QUASI-LINEAR APPROXIMATION IN 3-D ELECTROMAGNETIC MODELING

摘要

The Born approximation in electromagnetic (EM) numerical modeling has limited application for solving 3-D electromagnetic induction problems, because in structures with high conductivity contrasts and al high frequencies, this approximation is inaccurate. In this paper, we develop a new and relatively simple approximation for the EM field called a qyasi-linear approximation, which is based on the evaluation of the anomalous field E(a) by a linear transformation of the normal (primary) field: E(a) = <(lambda)over cap>E(n), where <(lambda)over cap> is called the electrical relectivity tensor. The reflectivity tensor inside inhomogeneities can be approximated by a slowly varying function that can be determined numerically by a simple optimization technique. The new approximation gives an accurate estimate of the EM response for conductivity contrasts of more than one hundred to one, and for a wide range of frequencies. It also opens the possibility for fast 3-D electromagnetic inversion. [References: 15]