Simplified electrical structure models at AEM scales, Lawlers, Western Australia

Abstract

Fixed wing airborne electromagnetic (AEM) data in Australia commonly exhibit numerous local responses, most of which have been attributed to regolith inhomogeneities rather than the isolated conductive targets of sulphide exploration. To define the regolith structures causing local AEM responses several steps are needed. The first involves selecting a geological mapping scale matched to the scale of an AEM system. Sensitivity analysis indicates that an AEM system has a limited range, and is generally insensitive to any features located more than 200 to 300 m from the system. Generally, any confined conductive target cannot be detected at a distance more than a few times its lateral dimensions. With a nominal transmitter altitude of 120 m, this would set (say) a 30 m or 40 m minimum size on any structure likely to produce an anomaly. Long narrow features can however gather current and be detectable. A mapping scale was therefore indicated in the range between 1:10,000 and 1:100,000. In the Lawlers district of WA, the electrical structure could be resolved into at most 3 layers, specifically a thin (commonly 0 to 10 m), moderately resistive layer of alluvium/colluvium overlying a thick (commonly 30 to 50 m) conductive layer of saprolite/sediments, overlying a relatively resistive basement. One or both of the upper layers are absent in some areas. Seven important structures were identified at Lawlers at the scales suggested above. Drillhole data, ground and airborne EM, and ground resistivity were used to define vertical and lateral variations of importance within this overall layering scheme. The three common electrical structures were classified as 1) Layer (locations far from lateral inhomogeneity), 2) Contact, and 3) Wedge models. Four less common structures were the 4) Variable basement topography, 5) Variable surface topography, 6) Lateral inhomogeneity, and 7) Palaeochannel models. Geological and physical reasoning constrain the permissible geometry and dimensions of each of these structures. When AEM data in map form is interpreted by a geologist or geophysicist using other constraints (derived e.g. from drilling, magnetics, radiometrics etc.), it is possible to attribute an appropriate model from the above set to virtually every local anomaly of regolith origin observed in the survey. Once this is done, the slow process of modelling and interpreting individual responses may be attempted, in particular to distinguish bedrock target responses from those due to regolith structures.