Preliminary investigations using a helicopter time-domain system for bathymetric measurements and depth-to-bedrock estimation in shallow coastal waters - a case study in Broken Bay, Australia

An airborne electromagnetic (AEM) survey over the shallow coastal waters of Broken Bay (Australia) provided data for bathymetric mapping and for mapping the coarse topographical features of underlying bedrock in areas of relatively shallow marine sediments. Inversion of AEM data assumed a crude one-dimensional layered-earth model consisting of a conductive seawater layer overlaying a less conductive sediment layer which in turn overlies an infinitely deep resistive basement. Known or assumed layer conductivities were used as fixed parameters in the inversion of data. The primary focus of this study is to test the accuracy of AEM for bathymetric mapping. A secondary objective is to determine the potential for mapping sediment thickness by sensing the depth to bedrock and to estimate sediment resistivity. The vertical accuracy of the AEM method was measured by comparing the depth profiles with those obtained from historical data - sonar soundings for water depths and marine seismic data for estimated depths to bedrock. Water depth accuracy of 1 - 2 m was achieved in water shallower than 25 - 30 m, and ~50 cm accuracy was obtained in water shallower than about 10 m. This study shows that the AEM method is suited as a reconnaissance technique for measuring seafloor gradients adjacent to beaches, and for rapid bathymetric mapping in turbid and surf-zone waters. The AEM data delineated a distinct paleovalley 60 to 70 m below sea level.