Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Seismic waves are waves of force that travel through the Earth or other elastic body, for example as the result of an earthquake, explosion, or some other process that imparts forces to the body. Seismic waves are also continually excited on Earth by the incessant 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.