A case study of deep electromagnetic exploration in conductive cover

Abstract

This paper describes the survey design, data collection and results from a geophysically driven exploration program in Northern Australia. The exploration project was a joint venture between Falcon Minerals NL and Anglo American. It was focussed on finding massive to net textured nickel sulphides associated with a mafic intrusive body. The extent of the intrusive body was inferred from interpretation of a significant 14mgal gravity anomaly. Limited previous exploration indicated any drill targets were likely to be covered by 400 to 500m of mesozoic and recent sediments. These sediments are estimated to have a conductance of 200Siemens, making it difficult to use conventional electrical geophysics for effective exploration. Anglo American, together with its research partner IPHT of Germany have developed a low temperature superconducting quantum interference device (LTS) that can be used as a sensor in transient electromagnetic (TEM) surveys. The LTS is a ?B field? sensor with significant advantages in noise levels over other ?B field? sensors and the ability to detect targets with time constants up to 3 orders of magnitude larger than conventional dB/dt coils (Le Roux et al 2007). The LTS sensor is particularly suited to finding highly conductive targets beneath conductive cover. After forward modelling and survey design, the JV completed some 270km of reconnaissance moving loop ground electromagnetic surveys. Selected anomalies identified with this survey were then followed up with more detailed moving loop and some fixed loop surveys. These results were then modelled using a variety of software packages to try and fit the late time TEM field data. Drilling of selected targets and some downhole TEM surveys are in progress.