Numerical modeling of the electromagnetic response of three‐dimensional conductors in a layered earth

Abstract

This paper describes a numerical modeling program which was developed for computing the electromagnetic response of a three‐dimensional conductor in a layered earth. The algorithm is a hybrid one, in the sense that a finite‐element method is combined with an integral equation approach for the complete solution of the problem. In this approach, two relatively small matrices are inverted to obtain the secondary magnetic fields at the interior nodes of the subdivided volume. The finite‐element method requires special attention at low frequencies, i.e., when the body dimensions are small compared with the skin depth. The method was tested with various models involving spheres and rectangular dikes, and comparisons have been made with analytical, numerical, and scale‐modeling data. These comparisons generally show good agreement between our numerical model and the other models tested. Examples are given which show the flexibility and usefulness of this modeling algorithm when applied to a ground or airborne prospecting system with a coil separation of 10 m.