A case study of AEM bathymetry in Geographe Bay and over Cape Naturaliste, Western Australia, Part 1: 25 Hz QUESTEM

Abstract

Airborne electromagnetic (AEM) data were collected in the Geographe Bay region, Western Australia, using the QUESTEMTM 450 system operating at 25 Hz. The survey covered a region with shallow water up to 35 m depth (Geographe Bay), including a steel wreck, and two overlapping tie lines skirting Cape Naturaliste extending into deeper water. We interpreted the survey data using layered-earth inversion and Conductivity-Depth Imaging (CDI) processing to estimate sea depths and compared these findings with accurate bathymetric data. The root mean square (RMS) residual error of the estimated sea depths was found to be 3.2 m using layered-earth inversion of inline-component data measured in the depth range of 25 to 93.5 m off Cape Naturaliste. The CDI processing of vertical-component data in this region underestimated the sea depth but displayed conductivity variations representing seawater to depths of 60 to 70 m. In Geographe Bay, the RMS residual error of the estimated sea depths was found to be 0.9 m using layered-earth inversion of selected line data in the depth range of 25 to 35 m. In shallower regions, adjacent to the shoreline, the residual error of the estimated sea depth increases because of the influence of the edge effects at the boundary of the conductive (seawater) and resistive (land) layers.