Forward and Inverse Modeling of Large Loop TEM Data Over Multi-layer Earth Models

Abstract

The Transient electromagnetic method (TEM) represents a class of relatively new electromagnetic (EM) methods, which are widely being used for ground water, mineral exploration and environmental and geotechnical problems. The present work describes a forward computation and an inversion schemes for the interpretation of large loop transient electromagnetic (TEM) data acquired using central loop, in-loop and offset-loop configurations over multi-layer earth models, which are still useful for getting an initial guess about the subsurface resistivity and are used as initial model for advanced inversion schemes. The inversion program makes use of non-linear least square approach to generate a smooth inverted model from the data on the basis of criteria of minimization of misfit function and/or convergence of residual in two successive iterations. The forward problem is first formulated in frequency domain, and then Fourier cosine or sine transform is used to transform it into the time domain. The accuracy and robustness of algorithms are tested by computing the forward TEM response and inverting the large loop TEM data acquired using central loop, in-loop and offset loop configurations with or without the addition of random noises over homogeneous, two layer, three layer and four layer earth models. Both the forward and inverse programs work satisfactorily for the TEM response of a large loop source over layered earth models. The programs in their present forms are meant for computation and inversion of voltage response data, but have option for forward computation and inversion of apparent resistivity data with small changes in constraints of the program and input parameters.