Seismic exploration is the search for deposits of crude oil, natural gas, and minerals. It consists of a seismic data set measured and recorded with reference to a particular area of the Earth's surface, to evaluate the subsurface. During the process, artificial seismic energy is generated on land and transmitted via seismic wave energy into the subsurface rock layers. Seismic waves reflect and refract off subsurface rock formations and travel back to acoustic receivers called geophones (on land) or hydrophones (in water). The travel times of the returned energy, aid in estimating the structure and stratigraphy of subsurface formations.