CA1185689A - Seismic exploration with a swinging-weight vibrator - Google Patents

Abstract

ABSTRACT
The invention pertains to seismic exploration for petroleum by the Vibroseis method using swinging-weight vibrators. A seismic vibrator provides a measure of automatic compensation of frequency-selective action in the vibrator-ground coupling, and comprises in combination a rotatable eccentric weight, a baseplate on which the eccentric weight is mounted and by which the forces generated by the rotation of the eccentric weight may be coupled to the ground. Phase-measuring means are operatively coupled for recording the phase of the rotating weight, and a flywheel with means for coupling the flywheel to the eccentric weight whereby flywheel energy is used to rotate the eccentric weight.
The flywheel is mounted to decelerate freely between desired frequency limits as the energy is radiated.

Description

Title of the Invention SEISMIC EXPLORATION WITH A SWINGING-WEIGHT VIBRATOR
Cross Referenc~ to Re1ated Application~
The present application is r~lated to Canadian Patent Application Seri~1 No. 394,254, filed January 15, 1982, and entitled "Seismic Exploration Uslng Compressional and Shear Waves Simultaneously'.'.
Technical Field This invention is concerned with seismic exploration for petroleum by the Vibroseis method, using swinging-weight vibrators.
Background Art _ _ The Vibroseis method was initially implemented with a swinging-weiaht vibrator (Crawford, Doty and Lee; Geophysics v.25 p.95, 1960). Very soon this wa~ abandoned for hydrau-lic vibrators. Several hydraulic units could be used in unison and theix characteristics were better suited to the recording and correlating technology of the day.
Since then the hydraul.ic vibrator has become very com-plex. It i5 fairly reliable, but very expensive. Further, its limitations of bandwidth h~ve become onerous, as demands increase for better geological resolution. The limitation at the high-frequency end is because the vibrator is basically a constant-force device, ~ecau~2 of the com-pressibility of the hydraulic oil, ~ecause of limitations in the servo-valve, and because of the heavy baseplate.
These limitations can be offset by sWQeping for longer times at the highex frequencies, but this proves to be very expensive. The limitation at the low-frequency end is because of stroke constraints, and because of serious even-order distortion. A9ain these limitatiolls can be offset

2 ~ ~5dEi~
by sweeping for longer times at the low frequencies; this is less exp~nsive than at the high Erequencies. Particularly at the low frequencies, major phase shifts occur between desired and obtained signals; these must be eliminated by an expensive phase-compensa-tion system.
In contradistinction, the swinging-weight vibrator is essentially simple and inexpensive. Since it is a constant~
displacement device, its useful seismic output tends to rise with frequency. Its baseplate can be made light. Its output at low frequency contains little even-order distortion, and although this output is small it can be supplemented by simple changes to the swinging weight.
Objects of the Invention Accordingly, this invention seeks to provide a novel Vibroseis vibrator which is inexpensive, simple and reliable, and to provide a ~ibrator whose use~ul output rises at high and/or low frequencies, in controlled manner.
The invention also seeks to provide a method by which any desired signal spectrum may be achieved with the vibrator, substantially independent of the vibrator-ground coupling.
Disclosure of the Invention The invention in one broad aspect pertains to a seismic vibrator providing a measure of automatic compensation of frequency-selective action in the vibrator-ground coupling, and comprising in combination a rotatahle eccen-tric weight, a base-plate on which the eccentric weight is mounted and by which the forces generated by the rotation of the eccentric weight may be coupled to the ground, phase-measuring means operatively coupled for recording the phase of the rotating weight, and a flywheel and means for coupling the flywheel to the eccentric weight ~hereby flywheel energy is used to rotate the eccentric weight, the fly-wheel being mounted to decelerate ~reely between desired frequency limits as the energy is radiated.
The invention also broadly comprehends a method of seismic exploration in which the source of seismic vibrations comprises an eccentric weight, the method comprising rotating the eccentric weight over a frequency range sweeping from a high frequency down to a low frequency using the energy stored in a ,, ~

2a rotating flywheel, measuring the variation with time of the phase of the eccentric weight and generating a swept-frequency signal derived from the phase measurements, and correlating reflected vibrations against the swept-frequency signal.
The invention in another asp~ct pertains to a seismic exploration vibrator providing automatic compensation for frequency dependent impedance at the vibrator-ground coupling comprising a baseplate for coupling vibrations to the ground and at least one pair of eccentric weights mounted on the baseplate for rotation ln opposite dlrections relative to each other thereby to impart periodic forces through the baseplate to the ground. Primary flywheel means is provided for storing energy and operative coupling means couple the flywheel and Elywheel energy to the eccentric weights. Phase measuring means provide a record of the phase of the rotating eccentric weights for use in correlation with reflected vibrations.
A further aspect of the invention comprehends a method of seismic exploration using a pair of contra-rotating eccentric weights comprising rota-ting the contra-rotating eccentric weights by imparting to the weights the energy stored in a freely rotating flywheel thereby sweeping the frequency of rotation from a high frequency down to a low frequency as the energy stored in the fly-wheel is dissipated, and measuring the time variation of the phase of rotation of the eccentric weights for correlation with reflected vibrations.
Briefly, the invention disclosed provides a contra-rotating swinging-weight vibrator dr:iven by energy stored in a flywheel. The phase of the vibrator, as a function of time, is used to define a master sweep whose amplitude is made a function of the sweep rate.
The use of a free downsweep provides a measure of automatic compen-sation for resonance in the vi~rator-ground coupling. Supplementary output at low and/or high frequencies is obtained by controlled variation of the eccentricity. A plurality of vibrators may be arranged to have complementary acceleration and deceleration periods, or to be optimized for different parts of the frequency range, or for simul-taneous use at spaced-apart frequenciesO
More particularly the vibrator comprises a base for coupling energy to the ground and a pair of contra-., ,~.

S6~

rotating rotors or secondary fly-wheels mounted for rota-tion on the base. Weights are disposed eccentrically on the wheels, rotors or secondary fly-whee]s with specified eccentricity according to the amplitude of periodic forces or vibrations to he coupled through the baseplate to the ground. In the preferred form, the eccentricity of the weights may be varied during the sweep of frequencies, for example, from high to low frequency as a function of frequency according to the impedance characteristics of the 10 earth. Thus, in regions of the downsweep frequency band width with good impedance matching, coupling and radiation of energy into the earth the eccentricity of the wheels and therefore the amplitude of energy delivered is decreased while at the high and low frequency extremes characterized lS by impedance mismatch the eccentricity may be increased for imparting greater periodic force and amplitude of vibration.
The contra-rotating eccentric weights in the form ~f secondary rotors or fly-wheels are driven by the energy 2Q stored in a primary fly-wheel driven up to speed by a separate motor. During downsweep of the vibrator from high to low frequency within a specified range the primary fly-wheel is free-wheeling and decelerates as energy is im-parted to the fly-wheels. In portions of the frequency 25 spectrum characterized by good impedance matching with the earth and therefore high energy coupling and radiation of vibratory energy, the downsweep rate of the primary fly-wheel and the rotati~g weight vibrators increases. In the extremes of high and low frequency with poor impedance 30 match and energy coupling with the earth the frequency sweep rate decreases for radiation and irnparting of enerc3y over a longer period aP tlme. Thus, the coasting primary fly-wheel drive of the present inVe~ltion affords at least partial compensation for the frequency dependent impedance 35 coupling between the vibrator and the earth. Further compensation is afforded by varying the eccentricity of the fly-wheels as a function of frequency in order to achieve a more uniform frequency response, or frequency response of other desired characteristics.

Thus, the invention contemplates the method of provicling one or more primary drive fly-wheels, driving the fly-wheels through a frequency upsweep to a desired maximum by means of an external drive motor and then using the stored energy of the primary fly-wheel while in free rotation to drive the vibrators during the downsweep within a specified sweep frequency range. The phase of the rotating eccentric weights is measured to generate a swept-frequency signal used for correlation with ref]ected vibrations.
Multiple or plural vibrators may be used with eccentric weights of different characteristics in order to cover different bands or portions of the desired frequency spec-trum. The outputs of the plural vibrators may be so ar-ranged and coordinated to act sequentially and provicle 15 effective and continuous overlapping coverage for wider frequency ranges.
Other features and advantages of the invention will be apparent in the following specification, Brief Descri~_on of Dra~in~s The apparatus and method are now described with reference to the drawings, in which:
Figure lA is a diagrammatic side view of a swinging-weight vibrator of the prior art, while Figure lB is a diagrammatic plan view of the same swinging weight vibrator.
~igure 2 is a graph illustrating the variation of sweep frequency with time, with and without resonance in the vibrator-ground coupling.
Figure 3 is a diagrammatic plan view showing desirable changes to the vibrator of Figures lA and lB according to 30 the invention.
Figure 4 is a plan view illustrating a variable eccentric weight.
Figure S is a side cross section illustrating a variable-eccentricity vibrator according to the invention.
Figure 6 shows a detail plan view of the eccentric weight.
F`igure 7 is a graph showing a cycle of upsweep and down-sweep when the vihrator is usecl on the upsweep.

Figure 8 is a yraph illustrating a variation of Figure 7 in which the eccentricity o the weight is varied.
Figure 9 is a flow chart depictiny a method of control-ling the eccentricity as a function of frequency.
Figure 10 is a graph of a method using only downsweeps, achieved by the alternation of two vibrators.
Figure 11 is a block diagram of a two-vibrator system in which the two vibrators are altexnately accelerated by a single motor.
Figure 12 is a block diagram of a variation of Figure 11, using a differential instead of two clutches or separate drives.
Figure 13 is a block diagram corresp~ding to the left half of Figure 11 and in which the phase between two vibrating uni-ts 15 ene~rgized by a single flywheel can be varied to maximize or annul their combined output.
Figure 14 is a graph illustrating how six vibrators on three trucks can be used simultaneously without requiring synchronization.
Figure 15 is a graph showing how two vibrators may cooperate to furnish different parts o the frequency range.
Figure 16 is a graph of the method using six vibrators~
Best Mode for Carr ~ ~
Figures lA and lB show the basic swinging-weight vibra-25 tor known in the prior art- Equal weiyhts 1 are caused to rotate eccentrically by equal gear wheels 2. The wheels are supported in a framework 3 on a baseplate 4, and driven by a motor 5. Isolated hold-down weight may be provided through springs 6. The wei~hts rotate in opposite direc-30 tions, and the points of attachment are such that thehorizontal components of the centrifugal forces cancel while the vertical components reinforce. The frequency of the vibration is changed to achieve a sweep signal, by varying the speed of the drive motor. Drive motor 5 may 35 be an electric motor, a hydxaulic motor or an internal com-bustion enyine. The resultant force varies with the mass of the weights, their eccentric radius, and the square of the frequency.

The coupling between the baseplate and the ground is likely to be a resonant system, and this reasonance is probably arranged to be in mid band. While the motor is driving the vibrator through the resonant range of fre-quency, much power is drawn from the motor. At otherfrequencies the power is modest, and little energy is being transmitted into the earth. If the motor is of standard (non-synchronous) type, the sweep rate tends to be slow in the resonant range, and fast away from resonance. This is 10 the opposite of the requirement for good signal bandwidth, which is that the sweep rate should be fast where the signal obtains good amplitude, and 510w where less signal is usefully transmitted into the earth~
A first modification of the prior art according to the 15 present invention is obtained, therefore, by providing one or more flywheels, by driving them up to a maximum desired frequency by means of the motor, and then by using the stored energy to drive the vibrator.
In this mode the vibrator emits only a weak signal away 20 from resonance, ~ut it emits it for a long time. Near resonance, energy is withdrawn very quickly from the vibra-tor7 and the sweep rate is therefore fast. On the low side of resonance the vibrator returns to a slower sweep rate.
Since with a well-constructed vibrator the efficiency is 25 very large, the controlling factor on the frequency sweep rate is the rate of emission of seismic energy. There is therefore an automatic agency tending to counteract the undesirable effect of the resonance.
In Figure 2, if curve 7 shown by the solid line repre-30 sents the run-down of the vibrator in the absence of a coupling resonance, the dashed line curve 8 shows the effect of the resonance. To offset the higher output, the time spent in the zone of resonance 9 is automatically reduced.
This system takes full advantage of the resonance in 35 reducing the time spent transmitting the necessary energy, but also achieves a good bandwidth of the radiated signal.

One approach might be to make the year wheels 2 very heavy, so that they become 1ywheels. The disadvantage of this is that it adds to the weight of the vibrator base-plate. Accordingly Figure 3 shows a suitable compromise arrangement according to the invention in which the wheels 2 are given enough mass to have some flywheel action, but the major flywheel 10 is isolated from the vibrator by a drive shaft 11 fitted with ~Iniversal joints. Figure 3 corresponds with the prior art view of Figure 1~, suitably 10 modified and with corresponding elements designated by the same reference numerals. The flywheel 10 and the motor 5 can now be carried on the vehicle separate from the vibra-tors, as part of the hold-down weight. It may be desirable to split the flywheel 10 into two contra-rotatiny parts to 15 avoid gyroscopic problems with the vehicle.
It must be stressed that the difference between Figure 3 and Figure lB is not just the addition of a flywheel. In Figure lB the motor supplies the drive energy to work the vibrator, whereas in Figure 3 it is important that m~st or 20 all of the drive energy should come from the flywheel.
Only in this manner is compensation ob-tairled for the Ere-quency dependence of -the radiation imp~dance of the vibrator-ground coupling.
In the system of the present invention, no attempt is 25 made to control the sweep in the manner used for hydraulic vibrators. The vibrator just runs down as its energy is radiated. The run-down (between given high and low fre-quencies) is fast if the coupling into the earth is good, and slow i the coupling is poox. It is correct, of course~
30 that if the coupling is poor we must make a choice between increased time (that is, cost) and decreased bandwidth (that is, resolutionJ.
Because the vibrator is not controlled, it is necessary to take a record of its vibration~ for use in the correla-35 tion process. A suitable way to do this is to attach apseudo-random code generator 12 to the flywheel 10, the drive shaft 11 or one of the wheels 20 This unit transmits to the recording instruments, over a radio link, one or more fade-resistant signals for each cycle of revolution. It is not necessary to transmit a continuous record of the vibrator phase. A reasonable compxomise is to generate a unique code signal at each zero-crossing of the force vector, and perhaps at each peak and trough. A simple operation in the instruments can then erect a half-cycle or a quarter-cycle of a sinusoid between the spikes derived from demodulation of the codes, and so reconstitute the complete quasi-sinusoidal sweep. The code generator 12 can 10 be of magnetic, optical or other type well known in the art.
The radio link is conventional.
Because the sweep rate, and its variation with requency, contain information on the output of the vibrator as a function of fre~uency, manipulations based on this informa-15 tion (including source-signature deconvolution) can be effected without requiring a transducer on the baseplate.
For example, the amplitude of the s~eep constructed as above can be modulated, as a function of frequency, before the sweep is used in correlation. Any desired spectrum 20 for the correlated signal can be achieved in this way.
Alternatively the information contained in the sweep rate can be used to derive a shaping operator to be applied in later processing. In effect, the sweep rate and its varia-tion define, to a first approximation, the radiation impe-25 dance of the vibrator-ground system, the dependence of this impedance on frequency, the energy radiated at each frequency, the total enerqy radiated, and the additional tirne to extend the bandwidth at the low frequencies. All this is achieved very simply, merely by sending to the instruments the code 30 or codes indicative of the phase of the vibrator. This is necessary, in any case, to obtain the sweep against which to correlate, but with this system much additional benefit is obtained also. These matters are further discussed hereinafter.
As noted above, the useful output of the vibrator ordi-narily rises with frequency~ The degree to which it does so contributes a desirable pre-compensation of the high frequencies, in anticipation of a loss of these fre-quencies in the earth. For some targets, this pre-compensation may be sufficient to keep the reflected signal spectrum substantially flat over a considerable range. For others, the pre-compensation may be inadequate.
S In ~ny case, of course, the output from a swinging-weight vibrator tends to be deficient at the very low frequencies.
Accordingly, there is merit in modi~ying the vibrator -to - provide pr~gressively augmented output at the low fre-quencies (perhaps 5-25 Hz) and possibly at the highest 10 frequencies (perhaps 50-100 Hz).
Figure 4 illustrates diagrammatica31y a means for doing this. The eccentric weight 13 is here formed in a w~dge shape, into which a slot 14 is cut to take -the axle of rotation. When the axle occupies position 15 the weight 15 is balanced. Minor adjustments to perfect the balance can be made by additional masses 16. In this condition the machine generates no vibration, and the only input required to keep it running at constant speed is the minor amount necessary to overcome friction. As the axle is 20 caused to occupy other positions in the slot, culminating in the position 17, it generates progressively stronger vibrations. This is both because the eccentric mass increases and because the radius of eccentricity increases.
Figure 5 shows diagrammatically a vibrator in which the 25 relative position of the axle and the slot may be changed while the vibrator is in operation. A strong shaft 18 is mounted in robust bearings 19 supported from the baseplate ~not shown). The shaft is driven through coupling 20.
The minor flywheel of Figure 3 becomes flywheel 21. The 30 eccentric weight 22, together with leadscrews 23 by which its eccentricity may be changed, is mounted within a space milled out in the flywheel 21. The flange 24 provides containment in the event the weight should break and fly wild. Figure 6 shows a view looking into the space 35 containing the weight and the leadscrews. The boss 25 of the flywheel has hardened flats on which the weight can slide, and by which the rotary motion is imparted to the weight. The threads contacting the leadscrews may be permitted some lateral movement.

The remainder of the apparatus shown in Figure 5 represents one ~eans by which the eccentricity of the weight may be changed, during operation, by rotation of the leadscrews. The leaclscrews are fitted with gears 26 meshing with a ring gear 27 mounted on wheel 28. Relative motion between wheel 28 and flywheel 21 therefore moves the weight. The wheel 28 carries two planet wheels 29, rota~
ting about a sun wheel 30 within a ringwheel 31. These three units constitute a differential. Gears 32, 33, 34 lO and 35, together with layshaft 36, provide gearing to offset any gear ratio inherent to the differential. By this means the wheel 28 rotates at the same speed as the flywheel 21 if the rin~ wheel 31 is maintained stationary. A servo-motor 37 drives the ring wheel 31 through a gear 38. A
15 portion of the ring wheel 31 has meshing teeth to engage - this gear. The ring wheel 31 may be supported on peripheral bearings (not shown) or from an additional wheel ~not shown) supported concentric to the shaft 18. Its position may be continuously detected by means of a potentiometer 20 or displacement transducer (not shown), whose output represents one input to the servo loop driving the motor 37.
The phase of the vibrator may be obtained, for example, by the combination of a magnet 39 (attached to the flywheel 21) and one or more pick-up units 40. The pick-up may have 25 several armatures and several coils disposed in such a way as to generate the unique pseudo-random codes described earlier. These are then transmitted to the recording instruments by radio 41. If a plurality of vibrators is used (as hereinafter described), the pick-ups 40 may Aiffer 30 between vibrators, so that only one radio channel is re-quired. The output of the pick-up 40 is also used, at the vibrator, for a direct measurement of the frequency being radiated. The use of this, and the servo loop described above, will be discussed later.
Figure 5 shows only one of the pair of vibrating units required according to Figure 3. The second unit of the pair does not need its own differential and servo motor. Its flywheel may be driven from the identical flywheel 21 b~
means of peripheral gearing ~not shown), and its wheel from the identical wheel 28 by means of similar peripheral gearing ~not shown). This gearing must be engaged, of course, so that the leadscrews of both units are vertical at the same time. The term "vibrator" is now used to signify the pair of vibrating units assembled into th~ form of Figure 3.
A first manner of use of this vibrator is illustrated in Figure 7. The vibrator is swept up and down consecutive-ly, driven up by the motor and coasting down by itself (or 10 with reduced drive from the motor). This manner of oper-ation loses part of the automatic compensation of the vibrator-ground coupling, as described previously. One cycle of the sequence is illustrated from low point 43 through high point 44 to low point 45. Because the Vibro-15 seis signal demands a continuous change of fre~uency, theemission in the re~ion of points 43, 44 and 45 should not be used. This is easily achieved by truncating the reconsti-tuted sweep ~against which the correlation is made~ at frequencies fO and fN~ where the sweep rate is still 20 suitable. The actual sweeps used are then as suggested in heavy lines. The upsweep 46 and the downsweep 47 are not identical. The downsweep provides the automatic com-pensation discussed earlier, but compensation of the up-sweep requires a separate manipulation of the power drawn 25 by the motor as a function of frequency, or inverse appli-cation of the sweep-rate information obtained from the downsweep, applied to the amplitude of the sweep used for correlation.
The only power wasted, in this manner of operation, is 30 that supplied and released above frequency fN, in the vicinity of point 44, and below frequency fO. The system is very simple, and it may be used without the variable-eccentricity devices of Figure 5. ~owever, in this case it is not possible to move the vibrator, between positions, 35 without allowing it to come to rest. One solution to this problem is to make the "baseplate" in cylindrical form, so that the unit can roll while vibrating.

The basic technique described above illus-trates -the design considerations involved in choosing the variables.
The weight and its eccentricity are selected on considera-tions of practicality. The desired frequency range is dictated by the earth. The useful length of the sweep is dictated by the correlating equipment, and should pre-ferably be many seconds (for example, 30 s). The moment of inertia of the major flywheel 10, supplemented by the rninor flywheel 21, should be sufficient to give this length 10 to the downsweep (on ground of fairly high impedance).
The power of the motor should be sufficien-t to accelerate the vibrator from point 43 to point 44 in about the same time. ~Since the motor runs for only about half of the total time, this means that the peak power required of 15 the motor and of the prime generator is about double the minimum conceivable requirement.
A desirable modification to this basic system is given by increasing the eccentricity, at low and possibly very high frequencies, using the devices of Figure 5. This, 20 of course, accelerates the run-down, allows an increase in the size of the flywheels, and correspondingly dernands a higher peak power from the motorO After these adjustments, the cycle may take the same time as before, as suggested in Figure 8. Curve 48 now shows a faster sweep rate in ~5 mid-band (and slower at low and very high frequencies).
Curve 49 shows the opposite. The eccentricity employed is suggested in curve 50 (as a function of time, though of course the independent variable is frequency).
The manner of controlling the eccentricity is shown in 30 diagrarNnatic form in Figure 9. A frequency measurement is made (by counting the impulses obtained from the pick-up 40 in unit 42, or otherwise), and a comparison is made between successive values to determine whether the frequency is increasing or decreasing. A micro-processor of standard 35 configuration can perform this function and the next, which is to generate an output value dependent on frequency in accordance with the tables of Figure 9. The interpolations can be of any suitable type. A linear interpolation be-tween P and Q, for example, would involve the computation of ~ 3 P (P Q)(f-f ~/(fl-f ~- The values of fo~ f1~ f2~ N~
Q and R can be inserted manually at the vibrator location (for example, by thumbwheel switches), or may be under remote radio control from the recording truck~ In the latter case the observer (or an automatic substitute~ can increase the value of Q (the "normal" eccentricity~ or adjust the other variables if he finds that the sweep is not reaching its lowest planned frequency safely within the time allowed by the correlator.
A second manr-er of use of the vibrator is shown in Figure 10. In this 5cheme two vibrators are rnounted on one truck (possibly on one baseplate, if the baseplate mass is not thereby increased too much). The vibrator is used only on the downsweep, so that the advantage of automatic com-15 pensation of the vibrator-ground coupling is always obtained.
While the vibrator is being accelerated, between downsweeps, the eccentricity is reduced to zero. One vibrator is thus being accelerated while the other is delivering its down-sweep. The motor is runnin~ all the time, and the acceler-20 ation requires no radiated power. This improves the ratioof average power radiated to peak power consumed.
The upper part of Figure 10 shows the sweep cycles of the two vibrators A and B; the lower part shows the corres-ponding eccentricity programs which might be used. The 25 sweeps utilized are designated by 51, 52 and 53. The run-up periods are 54, 55 and 56. The eccentricity decreases slightly over the period 57 to 58, increases considerahly over the low-fre~uency period 59 to 60, and collapses to zero at 61. It stays at zero during the run-up, and 30 then rapidly increases between 62 and the repeat point 57.
Increases of eccentricity are easy, of course. The decrease from 60 to 61 is a lar~e one, but it occurs at low frequency, where the centrifugal force is small. The decrease from 57 to 58 occurs at very high frequency.
35 Therefore it must be either small or slow if the stresses on the gears, and the power of the servomotor, are to be reasonable.

At the time 61 (and its counterparts represen-ted by dashed lines 63), neither vibrator is eccentric. The cycle may be interrupted, the vibrator moved, and the cycle resumed. The ability tv move the vibrator is important, of course, for those applications requiring a source array.
Figure ll shows the two vibrators A and B, each with its major flywheel lO, driven from a single ~otor 64 by means of clutches 65. Alternatively, separate motors may he driven from a single prime generator, and the motors separately excited through switches. In either of these cases, one flywheel lO is engaged, and th~eother released, at the start of each new cycle, such times corresponding to dashed llnes 63.
An alternative is shown in Fi~ure 12, in which both flywheels are engaged continuously, but through a dif-ferential 66. When the weic3hts are eccentric in one vibrator, most of the motor power is directed to accel-erating the other. In this way no control i5 needed, except for the eccentricities of the two vibrators.
~ or the range of frequencies presently usu~l in Vibroseis, the motor 64 can drive the system directly.
60 Hz is equivalent to 3600 revolutions/minute~ For frequencieS of the order of lO0 Hz (which become attain-able with the enhanced high-frequency output of this vibrator) the motor may be geared.
Hydraulic vibrators currently used in the Vibroseis system have a nominal peak force of 135 kN~ However, they apply this full force only at frequencies between 40 and ~0 Hz~ A swinging-weight vibrator having an eccentric mass of lO kg and an eccentric radius of 68 mm generates this same force at 50 H~ ~with le55 below, and more above).
An alternative arran~emen-t which effectively allows the equivalent oE the eccentricity control described above is disclosed in ~S Patent No. 4,234,053 to Erich. In this system two vibrators are used in place oF each one des-cribed above, and a differential and a servo-motor allow the phase between the two vibrators -to be varied. Thus when the phase anyle is 0 the output of the compound vibra-tor is doubled, when it is 90 the output is reduced to ~ , 10 and when it is 180 -the output is zero. Yigure 13, follow-ing the left half of Figure 11, shows in diagrammatic form how this concept may be applied to the present invention.
The energy of the Elywheel 10 is applied to a differential 67, and thence to the two vibrators 68 and 69. The phase 15 between the two vibrators is changed by energizing the servo-motor 37a through the gear 38a. I'he servo-motor is driven, in a manner analogous to that described hereinbefore, to provide zero output durin~ acceleration and during move-ment of the compound vibrator, and to provide increased 20 output at those frequencies requirin~ addikional radiatecl power.
Usual Vibroseis practice involves 2-4 hydraulic vibra-tors. With swinging-weight vibrators usin~ the quoted values of mass and radius, it would again be desirable to 25 have a plurality of units. We come, therefore, to what has always been regarded as a compelling advantage of the hydraulic vibrator, and a cor3demna-tion of the swinging-weight vibrator - that hydraulic units can be synchronized, while swinging-wei~ht units cannot.
Figure 14 shows a technique by which this objection can be overcome. The first two levels of the diagram repeat the scheme of Figure 10, for a pair of vibrators mounted on a single truck. The ne~t two levels repeat the same scheme delayed by a time T, radiated by a second pair o$ vibrators 35 which may be on a second truck. Similarly the final two levels repeat the same scheme delayed by a time of approx-imately 2T, radiated by a third pair of vibrators which may be on a third truck. As an illustration, the complete cycle time in Flgure 14 might be 1 minute, so that the time between the vertical lines is 30 seconds. Then T might be about 10 s, so that the three trucks emit theix signals at approximately 10 5 intervals. In -this way, even in the presence of dif~erences between vibrators and between vibra-tor-ground couplings, there is virtua]ly no risk of genera-ting undesirable interference ef~ects between the vibrators (either in the downgoing waves or in the surface waves).
According to the discussion of Fiyure 10, a source 10 array can be built up by moving one pair of vibrators at the time of zero eccentricity, denoted 63. In Figure 14 the first vibrator truck may be moved at this timeO
Correspondingly, the second vibrator truck is ~oved at time 70~ and the third at time 71. (It is actually an 15 operational advantage to have the lead vibrator move first, ollowed by the others in turn.) With -the scheme of rigure 14, the emission and recep-tion of signals is continuous. For correlation purposes it is desirable to cut the signals into segments, typically 30s 20 in length. In the above example this represents the time between vertical lines in the Figure. Then the coxrelation operation is done by correlating the transmitted signal of length indicated at 72 against the received signal of length indicated at 73, where the difference between -these 25 two is the maximum reflection time of interest. Typically this requires accumulation o~ the received signal in t~o stores. The first stores 36 s of signal, and the second receives tand continues to receive for 36 s~ the signal 30 s after the first. As before, the signal against which 30 the correlation is made may be truncated at the desired limit frequencies.
With this system, it is desirable to correlate first, and to stack the correlated results over the desired number of emissions. This is in contradistinction to the usual 35 practice with controllable hydraulic ~ibrators, where it is usual to stack first and correlate after. In this respect it may be advantageous to use a sign~bit system tparticularly when many channels are desired).

Since the power output of a swlnging-weigh-t vibrator rises at high frequencies, these vibrators are excellent for modern hi~h-resolution studies. ~lowever, the fu]l requirement for good resolution includes a balanced need for the low frequencies also. We consider next how this may be achieved.
First, the uncontrolled downsweep ~rom a swinging-weight vibrator is inherently non-linear. Thus, althouqh the force developed is very large at high frequencies, the 10 sweep dwells in the high frequencies for only a short time.
Conversely, the force developed is small at low frequencies, but the sweep rate is also small.
Second, as explained hereinbefore, the arranyement of Figure 6 provides the ability -to increase both the eccentric 15 mass and the radius of eccentricity, under ~requency-dependent control.
Third, Figure 15 shows how two vibrators having un-equal weights can cooperate to form a substantially con-tinuous sweep in which the vibrator wi-th the smaller weight 20 contributes the high-fre~uency part 74 of the ~weep and that with the larger weight the low-frequency part 75.
To maintain the approximate 30-second cycle oE the previous illustrations, the two parts of the sweep may each be something less than 15 seconds. Each vibrator has its 25 own run-up time 76-77. The master sweeps used in correla-tion may be truncated exactly at -the cross-over frequency f2 The principle of Figure 15 can be extended directly to that oE Figure 16, in which (now in simpllfied diagrammatic 30 form) there is shown a scheme by which three pairs of vibrators, on different trucks, can cooperate to generate very long overlapping sweeps. Now each vibrator operates in only one-sixth of the desired frequency range, and is configured with a weight and an eccentricity appropriate 35 to that narrow range. At any one time, three signals o different frequency are being emitted, but the separation between them is sufficient to protect against mis-corre-lations. The disadvantage with this system, of course, is that any vibrator which Eails must be repaired or replaced before work can be resumed~ The systems of the earlier figures do not have this disadvantage.
Fourth, it remains possible to radiate the lowest frequencies from a separate "woofer" vibrator of different type. This might even be a single hydraulic vibra-tor, specially configured for the low Erequencies, operating simultaneously with the "tweeter" swinging-weight vibrators.
Clearly, many variations of detail (both in mechanical implementation and in manner of use) can be made without 10 departing from the scope of the invention. All such varia-tions, modifications and equivalents within the scope of the following claims are encompassed by the invention.
It is a particular feature of the invention that its spectrum-shaping facility remains fully effective when 15 multiple vibrators are used. The considerations are as ~ollows.
(a) We can visualiæe (or actually provide, by a framework carrying springs and dashpots) a "reference surface" on which a vibrator can operate. This reference 20 surface provides no resonances of vibrator-ground coupling within the desired frequency range. On this surface the vibrator yields the frequency-time curve 7 of Figure 2.
The sweep rate decreases monotonically with time.
(b) Since the amplitutde spectrum of the auto-25 correlation of the sweep is inversely proportional to thesweep rate, the efect of the changing sweep rate on the reflection pulse can be counteracted, if desired, by correlating against a sweep whose amplitude is directly proportional to sweep rate. The amplitude varlations can 30 be inserted at the time the master sweep is fabricated from the phase codes discussed earlier.
(c) In the same way we could, if we chose, remove the spectral consequences of the change of sweep rate introduced by a resonance in the vibrator-ground coupling (curve 8 35 in Figure 2).
~ d) Again, we could compensate the change of sweep rate introduced by imposed variations in the eccentricity of the vibrator.

~ a~

(e~ We may also choose to modify the spectr~ to be that obtained from a constant-force or constant-velocity device. The 6 dB/octave slopes necessary for this can be incorporated into the amplitude of the master sweep as above, or implemented by a separate operator at a later stage of the processing.
(f) If we are using a driven upsweep as well as a free downswcep (Figure 7), we can use the sweep-rate variations observed on the downsweep to compensate the 10 undesirable effect of the variations on the upsweep.
(gj Whichever compensation (or combination of com-pensations) we care to ma~e, we can choose to make :it on the amplitude of the reconstituted master sweep before coxrelation. Since each vibrator output is correlated 15 separately, and s~bsequently stacked, we have total control of the compensation of each vibrator and its coupling with the ground. Thus the common situation where one vibrator is on a road, and another on a grass verge (or one on an outcrop, and another on pasture) can be fully compensated;
20 this is a significant advantage over hydraulic vibrators.
In fact, of course, we may choose to apply only partial compensation for the above effects. Thus the general monotonic decrease of sweep rate increases the useful output at low frequencies (as discussed hereinbe-25 fore), and we may elect not to compensate it entirely.The changes of sweep rate introduced by a resonance in the vibrator-ground coupling are beneficial, and in general we would not compensate them. The increase in force OlltpUt at the hlgh frequencies may be benefici.~l iE our problem 30 is resolution, and we may elect to ret:ain its benefit.
The increase in sweep rate when we increase the eccentri--city, however, acts tonegate the advantage. We wo~ld usually choose to compensate this.
There are thus excellent possibilities, with this 35 system, for attaining a desired signal spectrum whether that desired spectrum is flat between wide limits, or whether it incorporates compensation or absorption or other frequency-selective effects along the seismic path.

And all this is done by usin~ the measurement of frequency and sweep rate as a function of time to generate amplitude variations of the master swe~p as a function of frequency.
Also noteworthy is the possihility that this vibrator could be made portable, and thus bring Vibroseis (for the fixst time~ to areas impenetrable by heavy trucks. For this application the prime mover(s3 would run at very high speed, to minimi2e the need for mass in the flywheel. The vibrators, the prime movers and the flywheels would repre-10 sent separate packages, each capable of being man-handled.
A sprung platform would allow the men who carry the equip-ment to double as hold-down weight.
The present invention has been described in terms of a vibrator generating compressional waves. The companlon 15 application Canadian Serial No. 394,254 entitled "Seismic Exploration ~sing Compressional and Shear Waves Simulta-neously" describes a development of this invention for gene-ating cvmpressional and shear WaVQs simultaneously, and a method of seismic exploration based thereon.

Claims (25)

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

1. A seismic vibrator providing a measure of automatic compensation of frequency-selective action in the vibrator-ground coupling, and comprising in combination:
a. a rotatable eccentric weight;
b. a baseplate on which the eccentric weight is mounted and by which the forces generated by the rotation of the eccentric weight may be coupled to the ground;
c. phase-measuring means operatively coupled for recording the phase of the rotating weight; and d. a flywheel and means for coupling the flywheel to said eccentric weight whereby flywheel energy is used to rotate the eccentric weight, said flywheel mounted to decelerate freely between desired frequency limits as the energy is radiated,

2. The vibrator of Claim 1 further including means to vary the eccentricity of the eccentric weight as a function ~ /

of the frequency of rotation.

3. A method of seismic exploration in which the source of seismic vibrations comprises an eccentric weight said method comprising:
rotating said eccentric weight over a frequency range sweeping from a high frequency down to a low frequency using the energy stored in a rotating flywheel, measuring the variation with time of the phase of the eccentric weight and generating a swept-frequency signal derived from said phase measurements, and correlating reflected vibrations against the swept-frequency signal.

4. The method of Claim 3, in which sweeping through the desired frequency range from said high frequency to said low frequency is accomplished by a plurality of vibrators, each of which comprises an eccentric weight driven by the energy stored in a flywheel, and each of said vibrators radiating through separate individual and different portions of the total bandwidth.

5. The method of Claim 3, in which sweeping through the desired frequency range from said high frequency to said low frequency is accomplished by a plurality of vibra-tors, each of which comprises an eccentric weight driven by the energy in a flywheel, and in which the said vibrators are energized in a sequence such that no two vibrators generate the same frequency at times separated by less than the maximum reflection time of interest.

6. The method of Claim 3 further employing a second similar vibrator, and alternately accelerating the flywheel of each vibrator in turn while the other is decelerating and imparting its energy.

7. The method of Claim 3 comprising modulating the amplitude of the said swept-frequency signal as a function of frequency and sweep rate in order to provide a desired overall frequency response.

8. A method of seismic exploration using a pair of contra-rotating eccentric weights comprising:
rotating said contra-rotating eccentric weights by imparting to said weights the energy stored in a freely rotating flywheel thereby sweeping the frequency of rotation from a high frequency down to a low frequency as the energy stored in the flywheel is dissipated;
and measuring the time variation of the phase of rotation of the eccentric weights for correlation with reflected vibrations.

9. The method of Claim 8 further comprising the step of varying the eccentricity of the weights as a specified function of frequency and rate of frequency sweep during the downsweep thereby modulating the amplitude of vibra-tions to provide a desired overall frequency response.

10. The method of Claim 8 further comprising the step of accelerating the flywheel with drive means for rotating said eccentric weight over a frequency range sweeping from a low frequency to a high frequency and for storing energy in said flywheel.

11. The method of Claim 10 further comprising the step of varying the eccentricity of said eccentric weight as a function of frequency.

12. A seismic exploration vibrator providing automatic compensation for frequency dependent impedance at the vibrator-ground coupling comprising:
a baseplate for coupling vibrations to the ground;
at least one pair of eccentric weights mounted on said baseplate for rotation in opposite directions relative to each other thereby to impart periodic forces through the baseplate to the ground;
primary flywheel means for storing energy and operative coupling means for coupling said flywheel and flywheel energy to said eccentric weights;
and phase measuring means to provide a record of the phase of the rotating eccentric weights for use in corre-lation with reflected vibrations.

13. The vibrator of Claim 12 wherein said primary flywheel means is mounted for free rotation whereby said primary flywheel decelerates while imparting energy to the rotating eccentric weights and thereby downsweeps from higher frequency to lower frequency.

14. The vibrator of Claim 12 wherein said at least one pair of eccentric weights comprises a pair of secondary flywheels or rotors mounted on said baseplate for rotation in opposite directions relative to each other said secondary flywheels or rotors affording secondary inertia for storing energy, each said secondary flywheel or rotor having eccentric weight means mounted thereon.

15. The vibrator of Claim 14 further comprising means for translating said eccentric weight means radially on the respective secondary flywheels thereby varying the amplitude of forces imparted to the ground.

16. The vibrator of Claim 15 wherein said means for translating the eccentric weight means radially on the res-pective secondary flywheel comprises lead screw means mounting said eccentric weight means on said secondary flywheel, servo motor means mounted in stationary position relative to said secondary flywheel for turning said lead screw means, and differential gearing means for coupling said servo motor means to the lead screw means on said rotating secondary flywheel.

17. The vibrator of Claim 16 further comprising servo motor control means responsive to the frequency of rotation of the respective secondary flywheels for driving said servo motor means as a function of said frequency of rotation.

18. A swinging weight vibrator for the generation of swept-frequency waves in seismic exploration, and which automatically provides a measure of compensation for the frequency dependence ] of the vibrator-ground coupling, comprising:
drive means to provide rotational energy;
flywheel means by which said rotational energy may be stored;
first coupling means between said drive means and said flywheel means, by which the flywheel can be accelerated to a desired first frequency;
a rotatable swinging weight constituting the vibrating element;
baseplate means by which the forces generated by the vibration element may be coupled to the earth;
disabling means by which the forces generated by the vibrating element may be effectively annulled during acceleration and spatial movement of the vibrator;
means to cause effective release of said first coupling, so that said flywheel and said vibrating element decelerate freely to a desired second frequency while radiating seismic waves into the ground; and monitoring means by which a signal can be derived representative of the rotation of the vibrating element.

19. A vibrator according to Claim 18, in which said disabling means includes means for controllably varying the eccentric mass and the eccentric radius of the swinging weight.

20. A vibrator according to Claim 18, in which said disabling means includes a phasing control between two like vibrating element, so that by variation of the phase the individual outputs of the two vibrating elements can supplement each other or cancel each other.

21. A compound vibrator formed of two vibrators each according to Claim 18 in which said drive means is common to the two vibrators and in which said first coupling means in each vibrator is a clutch engaged in one vibrator and disengaged in the other, thereby providing acceleration of one vibrator during decelerationof the other.

22. A compound vibrator formed of two vibrators each according to Claim 18 in which said drive means is common to the two vibrators and in which said first coupling means is a differential driving both vibrators and constructed and arranged for providing acceleration of one vibrator during deceleration of the other.

23. A compound vibrator formed of two vibrators each according to Claim 20 in which said drive means is common to the two vibrators and in which said first coupling means in each vibrator is a clutch engaged in one vibrator and disengaged in the other, thereby providing acceleration of one vibrator during deceleration of the other.

24. A compound vibrator formed of two vibrators each according to Claim 20 in which said drive means is common to the two vibrators and in which said first coupling means is a differential driving both vibrators and constructed and arranged for providing acceleration of one vibrator during deceleration of the other.

25. A vibrator according to Claim 18 and providing a very light baseplate, in which only said rotatable swinging weight is mounted on said baseplate and in which said second coupling means includes a flexible member.