Airborne electromagnetic detection of shallow seafloor topographic features, including resolution of multiple sub-parallel seafloor ridges*
Julian Vrbancich 1 3 Graham Boyd 21 Defence Science and Technology Organisation (DSTO), 13 Garden Street, Eveleigh, NSW 2015, Australia.
2 Geosolutions Pty Ltd, 16 Centre Way, Belair, SA 5052, Australia.
3 Corresponding author. Email: julian.vrbancich@dsto.defence.gov.au
Exploration Geophysics 45(3) 189-200 https://doi.org/10.1071/EG12041
Submitted: 12 July 2012 Accepted: 9 April 2014 Published: 15 May 2014
Abstract
The HoistEM helicopter time-domain electromagnetic (TEM) system was flown over waters in Backstairs Passage, South Australia, in 2003 to test the bathymetric accuracy and hence the ability to resolve seafloor structure in shallow and deeper waters (extending to ~40 m depth) that contain interesting seafloor topography. The topography that forms a rock peak (South Page) in the form of a mini-seamount 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 peak at the water surface, after an empirical correction was applied to the data to account for imperfect system calibration, consistent with earlier studies using the same HoistEM system. A much smaller submerged feature (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 in these two regions showed that the airborne TEM system, following empirical data correction, was effectively operating correctly. The third and most important component of the survey was flown over the Yatala Shoals region that includes a series of sub-parallel seafloor ridges (resembling large sandwaves 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 sub-parallel ridges provide an interesting topography because the interpreted water depths obtained from 1D inversion of TEM data highlight the limitations of the EM footprint size in resolving both the separation between the ridges (which vary up to ~300 m) and the height of individual ridges (which vary up to ~20 m), and possibly also the limitations of assuming a 1D model in areas where the topography is quasi-2D/3D.
Key words: airborne electromagnetic, bathymetry, EM footprint, seafloor topography, Yatala Shoals.
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