Sidescan sonar uses sound energy reflected off the seabed to show the texture or roughness of the seafloor. Unlike echosounders to measure depth, which are installed on a ship's hull, a sidescan sonar is typically mounted in a cylindrical pod towed behind a vessel on a long cable.
The torpedo-shaped pod is towed slowly so that it sinks close to the seafloor. Sound energy is transmitted from either side of the pod in a fan shape at 10-100s of 'pings' per second. The sound waves reflect off the bumps and holes on the seabed to reveal an image made up of bright areas or shadows.