An air gun is mounted in a reaction mass having an impulse-transmitting ground-contact plate movably mounted below the mass. This mass is in face-to-face contact with the plate over the major portion of the entire upper surface of the contact plate. Confined liquid is not employed. The air gun is charged with pressurized gas and fired, discharging the pressurized gas into the narrow horizontal interface region between reaction mass and plate. Rapid expansion of the discharged gas between mass and plate causes an intensely powerful seismic impulse to be transmitted into the earth. The air gun has 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 pedestal assembly carrying said shuttle is mounted from above to the reaction mass and extends down into the hollow shuttle member. When this air gun is fired, the lower piston moves away from a peripheral seal at the lower end of the firing chamber suddenly discharging pressurized gas from a central location radially outwardly into the narrow horizontal interface region. By suspending the pedestal assembly from above, powerful seismic waves can be generated using a relatively short shuttle stroke, and the gun can be positioned near the impulse-transmitting plate for directly discharging the pressurized gas into the interface region between plate and reaction mass.