Airborne electromagnetic detection of shallow seafloor topographic features, including resolution of multiple sub-parallel seafloor ridges

Presented at the 22nd ASEG Geophysical Conference and Exhibition, February 2012.

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.

An airborne EM survey over shallow waters in Backstairs Passage, South Australia, detected a mini-seamount and a series of sub-parallel seafloor ridges that vary in peak height and inter-peak spacing. The interpreted resolution of these ridges based on 1D and 3D inversion is consistent with the expected EM footprint.

Key words::

• airborne electromagnetic, bathymetry, EM footprint, seafloor topography, Yatala Shoals.

Acknowledgements

Julian Vrbancich (J.V.) thanks the Royal Australian Navy (Australian Hydrographic Service) for providing the lidar bathymetry data that was used as a ground truth for the water depths in Backstairs Passage. J.V. also thanks Wolfgang Preiss (Geological Survey of South Australia) for his advice concerning the formation of Backstairs Passage and the likely seafloor morphology of Yatala Shoals, and TechnoImaging for providing synthetic HoistEM response data from a 3D model emulating multi-peak seafloor ridges. The authors are grateful to a reviewer for helpful suggestions that improved the clarity of this paper.

Notes

Presented at the 22nd ASEG Geophysical Conference and Exhibition, February 2012.