A geological interpretation of observed electrical structures in the regolith: Lawlers, Western Australia

Abstract

SALTMAP airborne electromagnetic (AEM) data for the Lawlers District, W.A., show that areas of complex regolith cover are characterised by marked variations in electrical conductivity. Their interpretation against ground electromagnetic, petrophysical and drill hole data indicates that conductive zones lie within the regolith, usually between a thin, relatively resistive, surface layer and a resistive basement. The conductive layer is commonly associated with the saprolite and to a lesser extent with alluvium in palaeovalleys and drainage sumps. Interpretation of the AEM with aeromagnetic and field data indicates that there is a strong lithodependence in the observed conductivity structure. Structure is also important. One-dimensional layered earth inversions and conductivity depth sections showed that the conductivity and thickness of regolith materials (predominantly saprolite) varied with lithology. For example, the felsic volcanics located in the NE margin of the survey area are characterised by a thick, poorly conductive saprolite. This contrasts with a thinner, more conductive saprolite developed over adjacent mafic lithologies. Somewhat surprising were the electrical characteristics of the ultramafic units, which appeared to be similar to those of the felsic volcanic units, suggesting thick, relatively resistive, materials. This behaviour is at odds with that reported in other studies concerning the electrical properties of weathered ultramafics in other environments. The Lawlers study suggests that differences in the electrical properties of in-situ regolith materials can be attributed to the complex interplay between the manner in which particular lithologies weather, the character of the resulting regolith in terms of porosity and permeability, and to variations in the soluble salt content and quantity of the saturant waters. The "EM response map" of the Lawlers area is further complicated by transported cover which appears to impose a cross cutting conductivity structure over that of the underlying saprolites.