Target detection with multisector and multiswath echosounders

Abstract

Discontinuities in the ensonification pattern of certain multibeam echosounders can hamper the detection of targets in the water column and on the bottom. Such echosounders operate in single or multiswath modes with several independent transmit sectors per swath. Each sector has a unique acoustic frequency band and is steered across and along track to compensate for the vessel’s pitch and yaw and to achieve a specified spatial density of soundings. This sonar geometry is optimized for bathymetry but less favorable for processing and interpretation of acoustic backscatter intensities because the sectors have different acoustic absorption profiles and are disjoint across and along track. This paper presents a method of target detection in the water column that involves frequency- and depth-dependent compensation for transmission loss in each sector, equalization of gains across all the sectors, and adaptive removal of sidelobe interferences. Echo intensities in each sector are then aggregated along track and displayed as signal-to-clutter ratio vs. altitude and distance along track. This technique allows detection of targets that are in the water column between the bottom echo trace and the ring of sidelobe interference—a region that is often deemed “blind” or too noisy for target detection.Discontinuities in the ensonification pattern of certain multibeam echosounders can hamper the detection of targets in the water column and on the bottom. Such echosounders operate in single or multiswath modes with several independent transmit sectors per swath. Each sector has a unique acoustic frequency band and is steered across and along track to compensate for the vessel’s pitch and yaw and to achieve a specified spatial density of soundings. This sonar geometry is optimized for bathymetry but less favorable for processing and interpretation of acoustic backscatter intensities because the sectors have different acoustic absorption profiles and are disjoint across and along track. This paper presents a method of target detection in the water column that involves frequency- and depth-dependent compensation for transmission loss in each sector, equalization of gains across all the sectors, and adaptive removal of sidelobe interferences. Echo intensities in each sector are then aggregated along track an...