Sonar, which stands for SOund NAvigation and Ranging, is
usually used to "see" objects in the water or to determine the distance to
something. For example, people who fish sometimes use simplified forms of
sonar, called depth sounders or fish finders, to find how deep the water is
where they are or to locate schools of fish. A depth sounder sends out a pulse,
measures the time it takes to receive a return signal, and—using the speed
of sound in saltwater (about 1,500 meters per second)—calculates the
distance to the object or sea floor.
Marine scientists may use sidescan sonar to map the hills, valleys, and other
features of the ocean floor. This sophisticated sonar returns very detailed
"pictures" of objects in the water and can be used to locate submarines, underwater
mines, shipwrecks, or aircraft crash debris. Sonar can also be used to inspect
pipelines, cables, or bridge foundations.