Layered earth inversions of AEM bathymetry data incorporating aircraft attitude and bird offset – a case study of Torres Strait*
Peter Wolfgram 1 3 Julian Vrbancich 21 Fugro Airborne Surveys Australia, Locked Bag 6, Wembley WA 6014, Australia.
2 DSTO Maritime Operations Division, PO Box 44, Pyrmont NSW 2009, Australia.
3 Corresponding author. Email: pwolfgram@fugroairborne.com.au
Exploration Geophysics 38(2) 144-149 https://doi.org/10.1071/EG07013
Submitted: 6 September 2006 Accepted: 14 February 2007 Published: 15 June 2007
Abstract
The potential of obtaining water depths derived from AEM data that are accurate to ~0.5 m cannot be fully realised if the variable geometry between transmitter and receiver is neglected. The rigorous approach of monitoring the bird offset in three dimensions is currently too costly. However, neglecting the bird offset variations from the assumed fixed nominal offset is problematic because the measured decay shape is sensitive to the transmitter–receiver geometry. An alternative procedure is to invert both layered earth parameters and geometric parameters using a non-linear least-squares method. This inversion procedure has been tested on 25 Hz GEOTEM data from a survey flown in the Torres Strait. The accuracy of the inverted water depths was appraised using data from laser airborne depth sounding surveys. Good agreement, to within ~1 m was obtained between the different methods.
Key words: AEM, layered earth inversion, GEOTEM, bathymetry, bird offset.
Acknowledgments
Dr Daniel Sattel developed an early version of the technique and assisted with the initial tests. We are grateful to Fugro Airborne Surveys for sponsoring the research involving inversion of AEM geometric parameters. J.V. gratefully acknowledges the Australian Hydrographic Service for sponsoring the survey and for providing laser airborne depth sounding data; and Frederic Saint-Cast (Geosciences Australia) for providing raw conductivity data for marine surveys S266 and S273. Finally, we thank the editors and reviewers for their thoughtful suggestions and comments that helped improve the clarity of the paper.
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* *Presented at the Australian Earth Sciences Convention, June 2006, Melbourne.