Abstract: Apparatus for generating an acoustic pulse in a medium wherein an impact piston is driven downwardly along a cylindrical housing by compressed gas to strike a rigid target positioned in contact with the earth. In its firing position, the tapered upper portion of the piston extends above the top of the housing within a container occupied by the compressed gas, the top of the piston being at the same time recessed within a gastight mating enclosure at the top of the container. In this position, the tapered piston surface is constantly exposed to circumferential gas pressure so as to provide a positive breakaway force. Upon release of suitable piston latch means, the piston is first urged downwardly by this breakaway force and thereafter accelerated more powerfully when the top of the piston separates from the enclosure and becomes exposed to gas flow uniformly from all directions.

Abstract: Apparatus for generating an acoustic pulse in a medium including an upstanding cylindrical housing within which an impact piston retained in the upper end of housing by an electrically controlled latch is driven downwardly by compressed gas to strike a target plate positioned adjacent the bottom of the housing. The compressed gas is provided by high volume accumulators interconnected with large area ports penetrating the sidewall of the housing near its top. The tapered upper portion of the piston presses against static seals mounted on the correspondingly tapered housing bore above and below these ports. This taper enables the compressed air to exert a positive break away force on the piston. Quick release of the latch triggers the downward thrust of the piston to deliver a blow against the target plate.