Acoustic Pulse Generator

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.

2. Description of the Prior Art

In Applicant's co-pending, commonly assigned applications, Ser. No. 3,107,009 filed Dec. 21, 1979 entitled Acoustic Pulse Generator, truck-mounted apparatus is described wherein gas pressure may be employed to propel an impact piston downwardly against a target plate located adjacent the lower end of the housing. The compressed gas is provided by one or more accumulators and introduced through large area ports in the sidewall of the housing which are normally blocked by the wall of the piston in its upper firing position. Initial downward displacement of the piston in opposition to spring bias clears the ports and exposes the face of the piston to the gas pressure, thus driving the piston downward to deliver a blow to the target.

The invention to be described hereafter represents an improvement on the method and apparatus of the above prior application which is particularly concerned with means for more efficiently exploiting compressed gas as a driving force in an impact device of this character.

It is therefore a general object of the present invention to provide an acoustic pulse generator of the type described with improved mass acceleration means.

It is a more particular object of the present invention to provide improved means for applying pressurized gas as an energy source in an acoustic pulse generator of the type described.

Other and further objects and advantages of the invention will become apparent from a consideration of the detailed description and drawings to follow.

In accordance with the preferred embodiment of this invention a hollow cylindrical housing adapted to be supported above the ground in a substantially upright position slidably engages an elongated impact piston normally retained adjacent the closed upper end of the housing by means of an electrically controlled latch. The sidewall of the housing adjacent the upper end is slightly tapered and provided with large area ports interconnected with a source of compressed gas. These ports are initially blocked by a correspondingly tapered upper portion of the piston. Static seals mounted on the inner bore of the housing above and below these ports bear against the tapered portion of the piston. A component of force applied by the compressed gas is constantly exerted downwardly against the tapered piston wall to ensure positive breakaway from these seals. Quick release of the latch allows the piston to fall freely and suddenly applies the full force of the compressed gas to drive the piston so as to deliver a blow to a target plate positionable on the ground adjacent the lower end of the housing. At the same time the gas is vented upwardly through one or more vents in the upper end of the housing to compensate for any recoil forces on the housing itself. Lift cylinders are provided for returning the impact cylinder to its firing position.

The drawing is an elevational view of an apparatus in accordance with the preferred embodiment of this invention prior to firing.

With reference now to the drawing, there is illustrated an acoustic pulse generating apparatus 10 in accordance with the preferred embodiment of this invention. A hollow cylindrical housing 12 is adapted to be supported in an upright position. This may be accomplished, for example, by attachment of brackets 14 to the handling device of a transporting vehicle (not shown). An impact piston 18 is normally retained securely adjacent the upper end of the housing 12 by means of quick release electrically controlled latches 20. The lower end of the piston is provided with an enlarged diameter plate 22 having a convex central portion 24. The upper end of the piston 18 is provided with an upwardly tapered wall 28 terminating in upper face 30. Adjacent the lower end of the housing 12 a target plate 32 is positionable on the surface of the earth or other medium in axial alignment with impact piston 18 and in spaced relation thereto. The target plate 32 consists generally of the bottom plate 34 and an upstanding central pillar 36 from which a flange 38 extends radially to a point adjacent the lower sidewall 40 of the housing 12. The lower end of sidewall 40 is provided with inwardly extending ribs 44 and 46. Rib 44 extends radially beneath flange 38 to enable pick up of target plate 32 for transport or deployment from one position to another. Rib 46, in like manner, blocks undesired upward travel of target 32 relative to housing 12.

The sidewall of the housing 12 adjacent the upper end is penetrated by multiple large area ports 50 which are interconnected with high volume accumulators 52. As shown in the drawing, the tapered wall 28 of piston 18 blocks the entrance to ports 50 from within the housing 18. Mounted on the correspondingly tapered upper bore 54 of housing 18 above and below ports 50 respectively are static seals 56 and 58. Seals 56 and 58 bear against tapered wall 28 of piston 18, thereby preventing gas pressure from escaping upwardly or downwardly within the bore of housing 12. It is a feature of this invention that because of the taper of piston wall 28 a component of force due to the compressed gas is constantly exerted downwardly against piston 18. This taper may be, for example, at an angle of about 5.degree. to the axial direction, which is sufficient to provide an intial breakaway force on piston 18 in the event it tends to wedge against seals 56 and 58. The taper is not so great, however, that the downward pressure on latches 20 becomes excessive so as to risk leakage around seals 56 and 58.

On either side of the piston 18 there are positioned hydraulic lift cylinders 60 with lift rods 62 extending through apertures 64 in piston impact plate 22. Lower end of lift rod 62 are provided with lift rings 68. Preparatory to the operation of generator 10, lift rods 62 are lowered to assume the dotted position as shown. Upon completion of the firing operation lift rings 68 are effective to engage plate 22 and return piston 18 to its intial ready position.

In operation, latches 20 are electrically operated and withdrawn so that impact piston 18 is free to fall downward away from seal rings 56 and 58. This enables the continuously present gas pressure, to enter the bore 54 and drive piston 18 downwardly, the driving force reaching a maximum at the moment the piston surface 28 is separated from upper seal ring 56. This may occur with a net downward movement of a fraction of an inch. Piston 18 is then driven with high velocity so that convex impacting surface 24 delivers a blow to mating concave upper surface 70 of pillar 36 to generate the desired acoustic pulse. During the downward motion of piston 18, it is maintained in proper axial position by means of guide strips 72 affixed to and extending longitudinally along the surface of piston 18. The upper end of guide strip 72 are beveled to ensure proper mating with the bore of housing 12 on return of piston 18 to its upper position. Lower housing wall 40 is provided with suitable air vents 74 to ensure no air pressure build up occurs beneath the piston 18 to lessen the effective kinetic energy.

Simultaneously, with the downward motion of piston 18 compressed gas is expelled through flared pressure vent 76 in the top 78 of housing 12. By the principle of action and reaction this compensates for any upward upward recoil force that may be exerted against housing 12 during the operation of the device.

It is understood that the preceding description and the accompanying drawing are illustrative only of a preferred embodiment of this invention. Therefore, without departing from the scope of this invention as set forth in the appended claims those skilled in the art will be able to make many modifications in the configuration and relative position of parts of the apparatus as described and shown.