Abstract: A seismic signal generating apparatus comprising a vehicle including a rearwardly extending bed, a ram-supporting framework pivotally mounted on the bed for pivotation about a horizontal axis, a ram slidably and reciprocably mounted in the framework, and a plurality of catapult springs connected between the framework, and one end of the ram for forceably reciprocating the ram from an elevated status in which the catapult springs are tensioned, to a lowered, impacting position in which the springs are tensioned to a lesser degree. A pair of superimposed metal plates are positioned below the ram on the ground for receiving the impact of the ram.