Rapid estimation of shallow seawater depth from airborne electromagnetics

Abstract

Airborne electromagnetics (AEM) has been used on a test basis for determining water-depth (bathymetric sounding), with both fixed- and rotary-winged aircraft systems. Useful application is restricted to shallow water (up to a few tens of metres), and the systems can be used through sea-ice. Many processing methods to date, however, have involved extensive post-processing of data before bathymetric information can be extracted, with lag times of the order of months. We have investigated a number of methods for rapid bathymetric depth estimation that could be used in real time with AEM data acquisition. The fastest methods are all based on an initial rapid transform of data to conductance-depth sections. Once in conductance-depth space, it is possible to estimate depth from the maximum conductance encountered at each reading if the conductivity of seawater is known, and if the sea floor can be assumed electrically resistive. Alternatively, it is possible to attempt to solve for both seawater conductivity and depth independently using a thick-layer approximation, but this is a slower and potentially unstable process, but with the potential advantage that seafloor conductivity can be assessed. AEM and bathymetric sounding data have been used to appraise the validity of the approximate methods. These data included helicopter DIGHEM data collected over Sydney Harbour and fixed-wing GEOTEM data collected over and near Geographe Bay, Western Australia.