oa Transformation of coincident loop EM data to an equivalent potential field response

We aim to provide a novel approach to the inversion of EM data through transform of moving-source EM data to an equivalent fixed-source potential field. We call this potential field the Surrogate potential field. The proposed approach then provides accurate and efficient depth estimates by Euler deconvolution. Under the quasi-static approximation at each instant a “smoke ring” of induced current will be the source of a potential field, samples of which will be mathematically manipulated to create a surrogate potential field from multiple realisations of induced current. The surrogate potential field is proportional to the square root of a coincident loop response. Processing will involve inspecting data and correcting for the sign ambiguity at suitably stable locations.

Surrogate potential field inversion will be most useful for horizontal and steeply dipping plates that are of relatively small size, where stitched 1D layered earth EM inversions are slow and often overestimate the depth of a small near surface body. Conductive overburden introduces a “DC offset” which may be subtracted before transformation to a surrogate potential. Confidence limits on solutions are part of a least squares regression involving a cluster analysis. These form the basis for evaluation of the reliability with which the proposed method estimated target parameters from EM data. Successful estimations of depth and location have been performed for synthetic horizontal and vertical tabular plate models calculated by Maxwell. A successful inversion of field data has been achieved using the surrogate potential field method.