Improving shallow-water multibeam target detection at low grazing angles
Beyond approximately 60º incidence angle, many modern multibeam echo sounders have difficulty maintaining sufficient depth accuracy and seabed target detection to comply with International Hydrographic Organization (IHO) standards. In some cases, a target at such a low grazing angle is not detected due to limitations of the existing bottom detection method and filters applied by the manufacturer. In lieu of clear positive bathymetric indicators, a data gap within the bathymetric surface or a shadow in the backscatter image may be the only indication of the presence of that target. This research presents a refined bottom detection algorithm based on the Bearing Direction Indicator (BDI) method. The BDI approach can markedly improve target detection capability at low grazing angles in shallow waters by independently discriminating each echo’s direction of arrival irrespective of the beam spacing. The algorithm can be applied in post-processing as long as the water column data is retained. Two test datasets were collected using an EM 2040D employing angular sectors as wide as +/-82°. Data were acquired over a site with multiple IHO-compliant anthropogenic objects to assess the EM 2040D ability to detect low-grazing-angle targets prior to, and after, application of the newly developed algorithm. Results obtained clearly illustrate that the BDI algorithm can enhance low-grazing-angle target detection/definition.