Rapid approximate inversion of airborne TEM
Peter K. Fullagar 1 6 Glenn A. Pears 2 James E. Reid 3 Ralf Schaa 4 51 Fullagar Geophysics Pty Ltd, PO Box 1946, Toowong, Qld 4066, Australia.
2 Mira Geoscience Asia Pacific Pty Ltd, PO Box 1946, Toowong, Qld 4066, Australia.
3 Mira Geoscience Asia Pacific Pty Ltd, 45 Ventnor Avenue, West Perth, WA 6005, Australia.
4 ARC Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, Tas. 7001, Australia.
5 Present address: Centre for Geoscience Computing, University of Queensland, St Lucia, Qld 4072, Australia.
6 Corresponding author. Email: peter@fullagargeophysics.com
Exploration Geophysics 46(1) 112-117 https://doi.org/10.1071/EG14046
Submitted: 2 May 2014 Accepted: 2 October 2014 Published: 20 November 2014
Abstract
Rapid interpretation of large airborne transient electromagnetic (ATEM) datasets is highly desirable for timely decision-making in exploration. Full solution 3D inversion of entire airborne electromagnetic (AEM) surveys is often still not feasible on current day PCs. Therefore, two algorithms to perform rapid approximate 3D interpretation of AEM have been developed. The loss of rigour may be of little consequence if the objective of the AEM survey is regional reconnaissance. Data coverage is often quasi-2D rather than truly 3D in such cases, belying the need for ‘exact’ 3D inversion.
Incorporation of geological constraints reduces the non-uniqueness of 3D AEM inversion. Integrated interpretation can be achieved most readily when inversion is applied to a geological model, attributed with lithology as well as conductivity. Geological models also offer several practical advantages over pure property models during inversion. In particular, they permit adjustment of geological boundaries. In addition, optimal conductivities can be determined for homogeneous units. Both algorithms described here can operate on geological models; however, they can also perform ‘unconstrained’ inversion if the geological context is unknown.
VPem1D performs 1D inversion at each ATEM data location above a 3D model. Interpretation of cover thickness is a natural application; this is illustrated via application to Spectrem data from central Australia. VPem3D performs 3D inversion on time-integrated (resistive limit) data. Conversion to resistive limits delivers a massive increase in speed since the TEM inverse problem reduces to a quasi-magnetic problem. The time evolution of the decay is lost during the conversion, but the information can be largely recovered by constructing a starting model from conductivity depth images (CDIs) or 1D inversions combined with geological constraints if available. The efficacy of the approach is demonstrated on Spectrem data from Brazil.
Both separately and in combination, these programs provide new options to exploration and mining companies for rapid interpretation of ATEM surveys.
Key words: airborne EM, geological constraints, inversion, resistive limit, time domain.
References
Cox, L. H., Wilson, G. A., and Zhdanov, M. S., 2012, 3D inversion of airborne electromagnetic data: Geophysics, 77, WB59–WB69| 3D inversion of airborne electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Fullagar, P. K., and Oldenburg, D. W., 1984, Inversion of horizontal loop electromagnetic frequency soundings: Geophysics, 49, 150–164
| Inversion of horizontal loop electromagnetic frequency soundings:Crossref | GoogleScholarGoogle Scholar |
Fullagar, P. K., and Pears, G. A., 2007, Towards geologically realistic inversion: Proceedings of Exploration ’07, Fifth Decennial International Conference on Mineral Exploration, Toronto.
Fullagar, P. K., and Reid, J. E., 2001, Emax conductivity-depth transformation of airborne TEM data: 15th ASEG Conference and Exhibition (Brisbane), Extended Abstract, 1–4.
Fullagar, P. K., Pears, G. A., and McMonnies, B., 2008, Constrained inversion of geological surfaces - pushing the boundaries: The Leading Edge, 27, 98–105
| Constrained inversion of geological surfaces - pushing the boundaries:Crossref | GoogleScholarGoogle Scholar |
Fullagar, P. K., Vrbancich, J., and Pears, G. A. 2010, Geologically-constrained 1D TEM inversion: 21st ASEG Conference and Exhibition (Sydney), Extended Abstract, 1–4.
Nabighian, M. N., and Macnae, J. C., 1991, Time domain electromagnetic prospecting methods, in M. N. Nabighian, ed., Electromagnetic methods in applied geophysics: Society of Exploration Geophysicists, Investigations in Geophysics No. 3, Vol. 2, 427–520.
Oldenburg, D. W., Haber, E., and Shekhtman, R., 2013, Three dimensional inversion of multisource time domain electromagnetic data: Geophysics, 78, E47–E57
| Three dimensional inversion of multisource time domain electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Reid, J., Fitzpatrick, A., and Godber, K., 2010, An overview of the SkyTEM airborne EM system with Australian examples: Preview, 2010, 26–37
Schaa, R., 2010, Rapid approximate inversion of 3D transient electromagnetic data: Ph.D. thesis, University of Tasmania.
Schaa, R., and Fullagar, P. K., 2010, Rapid approximate 3D inversion of transient electromagnetic (TEM) data: 80th Annual International Meeting, SEG, Expanded Abstracts, 650–654.
Smith, R. S., and Lee, T. J., 2001, The impulse-response moments of a conductive sphere in a uniform field – a versatile and efficient electromagnetic model: Exploration Geophysics, 32, 113–118
| The impulse-response moments of a conductive sphere in a uniform field – a versatile and efficient electromagnetic model:Crossref | GoogleScholarGoogle Scholar |
Smith, R. S., and Lee, T. J., 2002, The moments of the impulse response: a new paradigm for the interpretation of transient electromagnetic data: Geophysics, 67, 1095–1103
| The moments of the impulse response: a new paradigm for the interpretation of transient electromagnetic data:Crossref | GoogleScholarGoogle Scholar |