From AMS Glossary
A method of surveying the bottom of the ocean or other body of water to obtain detailed acoustic images, frequently used to locate debris, such as from aircraft accidents or sunken ships.
The acoustic beam is directed perpendicularly to the direction of travel. The ship moves at a constant velocity, and each sonar ping insonifies a slightly different wedge of bottom. The apparent depth is actually the slant range from the ship to the target. The display is a map of slant range and distance along the track.