A geophysical prospecting method based on the fact that the speeds of transmission of shock waves through the Earth vary with the elastic constants and the densities of the rocks through which the waves pass. A seismic wave is initiated by firing an explosive 2820 charge (or by equivalent artificial sources) at a known point (the shot point); records are made of the travel times taken for selected seismic waves to arrive at sensitive recorders (geophones). There are two main subdivisions of seismic operations: the reflection method and the refraction method. The seismic method has been applied to a lesser extent to elucidate mining problems, partly due to its high cost. It has been used to investigate the base of drift deposits, and drift-filled channels have been successfully outlined.