Modelling an arbitrarily oriented magnetic dipole over a homogeneous half-space for a rapid topographic correction of airborne EM data
Julien Guillemoteau 1 4 Pascal Sailhac 2 Mickael Behaegel 3
+ Author Affiliations
- Author Affiliations
1 Universität Potsdam, Institut für Erd und Umweltwissenschaften, 14476, Potsdam-Golm, Germany.
2 Université de Strasbourg, Institut de Physique de Globe/EOST, CNRS-UDS UMR 75-16, 67000, Strasbourg, France.
3 Areva Mines, Mining Business Group, Geosciences Department, 92084 Paris La Défense, France.
4 Corresponding author. Email: julien@geo.uni-potsdam.de
Exploration Geophysics 46(1) 85-96 https://doi.org/10.1071/EG13093
Submitted: 19 November 2013 Accepted: 19 August 2014 Published: 15 October 2014
Abstract
Most airborne electromagnetic (EM) processing programs assume a flat ground surface. However, in mountainous areas, the system can be at an angle with regard to the ground. As the system is no longer parallel to the ground surface, the measured magnetic field has to be corrected and the ground induced eddy current has to be modelled in a better way when performing a very fine interpretation of the data. We first recall the theoretical background for the modelling of a magnetic dipole source and study it in regard to the case of an arbitrarily oriented magnetic dipole. We show in particular how transient central loop helicopter borne data are influenced by this inclination. The result shows that the effect of topography on airborne EM is more important at early time windows and for systems using a short cut-off source. In this paper, we suggest that an estimate be made off the locally averaged inclination of the system to the ground and then to correct the data for this before inverting it (whether the inversion assumes a flat 1D, 2D or 3D sub-surface). Both 1D and 2D inversions are applied to synthetic and real data sets with such a correction. The consequence on the ground imaging is small for slopes with an angle less than 25° but the correction factor can be useful for improving the estimation of depths in mountainous areas.
Key words: airborne geophysics, electromagnetic imaging, inverse problem.
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