The mechanism whereby an elastic pulse is generated in the ground due to the detonation of an explosive charge is discussed. Initially the pressure in the borehole is very high, of the order of several hundred tons per square inch. The sudden impact of this on the walls of the borehole causes a shock wave in the ground, the intensity of which is very much greater than the ground can bear, so that the wave rapidly loses energy in fracturing the ground, and degenerates into an elastic pulse at a distance defined as the critical radius. This elastic pulse is transmitted sensibly unchanged through the ground. The ground vibration detected by a geophone is the synthesis of this pulse modified by reflections and refractions at the interfaces of the geological formation of the site, spread out in time according to the paths along which the components have travelled. Although adequate quantitative information is lacking, an attempt is made to evaluate such a pulse at various stages of its career in order that the physical magnitudes involved may be appreciated.