CA1197312A - Seismic exploration using compressional and shear waves simultaneously - Google Patents

Abstract

Title of the Invention SEISMIC EXPLORATION USING COMPRESSIONAL AND SHEAR
WAVES SIMULTANEOUSLY
Abstract of the Disclosure In a method of seismic exploration using swept-frequency signals, compressional and shear waves are emitted simultaneously. Typically the waves are generated by swinging-weight vibrators acting through a single base-plate. If the frequency of the shear vibration is one-half that of the compressional vibration, the downward vertical forces can be phased to minimize the horizontal slippage of the baseplate. The sensitive axis of the geophones is inclined to the vertical for detecting both compressional waves and shear waves. For defined ranges of sweep rate, separate compressional and shear records are obtained by cross-correlating the geophone signal separately against the vertical and horizontal emissions.

Description

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Title of the Invention SEISMIC EXPLORATION USING COMPRESSIONAL AND SHEAR
WAVES SIMULTANEOUSLY
Cross Reference to Related Applications The present application is related to Canadian Patent Application Serial No. 394,255, filed January 15, 1982, and entitled "Seismic Exploration with a Swinging-~eight Vibrator".
~" Technical Field This invention is concerned with seismic prospecting for oil, using compressional and shear waves simultaneously.
Background Art Seismic prospecting by compressional ~P) waves, using the Vibroseis system, has been established for many years.
In this system a vibrator ~us~ally of hydraulic type) applies an oscillating vertical force to a baseplate in contact with the ground, and radiates a signal whose fre-quency changes with time in a prescribed manner. Vertically-sensitive geophones detect the resultant movement of the ground, and cross-correlation of their outputs with the original swept-frequency signal yields a seismic reflection record of the standard form.
In recent years the system has been modi~ied to use shear (S) waves. In this case the vibrator applies an oscillating horizontal force to the baseplate, and the geophones are horizontally sensitive. The wave motion clnployed is horizontally-polarized shear ~SH) with the particle motion horizontally transverse to the line joining vibrator and geophones. SH waves travel with a velocity smaller than that of P waves. The ratio of velocities is expected to have lithological significance, and typically ~7~

varies within the~ range 0.4 to O . 6 . Despite this lower velocity, the SH wavelength is not significantly different from that of the P waves. This is because most of the agencies affecting attenuation in the earth are proportion-ately more severe for shear waves. Thus a seismic recordmight employ a frequency band of 10-100 Hz with P waves, but only 5-50 Hz with SH waves.
A seismic reflector at a depth of 2000 m may be associated with an average P velocity (from the surface) 10 of 4000 m/s. It would therefore appear at a time of 1 s.
The same reflector may be associated with an average SH
velocity of 2000 m/s. It would therefore appear at a time of 2 s. If the seismic section obtained with SH
waves is displayed at half the time scale used for the 15 P section, the reflections look very similar, both in position and apparent frequency content.
To the extent that a reflection can be correlated on the two sections, and positively ascribed to the same reflector, the ratio of reflection times gives the inverse 20 ratio of a~erage velocities. This ratio, being deemed diagnostic of the lithology, is valuable. The ratio of interval velocities, which is even more valuable, is similarly obtained rom the difference in reflection times for the same pair of reflections on P and SH sections.
Much current seismic work, therefore, is done using both P and SH waves. However, since the P-wave method requires vertical vibrators and vertically-sensitive geophones, and the S-wave method requires horizontal vibrators and horizontally-sensitive geophones, the present 30 field practice involves repetition of the recording work for the two methods. The cost is almost twice the cost of traditional work using P waves only.
Further, whereas P-wave vibrators can work along roads, current Sl~-wave vibrators cannot. They require to dig into 35 the surface, anA the damage they do to agricultural and other land must usually be repaired~
SUMMARY OF THE INVENTION
Accordingly, the present invention in one aspect seeks to provide simultalleous recording of P and SH waves.

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3~2 ~urther the invention seeks to generate both P and Sf~
waves with an inexpensive vibrator, and to provide a mode of operation which allows the simultaneous use of a plurality of such vibrators.
It is a still further aspect to provide a shear-wave vib-rator which does not require digging into the surface, and which can be used on roads.
Another aspect is to provide geophones which can be used for the simultaneous reception of P and SH waves.
The invention in one aspect pertains to an improved method in the technique of seismic exploration using swept-frequency signals from a wave source and emitting a compressional wave signal and horizontally-polarized shear wave signal simultaneous-ly, in which the swept-frequency compressional wave signal and the swept-frequency shear wav~ signal occupy different frequency bands and are emitted so that the time between the emission of any compressional wave signal frequency and the emission of the same sheax wave signal frequency exceeds the greatest reflection time of interest.
The invention also comprehends a vibrator providing simultaneous emission of compressional and polarized shear waves comprising in combination a pair of contra-rotating eccentric weights phased to generate a sinusoidal vertical force, a pair of contra-rotating eccentric weights phased to generate a sin-usoidal horizontal forcel baseplate means to couple both forces to the earth, and means to drive the second pair of weights at one-half of the frequency of the first pair of weights.
The invention also comprehends a geophone for use in a new method of seismic exploration involving the simultaneous emission from a wave source of compressional waves and horizon~
tally-polarized shear waves in different frequency bands, comprising a single sensitive element whose axis of sensitivity is inclined to the ver-tical in a vertical plane subs-tantially perpendicular to the line joining the wave source and the geophone.
More particularly, the invention as disclosed provides Eor the simultaneous use of two vibrators, one configured to ~-~
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generate vertical P waves and one ~o generate horizontal SH
waves. The vibrators may be swinying-weight vibra-tors acting through a single base plate. At any given time, the vibratofs operate at different frequency. A preferred arrangement is for the SH vibrator to operate at half the frequency of the P
vibrator. In this case, and if both vibrators drive the same baseplate, the maximum horizontal forces are generated when the downward vertical forces are at maximum. The tendency for horizontal slip of the vibra-tor is thereby minimized. The 10 generated signals are received by geophones in which the sensitive element is inclined to the vertical so that the geophone detects both P and SH waves.
In the preferred form of the invention two swinging-weight vibrators are assembled for generatin~ both vertical 15 P waves and transverse SH shear waves through a single base plate. One swinging-weight vibrator comprises a pair of fly-wheels or rotors mounted for rotation in opposite directions on the common base plate. Weights are disposed eccentrically on the wheels and arranged to arrive at the 20 highest and lowest points in phase, simultaneously. The opposing direction of rotation cancels the horizontal com-ponent of ~forces which would otherwise be transmitted to the base plate by the first set of fly-wheels or rotors of the vertical vibrator The other vibrator comprises a pair of rotors or fly-wheels mounted for rotation in opposite directions with eccentric wei~hts disposed on the wheels and arranged to be .../4 . i ~

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in phase for arrival at the left-most and then right-most extremities in phase, simultaneously. The opposite direc-tion of rotation of the fly-wheels of the second vibrator cancels the vertical component of the forces which otherwise would be transmitted to the base plate thereby providing transverse or horizontal shear wave excitation of the ground. Further, according to the preferred form the fly wheels or rotors of the horizontal or shear wave vibrator are double the diameter of the vertical vibrator fly 10 wheels so that they rotate at half the frequency. As a result, the generated shear waves have ~ypically half the ~requency of the longitudinal vertical waves. The phaseS
of the two vibrators are acljusted so that the maximum downward force of the vertical vibrator occurs simultaneously 15 with ~e lef t mos~ and right most forces of the norizontal vibra~or. Thi~ phase reiationship of the two vibrators min;~izeS the tendency for the vibrator to slip horizontally and minlmizes the need for hold down weights.
View~d as a method the invention contemplates conduct-ing seismic exploration using swept frequency waves by generating compressional waves using a vertical vibrating means, generating shear waves of substantially half the frequency of the compressional waves using horizontal vibrating means, and coupling or transmitting said com-25 pressional waves simultaneously into the earth. Themethod further contemplates relating the phase of the com-pressional waves and shear waves so that the maximum left and right forces o~ the horizontally polarized shear waves coincide with the maximum -~
30 downward foxces of the compressional waves. According tofurth~r refinements of the method, the frequency of the compressional waves and shear waves respectively are swept or varied and the emission of a shear wave at the same particular fre~uency as a previously emitted compres-35 sional wave is delayed for a period of time to exceed the 3~

greatest reflection time of interest thereby avoidingspurious correlations. The frequency of sweep is carried out so that the time for emissions to pass through one octave exceeds the greatest reflection time of interest.
The compressional ~ave and shear wave reflections are received simultaneously for correlation.
Brief Description of the Drawin~s The invention will now be described by reference to the drawings, in which:
Figure 1 represents a vertical swinging-weight vibrator of the prior art;
Figure 2 represents a combined vertical and horizontal vibrator;
Figure 3 shows the time variation of the vertical and 15 horizontal forces, and of the resultant vector;
Figure 4 shows a geophone configured to receive P and SH waves simultaneously; and Figure 5 illustrates a typical frequency-time relation-Ship for the P and SH emissions.
20 Best Mode for Carrying Out the Invention Although featlres of this invention are applicable to several types of vibrator, the following descriptive mat-erial concentrates on swinging-weight vibrators.
Canadian Patent Applica~ion Serial No. 394,255, 25 concerned with P-wave generation, describes a development of the well-known swinging-weight vibrator shown diagram-matically in Figure 1.
According to this development, the vibrator generates downsweeps, ~sing the enerqy stored in a flywheel. Two 30 similar units may be coupled in such a way that one unit is being driven up to speed while the other is emitting its downsweep. Control of eccentricity is provided to eliminate vibration from the unit being accelerated, and to permit greater output at selected frc~uellcies. Alter-35 natively, the same ob~ects may be achicvcd by incorporatingtwo vibrators in each unit, and by controlling the relative phase between them in the manner of U.S. Patent No. 4,234, 053 to Erich. A plurality of vibrating units may be used, I

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in such a way that the times at which they radiate the same frequency are separated by at least the maximum reflection time of interest. Correlation is performed against a master sweep reconstituted from phase codes S obtained from the vihrator, and the amplitude of the rnaster sweep is adjusted as a function of frequency to provide any desired signal spectrum or signal-to-noise - sPectrum. A measure of automatic compensation is provided for resonance effects in the vibrator-ground coupling.
In Figure 1 two rotors suggested diagrammatically by wheels 1 rotate in opposite directions, for example, by peripheral gearing, on a common baseplate 2. Weights 3 are disposed on the wheels, as shown, to be at ~heir lowest and highest points simultaneously. This arrange-15 ment cancels the horizontal component of the forcestransmitted to the baseplate, and provides a vertical P
excitation of the ground. Phase codes to represent the position of the weights are derived from a transducer ~not shown) on the shaft of wheel 1. Isolated hold-down weigh~
20 may be provided at 4.
Figure 2 shows, also in diagrammatic form, the develop-ment of the present invention. Wheels 1, together with their weights 3, mounting and drive, are counterparts of Figure 1. Wheels 5 together represent a second vibrator 25 on the same baseplate. They may be driven through the wheels 1 by the same circumferential gearing, and rotate in the directions shown. The wheels 5 are typically double the diameter of wheels 1, so that they rotate at half the frequency~ The weights 6, in contradistinction 30 to weights 3, are disposed to be at their rightward points simultaneously and hence their leftward points also. They may be larger than the weights 3.
From this it is apparent that the wheels 1 constitute a vertical vibrator and the wheels S constitute a horizontal 35 vibrator, of typically half the frequency, acting on the same baseplate.
Figure 3 illustrates the vertical and horizontal com-ponents of the Force applied to the earth, as a function of the position o the rotating wheels. A sinusoidal vertical force of frequency f is applied by the small wheels. A
sinusoidal horizontal force, in thi~ case of frequency f /2, is applied by -the large wheels, with phase such that both maximum forces, left and right, are developed at the time when the small wheels are generating maximum downward forceO This phase is preferred since it minimizes the tendency for the vibrator to slip horizontally. For a given minimum coefficient of friction, the weights 6, 10 and their eccentricity, can be selected so that the max imum horizontal force does not cause the baseplate to slip when simultaneously transmitting the maximum downward force. It is this feature which allows the vibrator to be used without damage to the surface on which it operates.
15 This feature also minimizes the need for hold-down weight~
It is even possible for the vibrator to leave the ground during the time of maximum upward force, as suggested by the dashed line 8, without significant loss of the hori-zonta] output. The bottom line of Figure 3 shows the 20 direction of the resultant forces on the earth, as a function of time,by means of the vector arrows 9.
The geophones ~sed with this system must be capable of detectin~ both P and SH waves. One possibility is to use conventional vertically-sensitive geophones for the P waves, 25 and separate horizontally-sensitive geophones for the SH
waves. ~'he outputs of the two sets of geophones may be electrically combined, so that a single electrical signal is derived from each geophone station.
Alternatively, one geophone may be used to detect both 30 P and SH. Such a geophone is illustrated in Figure 4. The sens.itive element 10 is inclined (-typica]ly at 45~) to the case 11 and the spike 12.
Any geophone used for SH waves must be aligned properly.
Accordingly the top of the geophone bears a suitable mark 35 13 showing both the direction of the line betweer~ source and geophone, and t:he direction to the reference side of that line.
Although the sensitive unit 10 is basically conventional, spring modifications are desirable to minimize the adverse 73~

effects (particularly the distortion) introduced hy the tilt.
The basic use of the described ~eophones and vibrators is as follows. The geophones are disposed in arrays along one or more lines, as in standard practice. The vibrator unit occupies successive vibrator positions, at each of which a succession of recordings is made. The motor accelerates the vibrator to a first desired speed, and the vibrator is then allowed to run down to a second desired speed. In so doing, it generates P and SH waves, with the P waves 10 typically having double the frequency of the SH waves.
Figure 5 illustrates the rundown for the case where the first and second speeds represent a frequency range of 80-10 ~z for P waves and 40-5 Hx for SH waves, and where the rundown takes 30 seconds. The P-wave frequency as a 15 function of time is shown at 14, and the SH-wave frequency at 15, both solid lines.
As described in moxe detail in the aforesaid companion application, a swept-frequency quasi-sinusoid may be recon-stituted to represent the transmi-tted signal. In the present 20 case, one such signal represents the transmitted P waves and another represents the transmitted SH wave. The recelved signals from the geophones are then correlated against each of these transmitted signals, to obtain two separate records, one P and one SH, each in its own frequency band. There-25 after the separate records may be subjected to various enhance-ment processes, as is normal in the art.
The maximum time shift used in the correlation represents the maximum reflection time of interest. For the case where this is 6 seconds, dashed lines 16 and 17 represent the 30 frequency-time relations for the 6-second reflection. Fxom this the following facts are evident:
a. There is no risk of a spurious correlation between P and SH waves, provided that the ti~e taken for the vibrator unit to pass through any one octave is safely greater than 35 the maxlmum reflection time of interest.
b. The same condition ensures that there i5 no spurious correlation within the reflection time of interest, for either P or SH separately, caused by the generation of 73~

second or higher-order harmonics in the vibrator-qround coupling.
c. There is a spurious correlation between the second harmonic of SH and the fundamental of P at the same reflection time. Since the swinging-weight vibrator unit as described generates very little second-harmonic distortion in the SH output, this spuxious correlation is small. It is further reduced hy the large absorption suffered by SH
waves at the higher frequellcies. Toyether, these facts 10 offset the smaller geometrical spreading suffered by the SH refleotio~
Figure 5, therefore, repr~esents a practical scheme where only one vibrator i5 in use at any time. This is likely to be the situation whenever the target is shallow, 15 particularly in coal-mining and civil-engineering applica-tions. For deeper targets it is desirable to use several vibrators simultaneously, and the companion application explains how this may be done, In the present context, multiple vibrators are permissible provided that the time 20 between sweep-starts o~ any two vibrators safe~y exceeds the sum of the time to sweep through any one octave plus the maximum reflection tim~ of interest. For the illustra-tion of Figure 5, this means that the sweeps generated by several vibrators must be separated by at least 16s.
25 This is entirely reasonable whQn flywheels have sufficient energy to provide long sweeps.
Several other modifications of technique described in the companion application may also be used with the pre-sent invention, subject to conditions associated with sweep 30 rate and maximum reflection time. Yet other modifications will be obvious to those skilled in the art, and these are encompassed within the scope of the inver-tion. Specifi-cally, these include the technique where an individual emission from one vibrator is restricted to less than one 3S octave, so that: each vibrator is optimized for a narrow frequency ranye, and where different narrow frequency ranges may be emitted sequentially or simultaneously.
Finally, it should be stressed that there are many advantages to t:he simultaneous recordinq of P and SH waves.

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One, mentioned earlier, is the cost. A second, realized partially when the two types of waves are generated at the same source and received by the same geophone, is the improved ease o correlation between P and SH reflections.
In prior practice, this correlation was often difficult.
The problem is much eased by the certain knowledge that the reflection path is identical for the two waves. Thus studies of relative attenuation become more meaningful, phase changes associated with minor differences of field 10 layout are eliminated, and the irksome pxoblem of the near surface sorrections is simplified by the certainty of identical coupling and sur~ace consistency.

Claims (18)

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

1. In the technique of seismic exploration using swept-frequency signals from a wave source and emitting a compressional wave signal and horizontally-polarized shear wave signal simultaneously, the improved method in which the swept-frequency compressional wave signal and the swept-frequency shear wave signal occupy different frequency bands and are emitted so that the time between the emission of any compressional wave signal frequency and the emission of the same shear wave signal frequency exceeds the greatest reflection time of interest.

2. The method of Claim 1 in which the start and end frequencies of the shear wave signal are related to the start and end frequencies of the compressional wave signal by a factor between 0.4 and 0.6.

3. The method of Claim 2, in which the factor is 0.5.

4. The method of Claim 1, in which the said compressional and shear wave signals are emitted respectively from vertical and horizontal vibrators coupled to a single baseplate.

5. The method of Claim 4, in which the frequency of the horizontal vibrator is maintained at one-half that of the vertical vibrator.

6. The method of Claim 5, in which the relative phase of the vertical and horizontal vibrators is arranged so that the maxima of the horizontal forces occur at the time of the maxima of the downward vertical forces.

7. The method of Claim 4, in which the said vertical and horizontal vibrators are swinging-weight vibrators.

8. The method of Claim 1, further utilizing geophones whose sensitive axis is inclined to the vertical in a vertical plane substantially perpendicular to the line joining the wave source and the geophone.

9. A method of seismic exploration using swept-frequency waves, comprising:
a. generating compressional waves at the output of vertical vibrator means;
b. generating shear waves having half the frequency of the compressional waves, at the output of horizontal vibrator means;
c. relating the phase of the horizontal and vertical vibrator means so that each maximum of horizontal force imparted by the horizontal vibrator means coincides with a maximum of downward vertical force imparted by the vertical vibrator means;
d. recording output signals corresponding to the compressional and shear waves at the outputs of said vertical and horizontal vibrator means;
e. receiving the reflections of said compressional and shear waves and generating corresponding reflection signals;
f. cross-correlating the received reflection signals separately against the said output signals from the vertical and horizontal vibrator means;
g. thus obtaining separate reflection signals for compressional waves and for shear waves; and h. recording the results.
10. A method of seismic exploration using swept-frequency waves, comprising:
a. generating compressional waves using vertical vibrator means;
b. generating shear waves having substantially half the frequency of the compressional waves using horizontal vibrator means;
c. correlating the phase relationship of the compression-al waves and shear waves so that the maximum left and right forces of the shear waves coincide with the maximum downward forces of the compressional waves;

Claim 10 - cont'd ...
d. emitting said so generated compressional waves and shear waves simultaneously into the earth;
e. simultaneously receiving reflections of said compressional waves and shear waves;
f. varying or sweeping the frequencies of said respective compressional waves and shear waves; and g. delaying the time between the emission of a particular compressional wave frequency and the emission of a shear wave at the same frequency so that said delay time exceeds the greatest reflection time interval of interest between the emission of any particular wave and the receiving of its reflection.

11. The method of Claim 10 comprising the step of delaying the time for sweeping the frequency through one octave to exceed the greatest reflection time interval of interest between the emission of any particular wave and the receiving of its reflection.

12. A vibrator providing simultaneous emission of compressional and polarized shear waves comprising in combination:
a. a pair of contra-rotating eccentric weights phased to generate a sinusoidal vertical force;
b. a pair of contra-rotating eccentric weights phased to generate a sinusoidal horizontal force;
c. baseplate means to couple both forces to the earth;
and d. means to drive the second pair of weights at about one-half of the frequency of the first pair of weights.

13. The vibrator of Claim 12, in which the weights are driven in a relationship such that the peaks of horizontal force occur substantially at the time of the peak downward vertical force.

14. A vibrating unit for providing simultaneous emission of compressional and polarized shear waves comprising:
a. a base plate for coupling vibrations to the earth;
b. a first pair of fly-wheels or rotors mounted on said base plate for rotation in opposite directions, said first pair of fly-wheels having first weights disposed eccentrically on said fly-wheels, said first pair of fly-wheels arranged relative to each other for arrival of said weights in phase at the highest and lowest points simultaneously for generating maximum oscillating forces in the vertical directions, the opposite direction of rotation of said fly wheels cancelling the horizontal component of forces thereby providing a vertical vibrator;
c. a second pair of fly-wheels mounted for rotation in opposite directions on said base plate, said second pair of fly-wheels having second weights disposed eccentrically thereon, said second pair of fly-wheels arranged relative to each other for arrival of the second weights in phase at the left-most and right-most point simultaneously thereby generating maximum forces left and right, said opposite direction of rotation of said second pair of fly-wheels cancelling the vertical component of forces thereby providing a horizontal vibrator;
d. said second pair of fly-wheels constructed and arranged for rotating at substantially half the frequency of the first pair of fly-wheels; and e. said first and second pairs of fly-wheels arranged for rotation relative to each other so that the maximum down-ward forces of the vertical vibrator first pair of fly-wheels coincides with the maximum left and right forces of the horizontal vibrator second pair of fly-wheels thereby minimizing the tendency of the base plate and vibrator unit to slip horizontally.

15. The vibrating unit of Claim 14 wherein said second pair of fly-wheels are substantially double the diameter of the first pair of fly-wheels so that they rotate at sub-stantially half the frequency of the first pair of fly-wheels.

16. A vibrator providing simultaneous emission of compressional and horizontally polarized shear waves, comprising in combination:
a. a first pair of contra-rotating eccentric weights phased to generate a sinusoidal vertical force;
b. a second pair of contra-rotating eccentric weights phased to generate a sinusoidal horizontal force;
c. baseplate means to couple both the vertical and horizontal forces to the earth;
d. gear means operatively coupled to drive the second pair of weights at one-half the frequency of the first pair of weights; and e. phasing means operatively coupled between the first and second pairs of weights such that the maxima of horizontal force occur substantially at the time of the maximum downward vertical force.

17. A geophone for use in a new method of seismic exploration involving the simultaneous emission from a wave source of compressional waves and horizontally-polarized shear waves in different frequency bands, comprising a single sensitive element whose axis of sensitivity is inclined to the vertical in a vertical plane substantially perpendicular to the line joining the wave source and the geophone.

18. The geophone of Claim 17, in which the sensitive axis is inclined to the vertical at substantially 45°.