Inversion of concentric loop electromagnetic data by transformation to an equivalent potential field response

A novel approach to the inversion of concentric loop electromagnetic (EM) data is through transformation of moving-source EM data to an equivalent fixed-source potential field. This equivalent potential field has been named as the surrogate potential field. The proposed approach then provides for example, efficient target depth estimates through Euler deconvolution. Under the quasi-static approximation, at each instant in the earth, diffuse loops of induced current are the source of the recorded response. Successive samples of the magnetic field of confined current loops can be mathematically manipulated to create a surrogate potential field from an equivalent constant induced current, as the surrogate potential field is proportional to the square root of the concentric loop EM response. Processing includes inspecting data and correcting for the sign ambiguity at suitably stable locations.

Correct surrogate potential field transformation relies on induced current systems being geometrically fixed in location, and as such will be predominantly correct for tabular conductors that are of relatively small size. For these small targets, stitched 1D layered-earth EM inversions often overestimate the depth of a small near-surface body. Unconfined currents induced in conductive overburden introduce a ‘background’ which requires subtraction 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 has estimated the target parameters from EM data. Successful estimations of target depth and location have been performed for synthetic horizontal and vertical tabular plate models, and with limitations for compact bodies represented by a spherical conductor. Three case studies illustrate the characteristics of the proposed method, illustrating useful results with limitations arising in field data and its processing.