CA1125905A - Shear wave method of seismic prospecting - Google Patents

Abstract

ABSTRACT
The baseplate of a shear wave seismic vibrator is vibrated to generate a shear wave of predetermined waveform. The resulting seismic waves are recorded. The pressure waves also generated by the shear wave vibrator appear on the record as noise. The baseplate is then vibrated to generate a shear wave of predetermined waveform which is substantially the same as but substantially opposed in phase to the previous shear wave generated. The resulting seismic waves are then detected and subtracted from the previous record to reduce noise caused by pressure waves simulta-neously generated by the shear wave vibrator.

Description

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1 AN IMPROVED SHEAR ~AVE METHOD OF SEISMIC PROSP~CTING
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2 BACXGROUND OF THE INV~TION

3 1. Field of the lnvention

4 This invention relates generally to seismic prospecting, and in particulax, it relates to an improved method for induci~g shear waves in the earth for seismic prospecting.
7 2. Description of the Prior Art 8 A physical disturbance at the surface of the earth generat~s two 9 general types of waves that propagate through the earth, the pressure wave, and the shear wave. Both types of waves have been used in seismic 11 prospecting. The motion of the earth particles in a pressure wave is 12 along the direction of propagation of the wave. In a shear wave, the 13 motion of the earth particles is normal to the direction of propagation of 14 the wave. If the shear wave particle motion is oriented nor~al to the plane of incidence, it is called a horizontal shear (SH) wave. The plane 16 of incidence is defined as the vertical plane passing through the point of 17 origin of the physical disturbance generating the wave, and the point of 18 detection at the surface of the earth. If the shear wave particle motion 19 is oriented within the plane of incidence, it is called a vertical shear lSV) wave.
21 The advantages of using shear waves for seismic prospecting are 22 well known in the art. At a given frequencyj shear waves travel through 23 the earth at approximately one-half the speed of pressure waves, so that 24 the shear waves have wave lengths about one-half that of the pressure waves having the same frequency, resulting in better resolution in seismic 26 profiles. Furthermore, pressure waves are partially converted into SV
27 waves and vice versa upon striking an interface in the earth. What appears28 on a pressure wave seismogram as pressure waves may actually be SV waves 29 that have been converted into pressure waves upon reflection. SH waves, however, are not converted into a different wave type upon reflection from 31 horizontal interfaces. Thus, horizontal shear wave seismograms may be 32 simpler to interpret. Since SV waves may be converted into pressure waves 33 and vice versa upon reflection by an interface, SV wave seismograms are 34 not as simple to interpret as SH wave seismograms. Where additional seisrnic data are desirable, however, SV wave seismograms may be useful.

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1 In traveling through the earth, shear waves are attenuated ~ore than pressure 2 waves. Hence, it is important to reduce the noise component in shear wave 3 seismograms.
4 Reflections of shear waves are detected by detectors at the ground surface. Such detectors are oriented so that the axes of maximum 6 sensitivity of the detectors are aligned within the plane of incidence to 7 detect SV waves, and normal to the plane of incidence to detect SH waves.
8 Thus oriented, these detectors are able to detect reflections of SH w3ves 9 or SV waves from interfaces and discontinuitie~ in the earth for the purpose of seismic prospecting. Trpically, to generate a SH wave seismo-11 gram, such detectors are placed along a straight line that passes through 12 the shear wave source, and that is perpendicular to the d~rection of 13 vibratory motion of the source. To generate a SV wave seismogram, the 14 detectors are placed typically along the line of vibratory motion of the shear wave source.
16 While most detectors used in seismic surveys have axes of maximum17 sensitivity, they are also sensitive, although to a lesser extent, to 18 vibrations not along axes of maximum sensitivity. Most known sources of 19 shear waves also produce pressure waves in the process of producing shear waves. The reflections of such pressure waves are detected by shear wave 21 detectors as noise. Since shear waves are attenuated more than pressure 22 waves when traveling through the earth, noise caused by reflections of 23 pressure waves can be significant. To obtain a relatively uncontaminated 24 shear wave seismogram, it is desirable to reduce such noises.
The problem of pressure wave contamination of SH wave seismograms 26 has been recognized in the prior art and solutions have been proposed.
27 Lash, in U.S. Patent No, 3,208,546 (1965) disclosed a method wherein two 28 substantially equal charges are sequentially detonated in two holes that 29 are in close proximity, one charge in each hole. Each hole enters the ground vertically, but slants in opposite directions at the same inclination 31 to the vertical so that the two holes lie in two parallel vertical planes.
32 The horizontal directions of deviation of the holes are therefore opposite 33 to cach other. The two charges are covered to a shallow depth with a 34 tamping fluid, which transmits the explosive force of the charges horizon-tally as well as vertlcally wlthin the holes, thereby generating shear 36 waves.

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1 Since the two charges are of spproximately equal weight, they 2 produce similar pressure waves. The SH waves produced are also similar 3 for t~e same reasou, but are 180 out of phase with each other, because 4 the tamping fluids in ~he ~wo holes that generate the SH waves are caused to move in opposite horizontal directions by the detonations. The reflec-6 tions of waves generated by each detonation are each separately recorded.
7 The two records are then subtracted from each other. Since the two detona-8 tions generate similar pressure waves from ~ocations in close proximity, 9 the reflections of thes~ waves from underground interfaces are also substan-tially similar, and they cancel out in the subtraction. The SH waves 11 generated by the two detonations, while also similar, are opposed in 12 phase. Hence, they add when the two records are subtracted.
13 Another solution to the problem of presure wave noise in SH wave 14 seismograms is the SYSLAP method disclosed by Michon in Shear Waves and Pressure Waves, An Example of Field Results ~1977). Three ~ertical 16 holes are drilled in the earth on a line on the earth surface perpendicula 17 to the line of spread of detec~ors. A shot in the center hole drilled on 18 the line of spread creates a cave, which causes a dissym~etry when equal 19 shots are fired in the two lateral holes. The dissymmetry causes the SH
wave generated by the two later equal shots in the lateral holes to have 21 mirror image polarities. Subtraction of the two records of the waves 22 produced by the two shots eliminates the pressure wave reflections while 23 adding the SH waves in a similar manner as that disclosed by the Lash 24 patent.
Cherry et al, in V.S. Patent No. 3,286,783 (1966) proposed a 26 method to lessen shear wave seismogram noise where a shear wave vibrator 27 is used. The type of noise that the method was proposed to eliminate, 28 however, is that caused by near surface shear waves, and not pressure 29 waves generated by the shear wave vibrator.

SUMMARY 0~ THE INVENTION
31 Use of explosives may not be feasible in certain areas, such as 32 near roads or populated areas. Accordingly, this invention provides a 33 novel method for shear wave seismic prospecting at controlled frequen~ies 34 using vibrators wherein the effects of pressure waves simultaneously generated are reduced so that the signal to noise ratio of the shear wave 36 seismogram is improved.

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1 The baseplate of 8 ~hear wsve vibrator is first vibrated along a 2 substantially horizontal line in response to a sweep signal having a predetermined 3 waveform. The resulting seismic waves are detected by one or more dete~tors4 and the detected waves are recorded as a first record.
The baseplate is then activated to vibrate at substantially the 6 same frequency as the previous vibrational sweep, but substantially in 7 opposite phase to the earlier sweep. The resulting seismic waves are 8 detected and recorded as before as a second record.
9 Since the change in phase of the horizontal vibratory motion of the baseplate does not substantially affect the phase of the pressure waves 11 produced in the two vibrational sweeps, the pressure waves produced are 12 substantially the same in the two vibrations. The reflections from earth 13 interfaces and surface waves caused by the pressure waves are also substan-14 tially similar. If the two records are subtracted one from the other without relative record-time delay, then the noise caused by pressure waves 16 is substantially cancelled. The shear waves generated in the two sweeps 17 are, however, substantially opposite in phase. Thus the shear wave reflec-18 tions will add when the two records are subtracted. Random noise, on the 19 other hand, is not similarly reinforced in the subtraction. Therefore, noise in the shear wave seismogram is reduced and the signal to noise ratio 21 is improved.

23 FIG. 1 is a schematic, simplified view of a shear wave vibrator 24 that may be used in this invention.
FIG. 2 is a schematic drawing illustrating the method of the 26 invention utilizing SH waves.
27 FIG. 3 is a schematic drawing illustrating the method of the 28 invention utilizing SV waves.
29 FIG. 4 is a schematic, simplified view of an alternative con-figuration of a shear wave vibrator that may be used in this invention.

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~L~259Q5 1 DESCRIPTION OF THE PREFERRED EnBODIMENTS
2 FIG. 1 is a simplified schematic view of a shear wave vibrator 10 3 that may be used in this invention. Shear wave vibrators are known in the 4 art; the details of such a vibra~or may be found, for example, in U.S.
Patent No. 3,286,783 (1966) to Cberry et al.
6 For eaæe of operation, vibrator 10 is integrated with a truck.
7 When the proper location for seismic surveying is reached, vibrator 10 may 8 be lowered to come into contact with the ground surface 70. Typically 9 vibrator 10 includes a reaction mass 12, piston 14 together with piston rod 16, and a baseplate 18. The reaction mass has a cylindrical bore 22 11 wherein piston 14 may reciprocate sealingly. A pair of conduits 24, 26 12 extends through reaction mass 12 and permits fluid communication between 13 servovalve 28 and cylinder bore 22 on opposite sides of piston 14. Servo-14 valve 28 is operated electromagnetically to control fluid passage to cylinder bore 22 to reciprocate piston 14 relative to reaction mass 12.
16 Hydraulic fluid is fed from a suitsble power source (not shown) to servo-17 valve 28 through conduits 30.
18 Typically piston 14 is attached to two vertical plates 42, 44 by 19 means of piston rod 16, which passes through and maintains sealing contact with reaction mass 12. Plates 42, 44 are secured onto baseplate 18. As 21 piston 14 reciprocates relative to reaction mass 12, it pushes and pulls 22 plates 42, 44 horizontally to cause baseplate 18 to reciprocate relative to23 reaction mass 12. A plurality of projections 46 are fixed onto the bottom 24 face of baseplate 18 in contact with the earth surface 70 to penetrate the earth. As baseplate 18 reciprocates, a particle motion is imparted to 26 surface 70 of the earth, which will propagate downwards as a shear wave.
27 An electrical signal of the desired waveform may be generated by 28 signal generator 32. When the electrical signal is applied to servovalve 28 29 through cable 34, hydraulic power fluid will be introduced into cylinder bore 22 through one of the two conduits 24, 26. For purpose of illustra-31 tion, asæume that hydraulic fluid is introduced into cylinder bore 22 32 through conduit 24. The high pressure fluid will then drive piston 14 to 33 the right relative to the reaction mass 12, and drive reaction mass 12 by 34 an equal and opposite force to the left. The result is that vibrator 10 exerts a horizontal force on the earth surface 70 through projections 46 36 that maintain the coupling between earth surface 70 and vibrator 10.

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1 As the electrical signal from signal generator 32 changes polar-2 ity, the servovalve 28 responds in such a manner that hydraulic power 3 fluid is introduced into cylinder bore 22 through conduit 26 instead of 4 conduit 24, and withdrawn therefrom simultaneously through conduit 24.
Piston 14 is driven to the left and a horizontal force is exerted on earth 6 surface 70 in a direction opposite to the one exerted befors the signal 7 changed polarity. Therefore as ~he electrical signal periodically switches 8 servovalve 28, baseplate 18 reciprocates, and a particle motion will be 9 imparted to surface 70 of the earth in response to the electrlcal signal.
When the electrical signal from signal generator 32 has a pre-11 determined waveform, surface 70 will be urged to move in response thereto, 12 causing a shear wave of substantially similar waveform to be generated.
13 The foregoing description of the structure of a shear wave 14 vibrator is for the purpose of illustrating the method of the invention.
It will be appreciated that various changes in the size, shape, as well as 16 details of the illustrated construction may be made within the scope of 17 the appended claims without departing from the spirit of the invention.
18 FIG. 2 is a schematic drawing illustrating the method of the 19 invention utilizing SH waves that are generated by vibrator lO shown in FIG. 1. Usually several detectors are laid out in a pattern with their 21 outputs connected at each detector station. It is understood that the 22 term "detector" as used herein may also refer to a group of detectors 23 having commonly connected outputs. For ease of illustration, only one 24 detector is shown in FIG. 2 at each detector station. When a SH wave seismic profile in a vertical plane is desired, one or more detectors lOO
26 are placed in that plane on the ground surface. Vibrator lO is also 27 placed at a point on the ground surface in that vertical plane at a distance 28 from the detectors. The vertical profile plane of interest is thus also 29 the plane of incidence. Vibrator 10 is so oriented that the baseplate vibrates along a line 52 substantially normal to the plane of incidence.
31 Each detector is aligned so that its axis 56 of maximum sensitivity is 32 substantially parallel to the line of vibration of the baseplate. Since 33 the reflected vibrations detected will be substantially normal to the 34 plane of incidence, recording of such vibrations will produce a SH wave seismogram.

)5 1 Signal generator 32 is used to generate a first electrical 2 signal with a predetermined waveform. When the first signal is applied to 3 vibrator 10, a SH wave of a substantially similar waveform is generated as 4 described. It propagates downwardly along ray paths~ such as 62a and 62b, is reflected by interfaces in the earth, such as interface 60, along ray 6 paths such as 64a and 64b, and detected by detectors 100. The detected 7 signal is then recorded in a tonventional manner as a first record using a 8 recording system 58 connected to detectors 100 through cables 102. A
9 suitable recording system that may be used for this purpose is Model DGS-IV
produced by Texas Instruments Inc. of Dallas, Texas. The recording of the 11 detected signal starts as soon as the electrical signal is applied to 12 vibrator 10.
13 While the horizontal vibration of the baseplate in vibrator 10 14 generates shear waves, it usually also generates pressure waves. Such pressure waves are also reflected by interfaces and reach detectors 100.
16 While the common axis of maximum sensitivity of detectors 100 has been 17 oriented to detect SH waves, i.e., vibrations normal to plane of incidence,18 most detectors commonly used will detect to a lesser extent vibrations 19 within the plane of incidence s~ch as pressure waves and SV waves. Reflec-tions of pressure waves generated will therefore be detected and appear on 21 the record as noise. Since shear waves are attenuated more than pressure 22 waves as they propagate through the earth, such noise may be significant.
23 Without relocating vibrator 10 or detectors 100, a second record 24 is made in a manner described as follows. Signal generator 32 is used to generate a second electrical signal substantially the same as the first 26 signal used in the making of the first record except that it is substan-27 tially opposed in phase to the first signal. The baseplate in vibrator 10 28 is then urged to vibrate in response to the second elec~rical signal.
29 HencP baseplate 18 is urged to vibrate in substantially the same manner as in the vibration giViDg rise to the first record except that the two 31 vibrations are substantially opposite in phase.

~ : , , ~1259~)5 1 T~is change in phase of the baseplate motion will csuse the 2 shear waves generated ~o change to a substantially opposite phase. Thus 3 the SH wave reflections recorded in the second record are substantially 4 opposite in phase to that in the first record, but the two are otherwise substantially the same. Subtraction of the two records will reinforce the 6 SH wave reflections. The change in phase of the baseplate motlon does 7 not, however, substantially change the pressure waves genera~ed. Noise 8 caused by pressure wave reflections is then substantially the same in the 9 two records. Sub~raction of the two records will cause such noise to be reduced.
11 In both records, the recording started at the time the electrical12 signal is applied to vibrator l~; there is no relative record-time delay 13 between the two records. The two records may be electrically subtracted 14 in a conventional manner. The resulting trace may be recorded as a SH
wave seismogram wherein noise caused by pressure waves is reduced, and the 16 signal to noise ratio improved.
17 Normally more than one detector is used, and the two records 18 will contain multiple traces, with outputs of each detector contributing 19 to two corresponding traces on the two records. In the subtraction step, each of the two corresponding traces from the same detector are subtracted, 21 and the resultant trace is recorded. Recording of all resultant traces 22 produces a SH wave seismogram with improved signal to noise ra~io, with 23 multiple traces, each corresponding to a detector.
24 Where 5V wave seismograms in the plane of incidence are desired, vibrator lO and detectors lO0 in FIG. 2 are rotated so that the line of 26 baseplate vibration 52 and common axis of maximum sensitivity 56 are 27 substantially within the plane of incidence. The resulting orientation 28 and configuration are shown in FIG. 3. Since the reflected vibrations 29 detected will be substantially within the plane of incidence, recording of such vibrations will produce a SV wave seismogram.
31 A first and a second SV wave record are generated in the same 32 manner as the first and the second SH wave records described above. For 33 the same reason as explained in the case of SH wave seismograms, the 34 change in phase of the baseplate motion will cause the SV waves generàted to change to a substantially opposite phase, leaving the pressure waves 36 generated substantially the same. Subtraction of the two records will 37 reduce noise caused by pressure waves.

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.. , ~ ~, ~ZS9~5 1 From the application of ~he invention to noise reduction in both 2 SH wave and SV wave seismograms, it will be appreciated that the invention 3 is also applicable where the seismogram records a combination of both SH
4 and SV waves, as is the case when detectors lO0 are oriented so that the common axis of maximum sensitivity lies between the plane of incidence and 6 the line normal to the incidence plane.
7 To protuce a shear wave seismogram with improved signal to noise 3 ratio according to this invention, at least two shear wave records must be 9 made. If only one shear wave seismogram is desired, then only two such records are made in the same manner as described earlier for SH wave 11 seismograms. Subtraction of the two records reduces noise caused by 12 pressure waves. If more than one shear wave seismogram is desired, then 13 four or more such records must be made, wherein each even numbered record 14 and the immediately preceding odd numbered record may be made in the same manner as described earlier for S~ wave seismograms. The even numbered 16 record and its immediate predecessor are then subtractively combined to 17 reduce noise caused by pressure waves.
18 An alternative method of changing the phase of baseplate vibra-19 tions may be accomplished by using a configuration of a shear wave vibratorshown in schematic simplified view of FIG. 4. Switch 36 is attached to 21 servovalve 28, and connected to signal generator 32 by cable 34. This 22 alternative configuration is otherwise the same as the vibrator shown in 23 FIG. l and used in the method already discussed.
24 To change the phase of the baseplate vibrations, switch 36 may beused as an alternative to changing the signal generated by signal generator 32.
26 Switch 36 may be mechanically or electrically operated. For example, in 27 reference to FIG. 4, switch 36 may be a type that operates mechanically to 28 reverse the order of application of hydraulic fluid to conduits 24 and 26, 29 which in turn causes the vibra~ory motion of baseplate 18 to be reversed indirection. Alternatively, switch 36 may be a type that electrically changes 31 the electrical signal from signal generator 32 to one haYing a substantially 32 opposite phase. When the same electrical signal from signal generator 32 33 is applied to vibrator lO both before and after flipping switch 36, the 34 vibrations of baseplate 18 will be Ln response to the same signal botEI
3S before and after flipping switch 36, but that they will be substantially 36 opposite in phase. The seismic wave generated before reversing the mode of _g_ : :.
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1 baseplate vibration will have a waveform substantially the same as but 2 substantially opposed in phase to the waveform of the seismic wave generated3 after the mode reversion.
4 In operation baseplate 18 is vibrated i~ response to an electricalsignal of a predetermined waveform from signal generator 32 to generate 6 seismic waves, which are detected and recorded in the same manner as 7 discussed earlier, to obtain a first record. Switch 36 is then flipped.
8 Substantially the same electrical signal is applied to vibrator 1~ through 9 switch 36. Baseplate 18 is thus made to vibrate in substantially opposite phase to but otherwise in a manner substantially similar to the vibration 11 in the first record. The resulting seismic waves are recorded as a seco~d 12 record. The two records are subtracted to give a record wherein shear 13 waves reinforce but pressure wave reflections are reduced. From the 14 above, it will be appreciated that any method of reversing the mode of baseplate vibration to a substantially opposite phase may be used and comes 16 within the scope of this iavention.
17 The above description of method and structure used is merely 18 illustrative thereof, and various changes in shapes, sizes or other details19 of the method and construction may be made within the scope of the appended claims, without departing from the spirit of the invention.

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Claims (6)

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

1. A method of geophysical prospecting comprising the steps of:
coupling the baseplate of a shear wave seismic vibrator to the earth;
vibrating said baseplate along a substantially horizontal line to generate a first seismic wave having a predetermined waveform;
detecting the resulting seismic wave with a detector and recording the output of said detector;
vibrating said baseplate substantially along said horizontal line to generate a second seismic wave having substantially the same waveform as but substantially opposite in phase to said first seismic wave;
detecting with said detector the resulting seismic wave from said second seismic wave; and subtractively combining the resulting seismic wave from said first seismic wave with the resulting seismic wave from said second seismic wave to reduce unwanted signals.

2. In a method of geophysical prospecting wherein a plurality of seismic records are generated by sequentially vibrating a baseplate along a substantially horizontal line in a number of vibrational sweeps to create seismic waves with predetermined waveforms and by sequentially detecting and recording the seismic waves resulting from each vibrational sweep as a corresponding seismic record, the improvement comprising:
vibrating said baseplate such that the predetermined waveform of the seismic wave generated in each even numbered sweep is substan-tially the same as but substantially opposed in phase to the predetermined wavefonm of the seismic wave generated in the immediately preceding sweep; and subtractively combining each even numbered record with the immediately preceding odd numbered record to reduce undesired waves.

3. A method of geophysical prospecting comprising the steps of:
coupling the baseplate of a shear wave seismic vibrator to the earth;
vibrating said baseplate along a substantially horizontal line to generate a first seismic wave having a predetermined waveform;
detecting the resulting seismic wave with a detector placed on a line which passes through said baseplate and which is substantially perpendicular to said horizontal line, said detector being aligned with axis of maximum sensitivity substantially parallel to said horizontal line;
recording the output of said detector;
vibrating said baseplate substantially along said horizontal line to generate a second seismic wave having substantially the same waveform as but substantially opposite in phase to said first seismic wave;
detecting with said detector the resulting seismic wave from said second seismic wave; and subtractively combining the resulting seismic wave from said first seisimc wave with the resulting seismic wave from said second seismic wave to reduce unwanted signals.

4. A method of geophysical prospecting comprising the steps of:
coupling a baseplate of a shear wave vibrator to the earth;
vibrating said baseplate along a substantially horizontal line for two sweeps, to create seismic waves of predetermined waveforms, wherein the waveform of the seismic wave generated in the first sweep is substantially identical to but substantially opposed in phase to the waveform of the seismic wave generated in the second sweep;
detecting the resulting seismic waves from the two sweeps with a plurality of detectors having axes of maximum sensitivity aligned substantially along said horizontal line, at points spaced substantially along said horizontal line;
recording the output of each detector from the two sweeps as two corresponding traces; and recording the difference between each two corresponding traces so that unwanted signals are reduced.

5. A method of seismic prospecting comprising:
coupling a baseplate of a shear wave seismic vibrator to the earth;
generating a first electrical signal having a predetermined waveform;
applying said first electrical signal to the shear wave vibrator such that said baseplate vibrates in response thereto along a substan-tially horizontal line;
detecting the resulting seismic waves with a detector aligned with axis of maximum sensitivity substantially parallel to said horizontal line;
recording the output of said detector as a first record;
generating a second electrical signal having a predetermined waveform that is substantially the same as but substantially opposite in phase to the predetermined waveform of said first electrical signal;
applying said second electrical signal to the shear wave vibrator such that said baseplate vibrates in response thereto substantially along said horizontal line;
detecting the resulting seismic waves with said detector;
recording the output of said detector as a second record; and subtracting said second record from said first record so that desired waves reinforce and undesired waves are reduced.

6. A method of seismic prospecting comprising:
coupling a baseplate of a shear wave seismic vibrator to the earth;
generating an electrical signal having a predetermined waveform;
applying said electrical signal to the shear wave vibrator such that said baseplate vibrates in response thereto along a substantially horizontal line;
detecting the resulting seismic waves with a plurality of detectors aligned with axes of maximum sensitivity substantially parallel to said horizontal line;
recording the output of said detectors as a first multiple trace record;
reversing the mode of the baseplate vibration to a substantially opposite phase;
applying said electrical signal to the shear wave vibrator such that said baseplate vibrates in response thereto substantially along said horizontal line;
detecting the resulting seismic waves with said detectors;
recording the output of said detectors as a second multiple-trace record; and recording the differences between each two corresponding traces of said two records without relative record-time delay, so that undesired waves are reduced.