The use of airborne TEM for detection of various seafloor topographic features ? multiple quasi-parallel ridges (Yatala Shoals), a mini-mountain peak (South Page) and a submerged bank (Threshold Bank) in Backstairs Passage, South Australia

Abstract

Two helicopter TEM systems (HoistEM and RepTEM) were flown over waters in Backstairs Passage, South Australia, in 2003 and 2010 respectively to test the bathymetric accuracy and hence the ability to resolve seafloor structure in shallow and deeper waters (extending to ~40m depth) that contain interesting seafloor topography. The topography that forms a rock peak (South Page) in the form of a mini-mountain that barely rises above the water surface was accurately delineated along its ridge from the start of its base (where the seafloor is relatively flat) in ~ 30 m water depth to its quasi-submerged peak. A much smaller submerged peak (Threshold Bank) of ~ 9 m peak height located in waters of 35 to 40 m depth was also accurately delineated. These observations when checked against known water depths showed that the two airborne TEM systems were operating correctly. The third component of the survey was flown over a series of quasi-parallel seafloor ridges (resembling large sand waves rising up to ~ 20 m from the seafloor) that branch out and gradually decrease in height as the ridges spread out across the seafloor. These features provide an interesting topography because the interpreted water depths obtained from 1D inversion of TEM data highlight the effect of the EM footprint in resolving both the separation between the ridges and the height of individual ridges, and possibly also the limitations of assuming a 1D model in areas where the topography is quasi-2D.