The electromagnetic Response of a Conductive Inhomogeneity In a Layered Earth

Abstract

In this thesis, the integral equation technique is used to model the electromagnetic prospecting problem of a conductive inhomogeneity embedded in a layered earth environment. Basically, it consists of replacing the anomalous conductor, which is simplified to a finite vertical thin plate, by a surface of scattering currents. The plate is sampled on a sqare grid and the number of points is kept to a minimum by using a fifth degree spline moment function to interpolate between them. In order to bypass the numerical problems encountered when attempting to solve directly for the components of electric field at each grid point, the integral equation is expressed in terms of two potentials representing divergence free current flow through its surface. Calculations involving the layered earth are done with a new Fast Fourier transform approach which basically consists of solving this problem in the Catesian wavenumber domain, i.e. the 2-D Fourier transform of the horizontal plane.

The results show that a conductive host and/or overburden will affect the amplitude, phase and width of the conductor's free air anomaly. Both the host and the overburden will cause an attenuation and phase rotation of the source fields to the plate and the anomalous fields from the plate. When the target conductor is in contact with a conductive host, the channelling of currents induced in the host is found to increase the anomaly amplitude substantially.

Computation costs do not permit the generation of high accuracy type curves, but the model examples presented demonstrate the basic physics of conductive earth effects in electromagnetic prospecting.