horizontally midway between a source-detector pair from which said corrected trace was originally derived;
(c) by means of automated processing means, indexing said corrected traces in two dimensions whereby each of said corrected traces is identified in its relationship to neighbouring traces on the basis of progressive changes in horizontal offset value versus progressive changes in common centerpoint location;
(d) displaying a series of said traces of step (c) on a side-by side basis as a function of progressively changing horizontal offset values, said displayed traces all being associated with at least the same general common group of centerpoints;
(e) identifying progressive change in a high-intensity amplitude event from trace to trace of said displayed traces as a function of progressive changes in horizontal offset value whereby more likely than not said event relates to reflections from acoustic impedances associated with strata containing gaseous hydrocarbons.

2. The method of Claim 1 wherein the gas containing media are gas sands underlying shale, and the progressive change is an increase in amplitude.

3. The method of Claim 1 wherein the gas containing media include limestone underlying shale, and the progressive change is a decrease in amplitude.

4. The method of Claim 1 in which step (c) is further characterized by the substeps of:
(i) selecting a first series of indexed traces within a common offset, common centerpoint window of predetermined dimensions; and (ii) summing said first series of selected traces to form a summed trace.

5. The method of Claim 4 with the additional substeps of:
(iii) increasing the window at least in the common offset dimension to select a second series of traces;
and (iv) summing the second series of selected traces to form a second summed trace.

6. The method of Claim 5 in which step (d) is a side-by-side display of said summed traces as a function of progressively changing composite horizontal offset values whereby progressive changes in said high-intensity amplitude event of step (e) are more easily identifiable.

7. The method of Claim 4 in which the dimension of the window of step (i) is four offset values high by five centerpoint locational points long.