Modelling the airborne electromagnetic response of a vertical contact

Abstract

Airborne electromagnetic (AEM) surveying is an important exploration tool because it can map conductivity variations over large areas at a fraction of the cost of ground survey methods. Using rapid but approximate techniques, large volumes of data may be processed to show the variation of conductivity with depth beneath the survey. These approximate methods work well in regions with horizontal layering, but in certain circumstances they can imply the presence of false conductors in the vicinity of 2D and 3D structures. By comparing the AEM response of several 2.5D models, each of which contains a lateral conductivity contrast, we show that artefacts associated with conductivity contrasts can imply the presence of a false conductor when flight direction is towards the area of greater conductivity. When flight direction was away from the area of greater conductivity artefacts associated with the lateral conductivity contrast implied a false resistor. These artefacts were of sufficiently high magnitude that they masked the response of a genuine conductor (1.0 Wm) at a depth of 50 m. We show that multiple-component data sets utilising the inherent directional dependence qualities of AEM prospecting systems can be used to minimise interpretational errors in the presence of lateral conductivity contrasts.