Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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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~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
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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 -
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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
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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,
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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
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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
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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.
