A method of locating irregularities on the ocean bottom. A pulse of sonar energy (typically 120 kHz) is emitted from a fish which is towed 50 to 500 ft above the bottom, depending on the range and resolution sought. The sonar beam is narrow in the direction of traverse because the source consists of a line array of elements. Bottom irregularities (rock outcrops, pipelines, shipwrecks, boulders) and variations in bottom sediments produce changes in the amount of energy return; see Figure S-9. The arrival time measures the distance from the fish to the reflecting object. Other names include: asdic, basdic, sideways asdic, sideways-looking sonar, sideways sonar, echo-ranger, horizontal echo-sounder, and lateral echo-sounder.