Comparison of Airborne and Ground TEM systems for a conductor beneath conductive cover - an example from North-West Queensland, Australia

Abstract

Magnetite-pyrrhotite mineralisation at the Bull Creek Prospect in North-West Queensland is both magnetic and conductive. The mineralisation of Proterozoic age is buried beneath 30-50m of conductive younger cover. Overburden conductance values of 10 to 30 S make this a challenging area for airborne electromagnetic systems. The presence of this conductive cover and availability of ground EM and drilling information make this a valuable case study for evaluating the performance of airborne electromagnetic systems. This paper presents data from both ground and airborne time domain electromagnetic systems and compares conductivity sections derived from the data with drilling information. The moving loop ground EM system produced a very clear response from the basement conductor. This conductor was also detected by the 25 Hz GEOTEM and TEMPEST systems flown over the prospect in 1997 and 1998 respectively. Conductivity sections derived from measured response data were useful for visualising the response variations. Due to the width of the discrete conductor and the continuity of the conductive cover, conductivity sections based on a 1-dimensional approximation provided reasonable quantitative estimates for the thickness of conductive cover and the depth to the conductor.