Modelling the airborne electromagnetic response of a sphere underlying a uniform conductive overburden

Abstract

Electromagnetic geophysical methods are used in mineral exploration to detect conductors at depth. In igneous and metamorphic settings, the background half-space is often largely resistive. In such cases, it is important to consider the interaction between the target conductor and any thin, conductive overburden that might exist above the half-space. The overburden is often comprised of glacial tills and clays or the weathering of basement rocks to more conductive material. This situation can be approximated using a discrete conductor model consisting of a “dipping sphere” in a resistive half-space underlying a uniform conductive overburden. A semianalytical solution that considers the first-order interaction of the sphere and overburden has been derived to calculate the electromagnetic response. The simplicity and efficiency of this solution makes it well suited to be implemented when computation time and immediacy of results are desirable. To this end, we have developed a graphical user interface (GUI) based program to model the electromagnetic response of this model. The program allows users to change the parameters of the survey and target body and quickly view the resulting changes in the shape and decay of the electromagnetic response. The program was tested on airborne electromagnetic data from the Forrestania test range in western Australia. The sphereoverburden model as implemented in the program was able to fit the anomalous data with a spherical body buried 156 m deep and having a dip of 35 degrees to the north.