Detection and correction of SPM effects in airborne EM surveys
Terence Kratzer 1 3 James Macnae 1 Paul Mutton 21 RMIT University, GPO Box 2476, Melbourne, Vic. 3001, Australia.
2 Southern Geoscience Consultants, PO Box 694, Belmont, WA 6984, Australia.
3 Corresponding author. Email: terence.kratzer@rmit.edu.au
Exploration Geophysics 44(1) 6-15 https://doi.org/10.1071/EG12048
Submitted: 31 July 2012 Accepted: 13 November 2012 Published: 7 January 2013
Abstract
Recent noise reductions in airborne electromagnetic (AEM) systems have allowed detection of conductors at great depths, but systems now have also become sensitive to superparamagnetic (SPM) effects. We distinguish SPM effects in airborne electromagnetic survey data from the response of good conductors. In electromagnetic data processing, off-time data can be accurately represented as amplitudes of a set of basis functions that are comprised of decays that decrease exponentially as a function of time. The SPM impulse response can be approximated by a decay that is proportional to time to the inverse power, a time dependence associated with magnetic viscosity. We identify the presence of SPM effects, as distinct from the decay of good conductors, by using inverse power-law decays as additional basis functions in constrained least-squares fitting. Application of the method to airborne time-domain electromagnetic (TEM) surveys shows that the method allows correction of SPM and hence aids significantly in conductive target identification.
Key words: airborne EM, magnetic viscosity, SPM, superparamagnetism.
References
Barsukov, P., and Fainberg, E., 2001, Superparamagnetic effect over gold and nickel deposits: European Journal of Environmental and Engineering Geophysics, 6, 61–72Billings, S., Pasion, L., Oldenburg, D., and Foley, J., 2003, The influence of magnetic viscosity on electromagnetic sensors: Proceedings of EUDEM-SCOT2, International Conference on Requirements and Technologies for the Detection, Removal and Neutralization of Landmines and UXO.
Buselli, G., 1982, The effect of near-surface superparamagnetic material on electromagnetic measurements: Geophysics, 47, 1315–1324
| The effect of near-surface superparamagnetic material on electromagnetic measurements:Crossref | GoogleScholarGoogle Scholar |
Cole, K., and Cole, R., 1941, Dispersion and absorption in dielectrics I. Alternating current characteristics: The Journal of Chemical Physics, 9, 341–351
| Dispersion and absorption in dielectrics I. Alternating current characteristics:Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DyaH3MXit1Sktw%3D%3D&md5=52e264ff34c82f9b506d63dfd8214312CAS |
Dunlop, D., and Özdemir, Ö., 2001, Rock magnetism: fundamentals and frontiers: Cambridge University Press.
Hanneson, J., 1992, The transient EM step response of a dipping plate in a conductive half space: Geophysics, 57, 1116–1126
| The transient EM step response of a dipping plate in a conductive half space:Crossref | GoogleScholarGoogle Scholar |
Kratzer, T., and Macnae, J. C., 2012, Induced polarization in airborne EM: Geophysics, 77, E317–E327
| Induced polarization in airborne EM:Crossref | GoogleScholarGoogle Scholar |
Macnae, J., and Lamontagne, Y., 1987, Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data: Geophysics, 52, 545–554
| Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data:Crossref | GoogleScholarGoogle Scholar |
Macnae, J., King, A., Stolz, N., Osmakoff, A., and Blaha, A., 1998, Fast AEM data processing and inversion: Exploration Geophysics, 29, 163–169
| Fast AEM data processing and inversion:Crossref | GoogleScholarGoogle Scholar |
Mutton, P., 2012, Superparamagnetic effects in EM surveys for mineral exploration: ASEG Extended Abstracts, 2012, 1–6
Stacey, F. D., 1963, The physical theory of rock magnetism: Advances in Physics, 12, 45–133
| The physical theory of rock magnetism:Crossref | GoogleScholarGoogle Scholar |
Stanley, J., Sertsrivanit, S., and Clark, P., 1992, Magnetic exploration beneath a near-surface magnetic noise source: Exploration Geophysics, 23, 323–326
| Magnetic exploration beneath a near-surface magnetic noise source:Crossref | GoogleScholarGoogle Scholar |
Stolz, E., and Macnae, J., 1998, Evaluating EM waveforms by singular-value decomposition of exponential basis functions: Geophysics, 63, 64–74
| Evaluating EM waveforms by singular-value decomposition of exponential basis functions:Crossref | GoogleScholarGoogle Scholar |
Watts, A., 1997, Exploring for nickel in the 90’s, or ‘Til Depth us do Part’: Proceedings of Exploration 97: Fourth Decennial International Conference on Mineral Exploration, 1003–1014.
West, G., and J. Macnae, 1991, Physics of the electromagnetic induction exploration method (volume 2 of Investigations in Geophysics): Society of Exploration Geophysicists.