Bathymetry and sediment depth investigation in Broken Bay using a prototype AEM time domain system (SeaTEM)

Abstract

The use of airborne electromagnetic (AEM) methods for measuring water depth and estimating sediment thickness has been demonstrated using commercial AEM equipment that is not optimized for marine surveying. A new prototype helicopter time domain AEM system, SeaTEM(0), is under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, NSW, in shallow water up to ~ 30 m in depth. The SeaTEM(0) system was untested and the Broken Bay survey identified instrumentation problems that will be addressed in future surveys. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Waters and Pittwater which join in Broken Bay give rise to paleo-valleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ~ 200 m. Sediment thickness and water depth is predicted from stitched 1D inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ~ ±1 m with known water depths in areas less than 20 m deep. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleochannels to depth limits of ~ 80 m below sea level in 30 m water depth.