Towards seawater depth determination using the helicopter HoistEM system

Abstract

Interpretation of helicopter time-domain EM data from a 2002 HoistEM survey of Sydney Harbour is under way to define seawater depth. Forward modelling, conductivity-depth imaging (CDI), and ID inversion have been applied. The effectiveness of airborne EM bathymetry can be appraised by comparing interpreted seawater depths with accurate bathymetric soundings.An extensive comparison of modelled and observed HoistEM responses over deep (~33 m) and shallow (~6 m) seawater showed that good agreement could be achieved for models with the correct depth of water if the seawater conductivity was assigned an unrealistically low value of about 3.5 S/m. The actual seawater conductivity in Sydney Harbour is 4.65 + 0.15 S/m.CDI processing reinforced the conclusion that the measured HoistEM responses are incompatible with the known seawater conductivity. However, a qualitative agreement was observed between the CDI seafloor topography and the true bathymetry. In an effort to reconcile the data with the “ground truth”, transmitter moment and altitude were adjusted, for a given seawater conductivity, prior to CDI processing.ID inversion of HoistEM data yielded sub-metre water-depth accuracy for depths shallower than 22 m, if an unrealistically low seawater conductivity was allowed. Thus forward modelling, CDI, and initial ID inversion results all suggest that the HoistEM system is viable for bathymetric surveys but that rescaling of the Sydney Harbour dataset will be required in order to reconcile the measured responses with the known seawater conductivity.