Comparing responses from different AEM systems and derived models at the Sunnyside nickel project, Botswana

Abstract

The Sunnyside nickel deposit in SE Botswana is a shallow Selebi-Phikwe type deposit composed of disseminated, blebby and massive nickel sulphides. It was discovered by Anglo American in the 70’s but considered uneconomic to mine. It is associated with pyroxenite and gabbro, and is an extremely complex orebody. Since that time several nickel companies have explored the body further, trying to improve on the size and grade and confirm whether the body extends to depth. The deposit has been surveyed to date by 4 different AEM systems, being VTEM, Spectrem, SkyTEM and Xcite. In addition, detailed ground geophysics in the form of moving loop TDEM and AMT has been done. In this paper we present a detailed comparison of the EM data measured by the 4 systems, and of the models derived through quasi-3D spatially constrained inversions, and of full 3D inversion of the AEM data. The resulting models are also compared to drilling information and the resistivity models obtained from inversion of the AMT data. The AEM systems display different signal and noise levels, and various types of preprocessing. The inversion results are, in general, in good agreement with each other and with the ancillary drilling and AMT information. SCI results follow the plunging conductor to significant depth. Some systems produce inversion outputs with higher accuracy or depth of investigation than others. IP effects, present in portions of the AEM datasets, add another degree of complexity but can also provide an extra layer of information. Sunnyside represents, defacto, the AEM test site for southern Africa.