Method and apparatus for producing shear waves for subsurface geophysical investigation

Two or a larger even number of compressional wave producing vibrators are symmetrically disposed in two groups on opposite sides of a profile line in closely spaced relationship. The vibrators are energized by the same (or identical) sweep signals, the signals to one vibrator (or group) on one side of the line being separated in phase by 180.degree. from the signals to the vibrator or group on the other side. P-wave components are thereby cancelled, leaving the shear waves to be propagated.

BACKGROUND OF THE INVENTION 
It is customary to conduct reflection seismographic measurements by 
producing acoustic waves which pass through the earth and are reflected, 
received and analyzed. Normally, such waves are produced by blasting or 
with one or more vibrators in or on the earth's surface, the conventional 
sources producing vibrations which are essentially pressure waves 
(P-waves) and such waves are received with conventional geophones which 
can be, for example, vertically disposed plunger-coil assemblies or 
similar devices responsive to waves reflected from interfaces in the 
sub-soil. It is possible to supplement these measurements by the 
production and reception of shear waves in which case it is possible to 
calculate the Poisson constant for individual characteristic reflections 
from the quotient of the speeds of the P- and S-waves which, in turn, can 
be analyzed to produce further information about the lithological 
characteristics of the reflection interfaces. Exhaustive reports 
concerning the production and reception of shear waves are printed in 
"Geophysics 1968", pages 229-254, including additional references to 
literature. The efforts along these lines depend primarily on the use of 
special vibrators such as those discussed in U.S. Pat. Nos. 3,159,232; 
3,159,233; and 3,205,971. Such techniques employ instruments which are 
built for the special purpose of producing S-waves. They permit the 
production, of shear waves of the type SH, that is, with the polarization 
in the plane of the reflection horizon. With the use of correspondingly 
polarized geophones, good shear wave seismograms have been recorded in 
these reported experiments. 
The vibrators used in the known experiments as shown, for example, in the 
patents already mentioned, fulfill the physical requirements for this 
special type of reflection seismic recording in an almost ideal manner. 
However, in the practical application of these devices, the vibrators have 
significant disadvantages. Because of the horizontal movement of the 
vibrator mass, the elements used for the guidance of the mass are very 
much subject to wear. For the purpose of transferring the shear forces to 
the sub-soil, the known vibrators are equipped with a steel plate which 
has pyramid-shaped warts or projections on that surface by which the plate 
is coupled to the soil. When using these vibrators, especially on highways 
and roads, expensive damage to the coupling surfaces is caused. A further 
and very significant disadvantage of these vibrators is that, with the 
vibrator performance used at the present time and considering the mass 
necessary therefor, it is not easily possible to insert such a vibrator 
into equipment which is customarily available for the production of 
P-waves. 
Generally, the experiments, continued until the most recent times in the 
United States with shear waves for seismic trial operations, have shown 
that good results can be achieved under the most variable geological and 
terrain conditions. In almost all areas with the exception of loose sand 
and water-saturated sediments, shear wave vibrators per se may be used as 
a source for the production of shear waves of the type SH. At the same 
time, in many areas, shear wave signals are observed in sufficiently great 
multiplicity. However, hitherto, the multiplicity has always been at least 
twice as great as in the case of recording of compression waves. In 
connection with the recordings heretofore, evaluating and interpreting 
processes have been developed which, as pointed out above, contribute to 
the discovery of lithological anomalies. 
It will be apparent that special so-called horizontal geophones are needed 
for the recording of SH waves when they are the primary waves used for 
sub-soil investigation. Providing testing crews with additional horizontal 
geophones is much less of a problem that the need for providing special 
devices for the production of S-waves. 
BRIEF DESCRIPTION OF THE INVENTION 
Accordingly, an object of the present invention is to provide a simple and 
efficient method of producing S-waves for sub-soil investigation. 
A further object is to provide an apparatus for the production of S-waves 
for subsurface investigation. 
Yet another object is to provide a method of operating readily available 
apparatus such that S-waves are produced for sub-soil investigation. 
Briefly described, the invention includes a process for producing shear 
waves in the earth for geophysical examination of the sub-soil comprising 
the steps of providing at least two vibrators of the type usable for the 
production of compressional waves in the earth, locating the vibrators in 
spaced, adjacent relationship with the bottom surfaces thereof in coupling 
contact with the surface of the earth, and concurrently energizing the 
vibrators with identical sweep signals in counterphase relationship. 
In another aspect, the invention contemplates an apparatus for producing 
essentially horizontally polarized shear waves in the earth for sub-soil 
geophysical examination comprising an even number of compression wave 
vibrators, said vibrators being located in closely spaced symmetrical 
array with half of the vibrators being located on each side of a central 
line and coupled to the earth's surface; and means for energizing the half 
of said vibrators on one side of the line with a first sweep signal and 
the other half of said vibrators with a substantially identical second 
sweep signal separated in phase from said first signal by substantially 
180.degree.. 
The invention has the great advantage that one can operate with 
conventional vibrators. If one assumes that the vibrators radiate 
essentially sphero-symmetrically into a homogeneous half-chamber, in a 
situation in which two closely adjacent T-wave vibrators are energized 
with counterphase signals, the P-waves will be essentially extinguished. 
It is also the case that the amplitude of force is decreased in the 
horizontally stimulated waves as compared with the vertical stimulation in 
the equal phase operation. For this reason, the use of four vibrators is 
recommended, the vibrators preferably being located at the corner points 
of a rectangle for which the profile line constitutes a bisecting line and 
wherein the vibrators on one side of the line are energized with signals 
of the same phase and the vibrators on the other side of the central line 
are energized in counterphase, i.e., with signals separated in phase from 
the first side by 180.degree.. 
Contrary to the known instruments built especially for the production of SH 
waves, such as, for example, those described in the above mentioned U.S. 
Patents, it is possible when using the P-wave vibrators controlled in 
counterphase to produce SH waves even on soils which consist of relatively 
loose sand and water-saturated sediments. This results in an additional 
advantage besides the simplification through the use of customary 
instruments.

An arrangement for carrying out the method of the invention is illustrated 
in FIGS. 1 and 2 wherein the simplest arrangement employing two vibrators 
is shown. 
A vibrator 12 and a vibrator 14 are disposed on opposite sides of a profile 
or central line 22 or 24. Vibrators 12 and 14, which are illustrated only 
in a schematic, simplified manner, produce compression or P-waves as 
indicated by the double arrows above the vibrator plates in FIG. 1. Since 
the vibrators 12 and 14 are closely adjacent, the compression waves 
produced thereby are essentially cancelled and extinguished at least in 
the central area as a result of the counterphase energization thereof. The 
developing waves propagated in the direction of arrow 20 are SH waves with 
the direction of vibration characterized by the double-headed arrow 18. 
After reflection in the sub-soil from interfaces and anomalous formations 
therein, the signals returned to the earth's surface are received by the 
horizontally sensitive geophones 16 which are connected to conventional 
recording apparatus, not illustrated. 
From a central control location, at which the receiving apparatus would 
normally be located, and which is not shown in the figures, a signal is 
provided by way of a wire or wireless connection 29 which activates a 
sweep generator 28, causing the sweep generator to produce sweep signals 
which are supplied to a control arrangement 26. From the control 
arrangement 26 sweep signals are conducted through connecting channels 30 
and 32 to vibrators 12 and 14. The function of control unit 26 is to 
reproduce the sweep signal produced by generator 28 in two forms, one 
being in counterphase relationship to the other so that the signals 
produced on channels 30 and 32 are replicas of the originally produced 
sweep signal but separated in phase by 180.degree.. 
A preferred embodiment of the invention is shown in FIG. 3 wherein 
customary compression wave vibrators are used and are supplemented by 
simple additional devices. The two vibrators 112 and 114 are connected to 
the outputs of control units 126a and 126b, respectively, which receive 
sweep signals from sweep generators 128a and 128b, the sweep generators 
being stimulated into activity by a signal provided on a wire or wireless 
connection 129a or 129b, as mentioned in connection with FIG. 2. 
In order to make sure that the vibrators 112 and 114 will propagate signals 
of equal magnitude and with opposite phase relationship into the soil, a 
comparator 133 is provided. The comparator receives signals from sensors, 
transducers or scanning mechanisms 134a and 134b which are coupled to the 
vibrators and which produce signals representative of the actual vibratory 
signals produced by the vibrators. Signals representative of the signals 
radiated by the vibrators are thus compared with one another in comparator 
133 which then produces a control signal or voltage which is transmitted 
to a regulator unit 137 which performs the function of correcting the 
amplitude and phase of, in this case, vibrator 112. Regulator 137 assures 
that the sweep produced by generator 128a is in counterphase to, but 
otherwise equal to, the sweep of generator 128b. The arrangement shown in 
FIG. 3 additionally permits the possibility of taking into consideration 
any kind of influences exercised on the vibrators themselves by the 
conditions of the soil. 
Except for the additional arrangements to accomplish the counterphase 
control, only conventional devices, and in particular conventional P-wave 
vibrators are used in the performance of the method of the invention. 
While certain advantageous embodiments have been chosen to illustrate the 
invention it will be understood by those skilled in the art that various 
changes and modifications can be made therein without departing from the 
scope of the invention as defined in the appended claims.