Dive and Discover : Oceanographic Tools: Sonar Multibeam

Multibeam bathymetry is based on the fact that more beams are better than one. About 30 years ago, the US Navy developed a system that could send out many beams of sound simultaneously to get a series of water depth readings along the line of a moving vessel. Instead of just one transducer pointing down, “multibeam bathymetry systems” have arrays of 12 kHz transducers, sometimes up to 120 of them, arranged in a precise geometric pattern on ships’ hulls. The swath of sound they send out covers a distance on either side of the ship that is equal to about two times the water depth. The sound bounces off the seafloor at different angles and is received by the ship at slightly different times. All the signals are then processed by computers on board the ship, converted into water depths, and automatically plotted as a bathymetric map with an accuracy of about 10 meters.

In this way, ships traveling at speeds over 10 knots can produce a swath, rather than a line, of water-depth information. Multibeam bathymetry systems are now routinely used during research cruises to map areas of seafloor as large as thousands of square kilometers.