NOAA hydrographic survey units use multibeam echo sounders to acquire water depth information in a survey area; to determine least water depths over critical items such as wrecks, obstructions, and dangers to navigation; and to detect objects in general.
Multibeam echo sounders, like other sonar systems, emit sound waves in the shape of a fan from directly beneath a ship's hull. These systems measure and record the time it takes for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. In this way, multibeam sonars produce a “swath” of soundings (i.e., depths) for broad coverage of a survey area. The coverage area on the seafloor depends on the depth of the water, typically two to four times the water depth.