Electrical structure of the regolith in the Lawlers District, Western Australia

Abstract

The weathered rock and transported overburden, which covers much of Australia, reduces the effectiveness of electromagnetic (EM) surveys. This material, termed the regolith, is variably conductive, both laterally and vertically, and reduces the sensitivity and penetration of EM systems. AMIRA (Australian Mineral Industries Research Association) project 407 was set up to improve the effectiveness of EM and especially airborne EM (AEM) in Australia. One of the specific aims was to study the electrical structure of the regolith in the expectation that a better understanding might lead to a significant reduction, or even removal, of its effect on deep penetrating AEM methods. As part of that study, several sites around Australia were selected for a detailed study of the electrical structure of the regolith and its effect upon AEM systems. This report discusses the results from a SALTMAP survey over the Lawlers area, which lies within the Eastern Goldfields region of the Yilgarn Craton in Western Australia. To test the accuracy and resolvability of the AEM data, a number of AEM responses were followed up with ground EM surveys. Both sets of data were interpreted using layered earth inversion algorithms. It was appreciated that a stitched set of 1D layered structures would not accurately model the physics of all of the responses, however it was expected that the vast majority could be usefully treated this way. The results show that although there may be up to six or even seven geologically identified layers with distinct electrical properties in the regolith, neither the airborne or surface EM data can effectively resolve more than three layers. However, good fits to three layer models can be obtained and much can be inferred about the nature of the regolith. At Lawlers, these layers can be broadly represented as follows: (1) the (mostly) resistive surface layer of colluvium/alluvium; (2) underlying conductive saprolite, mottled clays, etc.; and (3) resistive basement.