Patent CA-1192987-A (CA)
Inventor: CHELMINSKI STEPHEN V
IPC: G01V1/133
Filed: 1982-06-25
Granted: 1985-09-03
Abstract: ABSTRACT OF THE DISCLOSURE            Land seismic exploration method and system are provided for generating and transmitting powerful seismic impulses into the earth from an air gun mounted in a reaction mass having an impulse-transmitting ground-contact plate movably mounted below the reaction mass.    Initially, this contact plate is posi-tioned down firmly against the earth so as to be coupled to the earth in good impulse transmitting relationship therewith, the reaction mass is in face-to-face contact with the plate over the major portion of the entire area of the upper surface of the con-tact plate, and the air gun is charged with highly pressurized gas.  Then, the gun is fired, abruptly discharging the confined pressurized gas into the narrow horizontal interface region bet-ween the reaction mass and the contact plate.  This released gas suddenly expands in the interface region for abruptly driving the contact plate downwardly, thereby generating a powerful seismic impulse transmitted downwardly into the earth.  The rapid expan-sion of the discharged gas between the reaction mass and the plate causes an intensely powerful seismic impulse to be transmitted into the earth.  The apparatus includes a novel air gun which comprises a container having a firing chamber for holding a charge of pressurized gas, a movable shuttle for suddenly discharging the pressurized gas, including upper and lower pistons with a hollow member connecting these pistons.   A novel pedestal assembly carrying said shuttle is mounted from above the reaction mass and extends down into the hollow member of the shuttle.          When this air gun is fired, the lower piston moves down away from a peripheral seal at the lower end of the firing chamber allowing the pressurized gas to be suddenly discharged grom a central location radially outwardly into the narrow horizontal interface region between the plate and the reaction mass.   A resilient pad or bumper is mounted to cushion the lower piston after the air gun has fired. By suspending the pedestal assembly from above, powerful seismic waves can be generated using a relatively short shuttle stroke, and the operating gun can be positioned closely ad-jacent to the impulse-transmitting plate for directly discharging the high pressure gas into the region between the plate and the reaction mass.   By virtue of the fact that confined liquid is not employed, there is no need to spend time separating the discharged gas from liquid, and thus this land source can be fired at a rela-tively rapid repetition rate, if desired, for example, once every 3 to 4 seconds.  Moreover, in the absence of the liquid, there is a reduced "rebound" action.  In the presently preferred embodi-ment, the reaction mass is free to travel upwardly essentially un-restrained until its upward momentum is dissipated against the force of gravity, thereby causing the powerful reaction to be entirely absorbed by the reaction mass for isolating the other components of the system from the stresses and mechanical shocks due to the reaction forces.   In the absence of the gas-liquid separator, a considerably lower overall profile may be achieved for the present land source, and so it is adapted for mounting beneath a vehicle.