Seawater depth determination using the helicopter HoisTEM system

Abstract

Interpretation of helicopter time domain data from a recent HoisTEM survey of Sydney Harbour has been undertaken to identify the boundary between seawater and seafloor on the basis of the expected conductivity contrast. The survey area overlies an area previously surveyed using a frequency domain DIGHEM (V) system. Both conductivity-depth imaging (CDI) and 1D inversion have been applied. Interpreted seawater depths are compared with accurate bathymetric soundings, in order to appraise the airborne EM bathymetry method for this HoisTEM dataset. Various strategies have been employed in order to optimise the interpretation of HoisTEM data using accurate sea depth ground truths and knowledge of the sea floor in Sydney Harbour. CDI processing, forward modelling, and 1D inversion are under investigation. The CDI processing involves determination of apparent conductivity at each delay time, followed by estimation of the apparent depth as the depth of maximum current in a half-space with conductivity equal to the apparent conductivity at the time in question. Variable transmitter current is taken into account during the CDI processing. For a given seawater conductivity, optimal transmitter moment and altitude can be determined prior to CDI processing via successive single parameter inversions of an early time channel. An extensive comparison of modelled and observed HoisTEM responses over deep and shallow seawater showed that very good agreement could be achieved for models with the correct depth of water provided the seawater conductivity was lowered from its measured value of 5 S/m to about 3.5 S/m. This modeling investigation also provided useful insights into the quantitative effects of variations in transmitter altitude. 1D inversion of HoisTEM data supports these findings. Initial results suggest that the correct seawater depth can be inferred provided a low seawater conductivity is adopted.