oa Preliminary investigations using a helicopter time-domain system for bathymetric measurements in shallow coastal waters – Port Lincoln and Broken Bay, NSW, Australia

Helicopter time-domain AEM is being investigated as a technique for bathymetric mapping in shallow coastal waters. Previous studies in Port Lincoln, South Australia, used a floating AEM system to provide an upper limit to the expected bathymetric accuracy based on current technology for AEM systems. The same survey lines were flown with an airborne system (SeaTEM) on two separate occasions. A comparison of the interpreted water depths obtained from the airborne and floating systems is presented. An empirical data correction method based on modelled and observed EM responses over deep seawater at varying survey altitudes can lead to improvements in interpreted water depths. A comparison between results of the two surveys in Port Lincoln shows that uncorrected data also gives good agreement with known water depths. The results of a full survey of selected areas in Broken Bay (NSW) undertaken with the SeaTEM system are presented. A marine seismic reflection survey together with vibrocore samples of sediment in Broken Bay were used to provide an estimate of the sediment thickness and resistivity of shallow sediments to assist in the interpretation of the EM response. Interpreted water depths and depths to bedrock from SeaTEM data in Broken Bay are generally in good agreement with known bathymetry and seismic estimates of bedrock depths shallower than ~ 90 m in water depths shallower than ~ 25 – 30 m.