MULTI BEAM BATHYMETRY

A multibeam echosounder is a type of sonar that is used to map the seabed. Like other sonar systems, multibeam systems emit sound waves in a fan shape beneath a ship’s hull. The amount of time it takes for the sound waves to bounce off the seabed and return to a receiver is used to determine water depth. Unlike other sonars, multibeam systems use beamforming to extract directional information from the returning soundwaves, producing a swath of depth readings from a single ping. 

A multibeam echosounder is a device typically used by hydrographic surveyors to determine the depth of water and the nature of the seabed. Most modern systems work by transmitting a broad acoustic fan shaped pulse from a specially designed transducer across the full swath across track with a narrow along track then forming multiple receive beams (beamforming) that are much narrower in the across track (around 1 degree depending on the system). 

From this narrow beam, a two way travel time of the acoustic pulse is then established utilizing a bottom detection algorithm. If the speed of sound in water is known for the full water column profile, the depth and position of the return signal can be determined from the receive angle and the two-way travel time. 

In order to determine the transmit and receive angle of each beam, a multibeam echosounder requires accurate measurement of the motion of the sonar relative to a cartesian coordinate system. The measured values are typically heave, pitch, roll, yaw, and heading. To compensate for signal loss due to spreading and absorption a time-varied gain circuit is designed into the receiver. For deep water systems, a steerable transmit beam is required to compensate for pitch. This can also be accomplished with beamforming.