Seismic waves are waves that travel through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. Seismic waves are also continually excite by the pounding of ocean waves and the wind. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone. For seismic studies of oil reservoirs, hydrophones may give additional information.