A new AEM imaging algorithm for use in conductive areas

Abstract

A major use of airborne electromagnetic (AEM) surveys is to identify two and three-dimensional scattering effects that may be indicative of mineralisation. Where the background is resistive, these effects can be interpreted from simple properties of the scattered field. In conductive areas however, this approach is not generally feasible and conductivity maps are usually generated instead. Such maps assume that the ground can be approximated with a one-dimensional model. However, representing two and three-dimensional scattering effects with a one-dimensional model can be misleading. This paper presents a new approach for processing AEM data to image two and three-dimensional structure in a conductive background. These structures are assumed to be discrete, compact impedance contrasts that are replaced by distributions of unknown induced-sources. Since the scattered field is linear function of the induced sources, a set of linear equations can be solved to find the impedance contrasts. An approximate solver is used in the solution. This solver is rapid enough to be applied routinely to large AEM data sets, and is tunable so unwanted artefacts can be rejected. Two examples are presented using synthetic data. The first example illustrates how the solver can be tuned, and is presented for a two-dimensional imaging problem with free-space scattering. The second example illustrates three-dimensional imaging in a conducting environment.