Improved seawater depth determination using corrected helicopter time-domain electromagnetic data

Two areas within Sydney Harbour were surveyed in 2002 with a helicopter‐borne time‐domain electromagnetic system to test its potential for bathymetric mapping in shallow seawater. As delivered, the data were improperly calibrated. Therefore a re‐calibration was performed to reconcile the measured data with ‘ground truth’. Synthetic electromagnetic transients were computed for two‐layer models representing the seawater and the sediment overlying bedrock at a number of locations within the survey area. The seawater depth in the models varied between 3 m and 70 m. The measured and calculated data were compared at each delay time, and were found to be linearly related. The slope and intercept of the line of best fit were used to correct all the measured data. Inversion of the corrected time‐domain electromagnetic data generally resolved the bathymetry to submetre accuracy down to depths of about 55 m.