Seismic signals are sensed by geophones in land acquisition or by hydrophones in marine acquisition. Typically, the signals are excited in the earth with some sort of energy source such as an explosion, vibrator, or air or water gun. These signals travel through the subsurface structures and are reflected from boundaries having distinct physical properties. Eventually they produce multiple reflections observed at each recording phone. Often the geological structure is not simple, and although simple models have sufficed for many regions of exploration, the recorded reflections are interpreted well only if the geophysicist has much experience and is familiar with other sources of information.