Remote characterisation of shallow seafloor using airborne TEM

Abstract

Helicopter-borne time-domain electromagnetic (EM) systems combine the advantages of portability with large transmitter moments. In order to demonstrate the potential of helicopter TEM for shallow bathymetric mapping, a HoistEM survey was conducted near the entrance of Port Jackson (Sydney Harbour) in March 2002. 1D inversion of rescaled HoistEM yielded accurate (± 1m) water depth predictions to depths of about 55m. The seawater depths inferred from HoistEM were in good agreement with the depths obtained from accurate single-beam and multi-beam sonar soundings. It was necessary to rescale the HoistEM data in order to account for improper calibration. Re-scaling was possible because a considerable amount of ``ground truth' was available. Synthetic TEM decays were computed for numerous models representing shallow water, sediment and basement at a number of locations within the survey area. Measured and calculated data were compared at each delay time, and were found to be linearly related. A rescaling factor was then applied to each decay time, irrespective of water depth. Given the successful bathymetric mapping demonstration, the potential of airborne EM for seafloor characterisation has been investigated. Accordingly, the sediment thickness inferred from 1D inversion of HoistEM has been compared with estimates based on marine seismic studies. Generally, the sediment thickness, and hence depth to resistive bedrock, inferred from HoistEM was in reasonable agreement with depths estimated from marine seismic data when the seawater was less than about 20 m deep and the sediment was less than about 40 m thick.