Development of a helicopter time domain system for bathymetric mapping and seafloor characterisation in shallow water

Abstract

Time domain airborne electromagnetic (AEM) data acquired from surveys over seawater in Australian coastal waters can be interpreted to obtain seawater depths (Vrbancich and Fullagar, 2007a; Wolfgram and Vrbancich, 2007; Vrbancich, 2009) and to identify the coarse features of bedrock topography (Vrbancich and Fullagar, 2007b; Vrbancich, 2009). A comparison of derived water depths in shallow areas (< 50 m) with known bathymetry has shown that sub-metre depth accuracies can be achieved but these accuracies are not maintained over the entire survey region. Furthermore, the quantitative interpretation of AEM data using 1D inversion methods may require data rescaling (Vrbancich and Fullagar, 2007a; Vrbancich, 2009) and the measured seawater conductivity as a known parameter. The rescaling coefficients in these studies were obtained from the slope and intercept of linear fits between modelled and observed decays at representative sites (control points) with known water depths. These restrictions limit the potential of AEM for accurate bathymetric mapping. A time domain helicopter AEM system (SeaTEM) is currently being developed for the Defence Science and Technology Organisation for shallow water bathymetric mapping. This system consists of a transmitter and receiver loop assembly mounted on a rigid structure referred to as a ?bird? that is towed as a sling load below the helicopter. Instrument stability, calibration (Vrbancich and Fullagar, 2007a; Brodie and Sambridge, 2006; Davis and Macnae, 2008) and the ability to accurately track both the swaying motion (i.e. bird swing) and the altitude of the AEM sensor system over seawater during survey (Davis et al., 2006; Kratzer and Vrbancich, 2007) are issues that need to be addressed in order to develop AEM as a reliable and accurate bathymetry mapping technique. System calibration, self-response, transmitter current waveform, and altimetry were investigated and preliminary findings are reported in this paper.