Layered earth inversions of AEM bathymetry data incorporating aircraft attitude and bird offset – a case study of Torres Strait*

Peter Wolfgram; Julian Vrbancich

doi:10.1071/EG07013

RESEARCH ARTICLE

Previous Contents Vol 38(2)

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 ²

+ Author Affiliations

- Author Affiliations

¹ 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.

References

Daniell, J., Hemer, M., Heap, A., Mathews, E., Sbaffi, L., Hughes, M., and Harris, P., 2006, Geoscience Australia Survey 273 Post-cruise Report – Biophysical Processes in the Torres Strait Marine Ecosystem II Geoscience Australia Record 2006(10),
Lane R. , Brodie R. , and Fitzpatrick A. , 2004, A revised inversion model parameter formulation for fixed wing transmitter loop – towed bird receiver coil time-domain airborne electromagnetic data: 17th Geophysical Conference of the Australian Society of Exploration Geophysicists, Extended Abstracts.

Macnae, J., Robb, T., and Vrbancich, J., 2004, Rapid estimation of shallow seawater depth from airborne electromagnetics Exploration Geophysics 35, 288–291.
Sattel D. , Lane R. , Pears G. , and Vrbancich J. , 2004, Novel ways to process and model GEOTEM data: 17th Geophysical Conference of the Australian Society of Exploration Geophysicists, Extended Abstracts.

Smith, R., 2001, On removing the primary field from fixed-wing time-domain airborne electromagnetic data: some consequences for quantitative modelling, estimating bird position and detecting perfect conductors Geophysical Prospecting 49, 405–416.
| Crossref | GoogleScholarGoogle Scholar | Valleau N. C. , and Holladay J. S. , 1987, Airborne electromagnetics for through ice bathymetry: Canadian Hydrographic Conference.

Vrbancich, J., Hallett, M., and Hodges, G., 2000a, Airborne electromagnetic bathymetry of Sydney Harbour Exploration Geophysics 31, 179–186.

Vrbancich, J., Fullagar, P. K., and Macnae, J., 2000b, Bathymetry and seafloor mapping via one dimensional inversion and conductivity depth imaging of AEM Exploration Geophysics 31, 603–610.

Vrbancich, J., 2004, Airborne electromagnetic bathymetry methods for mapping shallow water sea depths International Hydrographic Review 5, 59–84.

Vrbancich, J., and Fullagar, P., 2004, Towards seawater depth determination using the helicopter HoistEM system Exploration Geophysics 35, 292–296.

Vrbancich, J., and Smith, R., 2005, Limitations of two approximate methods for determining the AEM bird position in a conductive environment Exploration Geophysics 36, 365–373.

Vrbancich, J., Sattel, D., Annetts, D., Macnae, J., and Lane, R., 2005a, A case study of AEM bathymetry in Geographe Bay and over Cape Naturaliste, Western Australia, Part 1: 25 Hz QUESTEM Exploration Geophysics 36, 301–309.

Vrbancich, J., Macnae, J., Sattel, D., and Wolfgram, P., 2005b, A case study of AEM bathymetry in Geographe Bay and over Cape Naturaliste, Western Australia, Part 2: 25 and 12.5 Hz GEOTEM Exploration Geophysics 36, 381–392.

Won, I. J., and Smits, K., 1986, Characterisation of shallow ocean sediments using the airborne electromagnetic method IEEE Journal Oceanic Engineering 11, 113–122.
| Crossref | GoogleScholarGoogle Scholar |

Zollinger, R., Morrison, H. F., Lazenby, P. G., and Becker, A., 1987, Airborne electromagnetic bathymetry Geophysics 52, 1127–1137.
| Crossref | GoogleScholarGoogle Scholar |

^* ^*Presented at the Australian Earth Sciences Convention, June 2006, Melbourne.