Fast approximate 2D inversion of airborne TEM data: Born approximation and empirical approach
Résumé
Airborne transient electromagnetic surveying provides data sections with a sufficient coverage to perform 2D imaging of electrical conductivity within the ground. Full 2D inversion using numerical modeling with finite differences or finite elements is still a time-consuming method to process the large amount of data acquired during an airborne survey. 2D structures increase the complexity of eddy current patterns within the ground. Consequently, fast approximate imaging using 2D sensitivities of equivalent homogeneous media is not sufficient and causes strong artefacts in the resulting model. To overcome this problem, one prefers to use 1D inversion or 3D inversion using local sensitivity to process this kind of data. However, we consider a fast 2D inversion to be reachable. By estimating numerically the 2D sensitivity caused by 2D perturbations and showing that it differs considerably from the ones derived from homogeneous media, we propose an empirical model for in-loop configuration which describes the numerical 2D sensitivity. By applying this method to synthetic data, we show that it eliminates 2D artefacts which are often encountered when using approximate inverse methods based on the theory of equivalent homogeneous half-space. An application to real in-loop data illustrates this improvement for imaging a dipping layer of conductive graphite deposits in Canada. This method is relatively fast. It could provide a better understanding of the ground during the survey and would allow geophysicists to better manage the whole campaign.
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