Detecting small seabed targets using a high frequency multibeam sonar: geometric models and test results

The detection of small seabed targets has predominantly been achieved through the use of deep-towed side scan sonar technology. The target cannot be relied on to have distinctive backscatter strength but usually lies proud of the seabed, thus casting a shadow. Such technology fails at higher grazing angles where shadows are no longer cast. Thus repetitive, overlapping survey geometries are required to fill the “nadir gap”. An alternate technology that can detect topographic anomalies without relying on shadows is high frequency multibeam sonars (HFMS). If such systems can be combined with side scan methods, target search times can be considerably reduced. Traditionally HFMS sonars have been hull mounted and thus are not able to resolve small (<1.5 m) targets in continental shelf water depths (30-200 m). If such sonars were deployed on tow bodies however, small target detection at high grazing angles may be feasible. To this end, we have investigated the capability of one of these sonars using altitudes between 10 and 30 m (as might be achieved by mounting such a sonar on terrain-following tow fish). Our preliminary modelling suggests that the beam footprints (using 1.5 degree beam) and the beam sounding density are potentially sufficient to achieve the required objective. We identified two limiting conditions. We compared the results of our modelling with actual data taken from a small survey launch in water depths between 10 and 30 m using targets ranging in size from a few decimetres to 1.5 metres