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A case study of AEM bathymetry in Geographe Bay and over Cape Naturaliste, Western Australia, Part 2: 25 and 12.5 Hz GEOTEM

We interpret airborne EM response data recorded by the GEOTEM 12.5 and 25 Hz systems flown over shallow seawater, using conductivity-depth imaging (CDI) to estimate sea depths to 65 m. We observed non-monotonic decay in the vertical component of the 25 Hz data recorded at survey altitude. Nonmonotonic decay had an adverse effect on the CDI results when the processing was run with positivity constraints. The removal of positivity constraints resulted in significant improvements in the quality of CDIs for interpreting water depth. Layered-earth modelling further showed that this non-monotonic decay in the 25 Hz data was due to variations in the transmitter-receiver geometry over the highly conductive seawater. Currently, 12.5 Hz airborne EM is unsuitable for bathymetric mapping unless latetime system noise can be reduced. CDIs from dBx/dt component data at 25 Hz provide the most accurate interpreted sea depths. For this dataset, the use of B-field responses computed from dB/dt observations does not offer any clear advantage over dB/dt data for interpreting sea depths from CDIs.