THE ELECTROMAGNETIC RESPONSE OF A CONDUCTIVE INHOMOGENEITY IN A LAYERED EARTH

Abstract

A numerical model has been constructed to determine the three‐dimensional electromagnetic fields in the vicinity of a finite, thin, conductive plate buried in a horizontally stratified, conductive environment. The EM source is a rectangular loop. The problem is formulated as an integral equation for the electric field in the plate. However, to avoid certain numerical difficulties, the actual working variables are a pair of scalar potentials which represent divergence‐free and curl‐free current flows in the plate, and whose values are known at the nodes of a rectangular grid. The basic integral equation is then reduced to a set of linear equations which can be solved numerically. The cases modeled are a simulation of the Turam method. The models were a shallow plate and a deep plate in a conductive half‐space, a deep plate in an insulating host rock under a conductive layer, and a deep plate in a conductive host rock under a conductive layer. In all cases, the top of the plate was separated from the overburden, and the conductivities of the plate, layer, and host rock were varied widely. It was found that a conductive overburden layer alone causes a phase rotation and an attenuation of the local anomaly, while a conductive host medium causes, mainly, the addition of a “current gathering” component to the anomaly. The importance of the current gathering effect may vary from negligible to enormous as its amplitude and phase depend strongly on the conductivity of the host rock. When a conductive overburden and a moderately conducting host rock are present, both effects may arise. Anomaly enhancement by a conductive host rock is not likely to be advantageous in most prospecting situations, for while the detectability of a target bedrock conductor goes up, the ability to distinguish its anomaly from other weaker conductors is markedly decreased.