Multibeam Sonar

Multibeam sonar is a common offshore surveying tool that uses multiple sound signals to detect the seafloor. Because of its multiple beams it is able to map a swath of the seabed under the ship, in contrast to a single beam sonar which only maps a point below the ship. With the capability to cover 100 per cent of the seafloor, multibeam sonars are becoming the industry standard for hydrographic and geological applications. Different frequencies are used to map different water depths, with higher frequencies (>100kHz) used for shallow water and low frequencies (<30 kHz) for deep water. With different frequencies there is also a trade-off in resolution, with higher frequency shallow water systems providing greater spatial resolution than lower frequency deep-water systems.

Generally, the multibeam sonar transducer is mounted rigidly to the hull of the survey vessel and its position can be calculated very accurately. Other parts of the multibeam system include auxiliary sensors such as motion-sensing systems and Global Positioning Systems (GPS) to ensure accurate positioning, motion sensing and sound speed measurement system.

A modern multibeam sonar transducer typically uses the Mills Cross array. The sound is transmitted from transducers that are perpendicular to the survey track. Consequently, the sound pulses forms a transmit swath that is wide across-track and narrow along-track. The returning sound pulses, which are mainly recording the impedance contrast and seafloor topography, are received by the receivers which are mounted parallel to the survey track. These return beams are narrow across-track. Unlike the sidescan sonar which commonly produces only acoustic backscatter data (i.e. hardness), the multibeam sonar generates both water depth and seafloor hardness data concurrently.