A sonic device for roughly determining sea floor composition is described. It consists of a hydrophone encased in a watertight metal container designed to be dragged along the ocean bottom, and a suitable audio amplifier monitoring equipment located aboard a towing vessel. The frictional noises produced when the metal case is scraped along the bottom are picked up by the hydrophone and fed through cable to the amplifier. Since the noises caused by mud, sand, stony, and rock bottoms differ in character, it is possible to identify the bottom type by listening to the amplifier output. The device is especially useful for making a very detailed survey of a small area in shoal water for the purpose of constructing a sediment distribution chart.