GB2187553A - Seismic wave generator with shock absorber - Google Patents

Abstract

An impact type surface seismic source propels a heavy mass 24 from a raised position downwardly to strike an earth contacting base plate 12 which couples the resultant energy into the earth as a seismic wave. Hydraulic cylinders 52 have piston rods 50 which are linked with the mass via a flange 46 and engage it in its downward position, lifting it rapidly from the base plate to its original raised position. Cushioning means (hydraulic means and/or spring assemblies) are engineered in to the lift system in order to control the rate of increase of upward force exerted upon the mass as lift is applied. This significantly reduces the secondary seismic shock wave which would otherwise result from the rapid removal of the weight of the mass from the base plate. <IMAGE>

Description

SPECIFICATION Apparatus for generating a seismic wave in the earth BACKGROUND TO THE INVENTION 1. Field of the Invention This invention relates generally to the field of seismic prospecting and more particularly to a method and apparatus for injecting an acoustic pulse into the earth by means of a repetitively cycled impact mass.

2. Description of the Prior Art United States Patent Number 4,284,165 issued August 18, 1981 to Tom P. Airhart et al entitled "Acoustic Pulse Generator" describes an impactor for generating an acoustic pulse in a medium wherein a heavy mass is driven downwardly from a raised position with great force by means of compressed gas to strike an earth-contacting base plate which couples the attained kinetic energy of such mass into the earth. Reflections of the resultant seismic wave from subsurface layers are recorded remotely in a manner well known in the art. The apparatus provides means for retrieval of the mass after each shot. Such means include single rod-end hydraulic cylinders whose downwardly extending piston rods pass slidably through apertures penetrating a flange portion of the mass.Preparatory to operating the apparatus with the mass in a raised position above the base plate the piston rods are lowered sufficiently so that lift rings secured to their bottom ends lie below the mass flange when the mass has traveled to its downward most position against the base plate. The hydraulic cylinders are then operated to retract the piston rods, causing the lift rings to engage the mass flange and rapidly return the mass to its original position where it is secured by latching means for the next shot.

Observation of noise spreads taken after such apparatus is fired confirms that when the lift rings strike the mass flange and lift the mass away from the base plate considerable shock results which is transmitted to the earth thereby producing an unwanted secondary wave in the seismic range. If the mass is lifted within a time interval during which the reflected seismic wave is being recorded noise contamination of such record due to such secondary wave adds to that inescapably present due to surface effects and other environmental interference. Analysis of seismic records shows that the amplitude of this secondary wave is typically on the order of 18db below that of the primary seismic wave of interest.

Even with present state of the art seismic data processing techniques it appears that a signal of this amplitude cannot be entirely eliminated from the resultant record.

One solution to the above problem is to program a "listening pause" into the operation during which the mass remains in contact with the base plate. The recording instruments are then turned off and the mass is then retrieved for another shot. However, this interruption of the mechanical up and down cycle of the mass may easily increase the effective time between shots by a factor of 2. When time is of the essence in a seismic field operation such delay adds significantly to total operating costs and otherwise interferes with efficiency.

SUMMARY OF THE INVENTION The present invention contemplates an impact type seismic type source which employs means for suspending a heavy mass in a raised position above an earth-containing base plate, means for propelling the mass downwardly so as to strike the base plate, which couples the the resultant energy into the earth, means for engaging the mass and lifting it from the base plate to its original raised position and means for cushioning the acoustic shock transmitted to the earth by the application of such lift force to the mass.In a more limited aspect the invention comprehends apparatus of the type described wherein hydraulic cylinders means secured to the mass suspension means are slidably linked with the mass so as to enable the mass to travel freely to its downward most position, such hydraulic cylinder means being thereafter operable to engage and apply lift force to such mass. Hydraulic accumulator means are interconnected in the fluid system of such hydraulic cylinder means for gradually increasing the fluid pressure thereof responsive to the application of such lift force. In a still further aspect of this invention the hydraulic cylinder means engage the mass by means of lift rods which communicate lift force to the mass through elastically deformable springs.

It is therefore a general object of this invention to provide an improved apparatus for retrieving the downwardly-driven mass of a repetively operated impact-type seismic source.

It is a more particular object of this invention to provide such an apparatus wherein the mass retrieval is accomplished without producing unacceptable secondary noise.

Other and further objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRA WINGS Figure 1 is a simplified elevational view of the apparatus of this invention partially sectioned showing an impact mass suspended above an earth contacting base plate preparatory to firing.

Figure 2 is a further elevational view of the apparatus of Fig. 1 partially sectioned wherein the mass is in transit to the base plate and the piston rods of the hydraulic lift cylinders are in a downwardly extended position.

Figure 3 is a further elevational view of the apparatus of Fig. 1 rotated through an angle of 90 degrees to illustrate the manner in which dual hydraulic lift cylinders engage the opposite sides of the mass.

Figure 4 is a detailed elevational view showing the mass of the apparatus of this invention in its impact position against the base plate.

Figure 5 is a further detailed elevational view showing a moved position of the mass of the apparatus of this invention as it is lifted toward its original position.

Figure 6 is a diagrammatic illustration of the fluid circuit of the hydraulic lift cylinders of the apparatus of this invention.

Figure 7 is a detailed vertical section showing the construction of the bottom ends of the hydraulic lift cylinder rods in accordance with an alternate embodiment of this invention.

Figure 8 is a detailed vertical section showing the construction of the bottom ends of the hydraulic lift rods in accordance with a further alternate embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 1, there is illustrated a seismic source comprising actuator 10 supported above base plate 12 in any suitable manner on horizontally extending frame 14 of a transport vehicle (not shown). Actuator 10 is an impact type seismic device which utilizes high pressure air to accelerate a mass to a target as generally described in United States Patent 4,284,165 issued August 18, 1981 to Tom P. Airhart and entitled "Acoustic Pulse Generator". Actuator 10 consists generally of a hollow, upstanding cylindrical housing 16 having a bore 18 lined with bronze bushing 19 which receives the upper end of an impact piston 22, the lower end of which enlarges in diameter to form an impact mass 24.The operation of actuator 10 causes mass 24 to be proposed rapidly downward by pressure provided froma plurality of pressure vessels 28 so as to strike base plate 12 which couples the attained kinetic energy of mass 24 into the earth as a seismic wave of acoustic frequency.

Fig. 1 shows piston 22 iatched in its upper most or "top" position in which its top surface 30 is closely adjacent the closed upper end 32 of housing 16. Any air trapped between end 32 and surface 30 can be vented to atmosphere through passage 33 by operation of valve 34. In this position the sidewalls of piston 22 block ports 36 communicating between the interior of bore 18 and pressure vessels 28. Hermetic seal 37 recessed within bushing 19 at the top of bore 18 prevents high pressure air from leaking above piston 22.

The respective ends of piston rods 38 of a pair of trigger cylinders 40 supported on opposite sides of housing 16 are connected to a pair of hinged latches 41 which project beneath and suppott flange 46 of mass 24 in the top position of piston 22. Cylinders 40 and latches 41 are fixed to housing 16 respectively by mounting brackets 42 and 43.

When a firing signal activates trigger cylinders 40 their piston rods extend and in so doing swing hinged latches 41 away from under flange 46 as best seen in Fig. 2. As mass 24 drops under the force of gravity, high pressure air, typically on the order of 300 psi, enters bore 18 along path 47 through ports 36 and is exposed to top surface 30 of piston 22.

This provides a powerful downward force across the entire cross sectional area of top surface 30 which aids gravity so as to drive piston 22 and thus to propel mass 24 into base plate 12 with great force. Hermetic seal 48 recessed within bushing 19 adjacent the lower end of bore 18 prevents air from exiting donwardly from bore 18 and thus insures that actuator 10 operates as a closed air system.

In other words, air is continuously cycled back and forth between pressure vessels 28 and bore 18 with repeated firings.

As best seen in Fig. 3 the piston rods 50 of hydraulic cylinders 52 affixed to opposite sides of housing 16 by brackets 53 project slidably through vertical apertures 54 penetrating flange 46 and terminate in cylindrical lift caps 58. Preparatory to operation of actuator 10 rods 50 are lowered to assume the dotted position shown. Mass 24 necessarily slides downwardly on rods 50 as actuator 10 is fired. When mass 24 has impacted base plate 12, as seen in Fig. 4, the underside of flange 46 is positioned slightly above caps 58. Upon completion of a firing operation cylinders 52 are actuated to retract piston rods 50 causing caps 58 to engage the under surface of flange 46 and, as shown in Fig. 5 to lift mass 24 toward its original latched position of Fig. 1.

At the same time pressure vessels 28 are being repressurized to substantially their original air pressure. Hydraulic cylinders 52 are designed to "bottom out" just as the top surface 30 of piston 22 overlaps top seal 37.

In the operation described unwanted acoustic shock results not only from the impact of caps 58 against flange 46 but also by reason of the fact that base plate 12 is suddenly relieved of the weight of mass 24, which typically is on the order of 5,000 pounds. While the exact nature of the resultant secondary seismic wave is still being analyzed it appears to ge due, at least in part, to a sudden relief of elastic strain in the earth.

In order to cushion this shock the fluid circuit of each hydraulic cylinder 52 is preferably as shown in fluid supply circuit 59 illustrated in Fig. 6. In accordance with well known prin cipls a pump 60 provides hydraulic fluid under pressure alternately to opposite sides of piston 62 of cylinder 50 through two-way valve 64 so as to either extend or retract piston rod 52. Tanks 68 and70 are available to accommodate idling of pump 60 or fluid overflow through valve 64. A hydraulic accumulator 72 is connected across the fluid line to the high pressure, rod side of piston 62. When lift caps 58 impact flange 46 the acoustic shock is absorbed at least in part by accumulator 72 and the unwanted secondary seismic wave is snigificantly attenuated.This results from the fact that accumulator gradually increases the fluid pressure in cylinder 50 as lift force is applied until sufficient force is present to exceed the combined weight of mass 24 and piston 22. Accumulator 72 may be any of various conventional types utilizing either gas or springs as a compressible medium which is separated by a rigid or flexible means from the incompressible hydraulic fluid. In a typical case wherein the circuit of Fig. 6 utilizes a maximum fluid pressure of 3,200 p.s.i. a 2 1/2 gallon accumulator of the nitrogen bladder type is found to work well and operate within its elastic limits.

In accordance with an alternate form of structure within the scope of this invention the piston rods 50 of cylinders 52 are provided with respective cylindrical spring assemblies 74 instead of lift caps 58. Each assembly 74 includes a plurality of parallel coil springs 76 spaced around rod 50. The lower ends of spring 76 are recessed within upwardly opening sockets 78 fixed to disk 80 formed at the end of rod 50. The upper ends of springs 76 are fastened to a disk 82 which floats with respect to rod 50. With impact of disk 82 on flange 46 springs 76 are compressed to absorb the shock of such impact. The time rate of increase of the upward force exerted against mass 24 is a function of the particular spring constants selected. Preferably assembly 74 is designed such that the maximum lift force is equaled by the reaction of springs 76 without exceeding their elastic limits.

A modified version of the assembly 74 consists of a further spring assembly 84. In this construction an upper disk 86 and a lower disk 88 are interconnected by longitudinal pins 90. The upper ends of expansion springs 92 surrounding pins 90 are fastened to disk 94 at the lower end of rod 52. The lower ends of sprins 92 are fastened to disk 88. With impact of disk 86 upon flange 46 springs 92 are stretched by continued upward movement of disc 94 until equilibrium with maximum lift force is reached.

If desired both types of shock absorber means described above may be combined for a more positive control. For example, in a particular instance the hydraulic accumulator circuit of Fig. 6 was combined with the use of spring assembly 74. In this way the reduction in amplitude of the resultant secondary seismic wave relative to the primary seismic wave was increased from 18 db to 40 db. This reduction is sufficient to substantially eliminate the effect of such secondary seismic wave upon subsequent seismic analysis and interpretation.

An advantage of this invention lies in the fact that it is difficult to insure that lift caps 58 or alternatively spring assemblies 74 or 84 impact opposite sides of flange 46 precisely at the same instant. The introduction of shock absorber means as described significantly reduces the likelihood of unbalanced lift forces on mass 24 which could cause damage or malfunction.

The shock absorption techniques described and shown are not intended to be all inclusive. For example, within the scope of this invention one may employ the cushioning effect of air sprigns in conjunction with hydraulic lift means. In addition, it may be noted that techniques as described herein are applicable within the scope of this invention to the related problem of picking up the base plate itself when moving the transport vehicle from one location to another. Any seismic source, whether of the impact or vibratory type which utilizes a heavy mass, particularly in repetitive operations, may face the need to decrease the unwanted secondary shock of suddenly removing the weight of such mass from the earth. As the sophistication of seismic interpretation and analysis increases the need for minimizing such secondary effects becomes more significant. Within the scope of this invention appropriate shock absorption means may be instituted.

Claims (9)

1. Apparatus for generating a seismic wave in the earth comprising: (a) a base plate for coupling acoustic energy in to the earth as a seismic wave; (b) an impact mass; (c) means for suspending said mass in a raised position spaced above said base plate; (d) means for causing said mass to travel downward from said position of suspension to strike said base plate, thereby generating said seismic wave; (e) means for exerting a force upon said mass adapted to lift said mass from said base plate to said raised position; (f) shock absorber means through which said lift force is exerted such that the magnitude thereof is gradually increased to a value sufficient to exceed the weight of said mass.

2. Apparatus as in Claim 1 wherein said lift means comprises hydraulic cylinder means secured to said suspension means and adjustably linked with said mass so as to permit such downward travel and the exertion of said lift force.

3. Apparatus as in Claim 2 wherein said shock absorber means comprises hydraulic ac cumulator means which regulate the fluid pressure of said hydraulic cylinder means responsive to the application of said lift force.

4. Apparatus as in Claim 2 wherein said shock absorber means is mechanical spring means.

5. Apparatus as in Claim 2 wherein said shock absorber means is air spring means.

6. Apparatus for generating a seismic wave in the earth comprising: (a) a base plate for coupling acoustic energy in to the earth as a seismic wave; (b) an impact mass comprising an upstanding cylindrical body having a laterally projecting flange penetrated by circumferentially spaced vertical apertures; (c) means for suspending said mass in a raised position spaced above said base plate; (d) means for propelling said mass downwardly from said position of suspension to strike said base plate, thereby generating said seismic wave; (e) a plurality of double acting hydraulic cylinders having their blind ends affixed to said suspension means and having piston rods which project through said apertures in said flange in axial alignment with the path of said mass, the ends of said piston rods being extendable beneath the position of said flange in the downward most position of said mass; ; (f) an enlarged diameter lift member affixed to each of said piston rod ends and adapted to engage said flange upon retraction of each of said piston rods so as to exert lift force upon said mass; (g) shock absorber means operably interconnected between said flange and each of said enlaged lift members so as to attenuate the shock of such engagement.

7. Apparatus as in claim 6 wherein each said shock absorber means is mechanical spring means adapted to be elastically strained by the application of said lift force.

8. Apparatus as in claim 6 wherein each said shock absorber means is an air spring.

9. Apparatus as in claim 6 additionally including hydraulic accumulator means introduced in the fluid supply circuit of said hydraulic- cylinders so as to regulate the fluid pressure therein responsive to the application of said lift force.