VIBRATOR SEISMIC SOURCE This invention relates generally to improvements to seismic sources used for geophysical examination of underground strata. In particular this specification deals with a vibrator seismic source (VSS) which is powered hydraulically. The main application of the invention described in this specification is for the generation of seismic compression and shear waves at the surface of the earth but equally the invention could be useful for generation of seismic compression and shear waves underground in mines and at the ocean bottom surface and also could be useful for generation of acoustic waves underwater for transmission through water for the purpose of sound navigation and ranging. BACKGROUND ART

A VSS according to the invention is powered hydraulically to take advantage of the low compressibility of hydraulic oils or other working liquids in the transfer from hydraulic energy within the VSS to wave energy in a transmitting medium, for example the earth, including mineral deposits and water. The low compressibility characteristic of hydraulic oils naturally leads to simple control of an output member of the VSS which applies forces cyclically to the transmitting medium. Control of the flow rate of the hydraulic oil into one end of a single acting hydraulic cylinder fitted with a slidably mounted piston in turn rigidly connected to the fijrst end of a piston rod enables control of the motion of the output member which is connected rigidly or by thrust bearing means to the second end of the piston rod. A more complete and detailed description of a preferred form of VSS according to the invention is given later by reference to the main application of the invention for generation of seismic compression waves in a transmitting medium to obtain lineal resolution of strata in the transmitting medium of around two metres together with

seismic penetration into the transmitting medium of approximately one kilometre. This particular application of the invention is useful in geophysical examination of coal deposits, for example in the determination of coal seam burial depth, extent of seam faulting, extent of seam parting and extent of longitudinal seam continuity all of which are important in the economics of planning and operation of coal mines.

This invention incorporates different design and construction and a different central principle of operation as compared to existing vibrator sources.

One vibrator source in widespread use for oilfield exploration worldwide and also used for coal exploration in Western Europe is the "Vibroseis" (a trade mark of Continental Oil Company Inc.) system originally developed by Continental Oil Co. of America. The principal difference between the VSS of the present invention and the "Vibroseis" system is that the former uses a single acting hydraulic cylinder for generation of cyclic compression waves in the transmitting medium, whereas the "Vibroseis" system uses a double acting hydraulic cylinder for generation of cyclic compression waves in the transmitting medium. The double acting cylinder in the "Vibroseis" system involves the use of hydraulic oil injection into two mutually inclusive ends of a double acting cylinder whereas the VSS uses hydraulic oil injection into a single end of a single acting hydraulic cylinder.

The present invention and the "Vibroseis" system both may be used in conjunction with a common data processing method of reflection seismology, for example, in one application of the use of the VSS according to the invention, this is the cross-correlation method of signal processing, in which for time varying swept frequency operation of a VSS according to the invention and the "Vibroseis" system a pilot signal representing cyclic compressing waves, ie. time

varying swept frequency compression wave signal, ie. wave train, ie. input signal, ie. sweep, to said transmitting medium is cross-correlated against time varying signals received by geophones which sense reflections, of the input signal, from the strata. Cross-correlation of the pilot signal and the geophone signals together with a definition of zero time of the pilot signal then specifies in the time domain the location of reflectors, ie. strata in the transmitting medium. This method of reflection seismology processing is based, upon the original work on radar by Klauder.

DISCLOSURE OF THE INVENTION

The present invention consists in a vibrator seismic source consisting of an output member adapted to be brought into contact with a transmitting medium and apply forces cyclically thereto, a single acting hydraulic cylinder having a piston rod therein one end of which is connected to the output member, an inlet for hydraulic liquid under pressure into the cylinder, a source of hydraulic liquid under pressure, valve means for controlling the intermittent supply of hydraulic liquid into the cylinder to apply a force to the piston and for returning hydraulic liquid for repressurisation.

In a preferred form the invention consists in a vibrator seismic source incorporating the principle of single action operation and comprising an output member whose contact area is coupled with a seismic t ansmitting medium, for example the earth's surface, the output member including structural stiffening means to withstand generation of compression waves or shear waves or other waves in the transmitting medium, the centroid of the output member contact area being connected rigidly or by accommodating thrust bearing means to a second end of a piston rod, the first end of the piston rod being connected rigidly to a generally cylindrical piston which co-operates and seals with the internal cylindrical surface

of a vibrator cylinder whose second end is fitted with sliding bearing means for the piston rod to allow sliding and guiding of the piston rod in the direction of the common centreline of the piston and the piston rod between the extremities of the piston rod first end and the piston rod second end, the first end of the vibrator cylinder incorporating a liquid inlet port for liquid injection directly from the outlet port of flow control means whose purpose is to shape the flow rate versus real time of the liquid injection conveyed directly to the first end of the vibrator cylinder through the liquid inlet port, the inlet port of the flow control means being connected to a high pressure hydraulic liquid supply, and whilst the liquid injection occurs to the vibrator cylinder first end there being accompanying exit of air from the vibrator cylinder second end through an air outlet port, the vibrator cylinder first end being connected rigidly to additional mass so that the total mass of the vibrator cylinder and attachments thereto constitutes reaction mass whose inertia provides reaction to the liquid injection during generation of said waves by the output member, the output member contact area being coupled to the transmitting medium by a first coupling part force consisting of a near constant force applied to the output member after transmittal through spring suspension means whereby the line of action of the first coupling part force is predominantly in the direction of the common centreline of the piston ari'd the piston rod, the first coupling part force being supplied by gravitational means, the output member contact area being coupled to the transmitting medium by a second coupling part force consisting of the force on the output member as a result of the liquid injections so that the combination of the first coupling part force and the second coupling part force results in generation of desired waves in the transmitting medium, the vibrator cylinder and other reaction mass in

assembly with the flow control means and the high pressure supply of hydraulic liquid collectively, whose line of motion is guided by external means so that the collective line of motion is substantially the same as the common centreline of the piston and the piston rod, the vibrator cylinder being fitted with flow control means so that after completion of liquid injection the injection liquid is returned from the vibrator cylinder first end to hydraulic means for re-pressurisation and re-entry to the high pressure hydraulic liquid supply.

One embodiment of this invention is shown in fig. 1 and is used now in a detailed description of the operation of the

VSS by way of example. This detailed description illustrates the main application of the invention and also illustrates the best method of carrying out the invention for the purpose of generation of cyclic compression waves on the earth's surface. Said method of reflection seismology processing is also applicable in conjunction with the detailed description of the VSS, although this is only one of several methods of signal processing that may be used in conjunction with the

VSS, since said swept frequency operation of the VSS is only one of several methods of operation of the VSS.

In order that the nature of the invention may be better understood a preferred form thereof is hereinafter described, by way of example, with reference to the accompanying drawings in which:- ψ Fig. 1 is a partly sectioned elevation of a VSS according to the invention mounted on the rear of a vehicle.

Fig. 2 is a cross-sectional view on line A-A of Fig. 1 of a servo valve assembly controlling the flow of hydraulic oil, and.

Fig. 3 is a diagram showing the hydraulic circuit of the servo valve assembly of Fig. 2.

MODE FOR CARRYING OUT THE INVENTION Fig. 1 shows a partly sectioned exterior arrangement

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drawing of a VSS according to the invention mounted at the rear of a vehicle. A single acting hydraulic cylinder 1 is shown fitted internally with a slidable piston 2 connected rigidly to one end of piston rod 3. The other end of piston rod 3 is connected rigidly or by a spherical thrust bearing 4 to a vibrator output member 5 in the form of an earth contact plate. The piston rod 3 is guided slidably by neck bearing 6 at the lower end of the hydraulic cylinder 1. Whilst the VSS is operating, hydraulic oil 7 is injected into the upper end of the hydraulic cylinder 1 and occupies volume 8; also whilst operating there is air 9 occupying the internal volume at the lower end of the hydraulic cylinder 1. Air is exhausted from the lower end of the hydraulic cylinder whilst hydraulic oil 7 is injected into the upper end. The flow rate of hydraulic oil 7 is controlled by a servo-valve assembly 10 which is also shown in a cross-sectional view on line A-A in Fig. 2. The servo-valve assembly 10 consists of two major assemblies namely a flow valve 11 and an electrohydraulic servo-valve 12. Typically, the servo-valve 12 is, for example, a Moog Inc. type 760 Y902 valve using the well known electromagnetic effect to operate the first stage of this two stage valve which, for example, has built in means for mechanical feedback from the second stage to the first stage. The servo-valve 12 is predominantly a flow control device whose output flow rate of hydraulic oil is proportional to electric current applied to the electromagnetic means. * The servo-valve is capable of operating at up to frequencies of 500 Hz and higher to give controlled flow rates of hydraulic oil 13 and 14 from a high pressure supply of hydraulic oil 15. Hydraulic oil 16 exhausted from the servo-valve assembly 12, including any internal hydraulic oil leakage from the servo-valve, is returned to a separate hydraulic power supply for reuse. The controlled flow rate of hydraulic oil 13 acting in the direction shown in Fig. 3 controls the opening motion of the

flow valve 11 by acting on piston 17 which is either attached rigidly to a first end of poppet valve 19 or is attached by accommodating and sealing means to that end of the poppet valve 19 whereby motion of piston 17 in the direction of the longitudinal centreline of poppet valve 19 is transferred directly to the poppet valve 19 but whereby any motion of the piston 17 at right angles to the longitudinal centreline of the poppet valve is accommodated by the accommodating and sealing means so as to allow for geometry tolerances in manufacturing whose ^* effects are present in assembly of the flow valve 11. The controlled flow of hydraulic oil flow 14 acting in the direction shown in fig. 3 controls the closing motion of the poppet valve 19 by acting on piston 18 which is attached to the second end of the poppet valve 19 by means similar to those described for attachment of the piston 17 to the poppet valve 19. Alternatively, the piston 18 may be free of any mechanical attachments to poppet valve 19 and . therefore the piston 18 simply contacts the second end of the poppet valve 19 as a result of hydraulic force due to hydraulic oil flow 14 acting on piston 18. There is an internal hydraulic balance passage connecting the first end and the second end of the poppet valve 19 to prevent hydraulic locking of the poppet valve 19. In addition there is a helical spring 20 which maintains contact between the piston 18 and the poppet valve 19. The spring 20 is also useful to keep the poppet valve 19 in sealing contact with seat 21, during periods wh^ft a controlled flow of hydraulic oil 14 does not exist, thereby sealing the hydraulic oil injection 7 and thus preventing hydraulic oil 7 from entering cylinder 1 during periods when output member 5 is not required to vibrate on the earth's surface. Sealing of hydraulic oil injection has the important advantage that power wastage is eliminated because hydraulic oil leakage from high pressure hydraulic oil supply 22 is eliminated during periods when the output member is not required to

vibrate on the earth's surface. Poppet valve 19 has sliding bearing and sealing means 46 for the first end cylindrical portion and second end cylindrical portion to enable the poppet valve to slide longitudinally and seal in the body of the flow valve 11. Reduced cross-sectional areas of pistons 17 and 18, which are for example one quarter of the cross-sectional area of the first and second end cylindrical portions of poppet valve 19, enable enhancement of the hydraulic oil flow rate from flow valve 11 and therefore enable increased injection 7 due to increased longitudinal displacement of poppet valve 19 from seat 21 for a given flow rate 13, provided that the high pressure hydraulic oil supply 22 is adequate.

Sensitive and responsive operational performance of servo-valve assembly 10 to result in the desired hydraulic oil injection into the hydraulic cylinder 1 first end is achieved through the use of feedback control, for example, using well known electronic feedback control techniques.

The feedback control includes a first control for the pressure difference between hydraulic oil flow 14 and hydraulic oil flow 13 to ensure, during periods whilst the ground contact plate is not required to vibrate on the earth's surface, that poppet valve 19 is in sealing contact with the seat 21 to preven oil injection. The first control involves, for example, the use of a differential pressure transducer to convert the pressure difference to an electrical signal for feedb Ta-ck control. The first control ensures that the pressure of hydraulic oil flow 14 acting on piston 18 results in a small magnitude net force to keep poppet valve 19 in sealing contact with seat 21. This first control has the added advantage of ensuring that the second stage of the servo-valve 12 is sensitive and responsive to enable initiation of cyclic compression waves in the transmitting medium. The feedback control includes a second control which

incorporates the use of a linear voltage displacement transducer, for example, rigidly connected to piston 17 to measure the displacement of poppet valve 19 from the seat 21. The second control also incorporates the use of a transducer mounted rigidly on the ground contact plate to measure, for example, the acceleration, the velocity and the displacement of the ground contact plate while these parameters vary with time during a sweep. The second control collectively may be used for feedback control of cyclic compression waves in the transmitting medium. A commercially available vibrator controller manufactured by Texas Instruments Inc., for example, is capable of such second control collectively to enable the time varying amplitude of sinusoidal compression waves in the transmitting medium to be controlled through the measurement of time varying ground contact plate acceleration and comparison of time varying ground contact plate acceleration with a desired and previously specified time varying reference sweep frequency signal generated using microprocessor based instrumentation. The vibrator controller includes the use of phase compensation techniques to allow for phase differences between the time varying ground contact plate acceleration and the reference signal.

Fig. 1 shows a single acting hydraulic cylinder 1 with the servo-valve assembly 10 rigidly connected to the upper end of the cylinder 1. This arrangement provides the

T shortest hydraulic oil path ^' and the most direct hydraulic oil path for oil injection. It is advantageous to have the shortest hydraulic oil path for oil injection since this results in the highest resonant frequency when considering the hydraulic stiffness of the oil path and the associated resonating mass whether the mass is the total first mass consisting of piston 2 plus piston rod 3 plus thrust bearing 4 plus earth contact plate 5 and plus a stiffening frame 23 or whether the mass is the total second mass consisting of

cylinder 1 plus servo-valve assembly 10 hydraulic oil plumbing 24 plus accumulator mountings 25 and 26 plus hydraulic accumulators 27, 28 and 29 plus hydraulic plumbing 30 plus added reaction mass 31 plus various bolted attachments 32 plus sliding means 33. The resonant frequency determined by the shortest hydraulic oil path and the first mass is approximately 550 Hz and the resonant frequency determined by the shortest hydraulic oil path and the second mass is approximately 100 Hz. Hydraulic oil plumbing 24 conveys hydraulic oil to hydraulic accumulators 27 and 28 when the separate hydraulic power supply is charging the accumulators with hydraulic oil during times when the ground contact plate is not required to vibrate on the earth's surface. During times when the ground contact plate is required to vibrate on the earth's surface hydraulic oil plumbing 24 conveys a high flow rate of hydraulic oil to flow valve 11 for oil injection 7. The plumbing 24 together with accumulators 27 and 28 collectively provide the high pressure hydraulic oil supply 22. When the magnitude of the separate hydraulic power supply is large enough then the separate supply may provide a significant part of the high flow rate of hydraulic oil, though to reduce equipment costs the separate hydraulic power supply will not be large enough to provide a significant part of the high flow rate of hydraulic oil. A balance is needed in choosing the size of the separate hydraulic power supply so that the accumulators 27 and 28 may be recharged quickly enough to minimise the time between sweeps. The accumulators 27, 28 and a further accumulator 29 are the usual commercial nitrogen filled rubber bladder type. The accumulator 29 provides the high pressure hydraulic oil supply 15 for the servo-valve 12 and accumulator 29 is charged with hydraulic oil by the separate hydraulic power supply. Accumulators 27, 28 and 29 all help to dampen out resonance. The hold-down mass and low stiffness suspension is now

described which basically maintains contact between the vibrator output member 5 and the earth's surface during sweeps. The lower portion of stiffening frame 23 is rigidly attached to the ground contact plate 5 to increase the structural stiffness of the ground contact plate 5 to minimise deflections due to forces on the ground contact plate 5 so that the displacement of the earth's surface is nearly uniform over the area of contact of the ground contact plate and the earth's surface. The low stiffness suspension consists of multiple, for example four, rubber suspension members 34 or other low stiffness means whose stiffness is small compared to the stiffness of the transmitting medium. The simple reason for the use of a low stiffness suspension is to minimise the vibration displacements of hold-down mass compared to displacement of the transmitting medium in contact with the vibrator output member 5 during sweeps. The mathematical model of the hold-down mass motion is well known. Double acting hydraulic powered lift cylinders 35 fitted internally with pistons 36 and piston rods 37 are used to lift the vehicle clear of the earth's surface. Piston rods 37 are flexibly attached to a suspension frame 38 in turn rigidly attached to the upper ends of the low stiffness suspension members 34. The lower ends of the low stiffness suspension members 34 are rigidly attached to the upper part of the stiffening frame 23. The lift cylinders are rigidly attached to a frame 39 in turn rotatably connected to a γ trolley 40. The trolley 40 ^' is slidably attached to beams

41. The lower portions of the beams 41 are rigidly attached to the upper surface of a vehicle 42, for example a truck or a trailer. The vehicle 42 is fitted with the usual suspension springs 43 together with road wheels and axles 44.

The apportioned amounts of all masses making up the total of the hold-down mass on the ground contact plate consist of the sum of all the vertical reaction components on the ground contact plate of the individual masses of the road

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wheels and axles 44, the springs 43, the vehicle 42, the beams 41, the trolley 40, the frame 39, the lift cylinders 35, the pistons 36, the piston rods 36, the suspension frame 38 and if needed additional hold-down ballast 45. The frame 39 provides a slideway for the sliding means 33 as well as being part of the vehicle lift system which makes use of the cylinders 35 to lift the rear end of the vehicle so that the wheels 44 are clear of the earth during vibration on the transmitting medium. After completion of a sweep, hydraulic oil occupying volume 8 is returned to a separate hydraulic power supply through an electric operated solenoid valve not shown in fig. 1. The solenoid valve enables the volume 8 to return to its minimum value for the commencement of a further sweep. After completion of all sweeps required in any one location on the transmitting medium whilst the VSS is in the configuration as shown in fig. 1, the lift cylinders 35 are operated to lower the road wheels at the rear of vehicle down to the earth's surface. Continued operation of the lift cylinders then results in elevation of the vibrator output member 5 clear of the earth's surface. The rotatable connection of frame 39 to the trolley 40 then enables rotary movement of the frame 39 to a near horizontal position powered by a double acting hydraulic cylinder, i.e., a tilt cylinder, not shown in fig. 1. The trolley 40, complete with the frame 39 and all of the elements rigidly and slidably attached to the frame 39, is then able to slide in beams'" 41 to result in distribution of the total weight of the VSS more evenly on, for example, four road wheels of the vehicle; two of these four road wheels being the wheels 44 at the rear of the vehicle and the remaining two road wheels being at the front of the vehicle. The VSS may be represented by a mathematical model to take account of the dynamics of the VSS including the coupling of the VSS to the transmitting medium. The mathematical model includes the flow rate of hydraulic oil from the flow valve

11 to the first end of cylinder 1. The high pressure hydraulic oil supply 22 provides the flow rate by direct connection through the plumbing 24 to an inlet chamber 47 of the flow valve 11 shown in fig. 2. The previously described control of poppet valve 19 enables hydraulic oil from chamber 47 to flow initially to a chamber 48 before leaving flow valve 11 to become injection flow 7. The flow path taken by hydraulic oil is predominantly in a straight line from the supply 22 through flow valve 11 in the direction shown for the injection flow in fig. 1.

During an injection flow 7, whilst ground contact plate 5 results in one compression wave, i.e., one cycle, for example on the transmitting medium, the displacement motion of the plate 5 may be sinusoidal, for example, when graphed against real time. During the compression cycle the pressure of hydraulic oil in the upper end of hydraulic cylinder 1 varies. The lowest magnitude of this pressure does not cause cavitation of hydraulic oil in volume 8 or in chamber 48 because of the existence of the pressure for an insufficient length of time. Experimental tests have shown for example that cavitation does not occur at any time during operational performance of a VSS according to the invention on the transmitting medium for swept frequency operation during generation of the cyclic compression waves over a range of operational frequency of the cyclic compression waves of from 20 Hz to 500 Hz. The operational performance of the invention therefore involves a different principle of operation hereby defined as single action operation as compared to all double acting vibrator sources for example "Vibroseis" apparatus.

It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention as described above without departing from the spirit or scope of the invention as defined in the succeeding

claims.

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