An efficient method for reducing the sound speed induced errors in multibeam echosounder bathymetric measurements

Nowadays extensive use is made of multibeam echosounders (MBES) for mapping the bathymetry of sea- and river-floors. The MBES is capable of covering large areas in limited time by emitting an acoustic pulse along a wide swathe perpendicular to the sailing direction. The angle and the corresponding two-way travel-time of the received signals are determined through beamsteering at reception. Water depths along the swathe can be derived from this angle and travel-time combination. In general, two sets of sound speed measurements are taken when conducting MBES measurements. The first set is used for the beamsteering and consists of the sound speeds at the MBES transducer. The second set is used for determining the propagation of the sound through the water column, needed for correctly converting the measured travel times to a depth. In general, this set of sound speed measurements consist of the complete sound speed profiles (SSPs). The quality of the sound speed measurements at the transducer position sometimes gets degraded, resulting in beam steering angles that differ from those aimed for. Also sometimes the SSPs used for converting the beam travel times to depths deviate from the true prevailing SSPs due to the, in general, limited amount of SSP measurements taken during a survey. Both above mentioned effects result in an erroneous bathymetry. Here, we present a method for eliminating these errors, without the need for additional sound speed information.