The Application of Airborne Electromagnetics to the Search for High Conductance Targets

Abstract

The airborne electromagnetics (AEM) technique has been used very successfully for over 50 years to locate conductive ore bodies. For most users of the technique, the primary purpose of AEM has been to provide a rapid and inexpensive means to locate targets, whereupon the best features were then followed up with ground geophysical techniques prior to possible drill testing. In this role as a first-pass detection tool, AEM equipment development has focused primarily on spatial resolution, depth of detection and rejection of conductive cover. Traditionally little emphasis was placed upon the need to provide quantitative information about a target?s conductance. This situation has been changing in the last 10 years, driven in large part from concerns expressed from those groups exploring for magmatic nickel deposits. Drawing upon both petrophysical studies and field observations, a case is developing that magmatic nickel deposits have conductances that place them far outside the conductance bandwidth of effectively all AEM systems. This being the case, then the use of AEM as a primary target identification tool for new nickel deposits is a seriously flawed strategy. So as to better understand this issue, AEM data sets acquired over the Voisey?s Bay deposits, Labrador, Canada have been analysed using standard commercial processing techniques such as CDIs and time constant analysis. Additionally, a series of numerical models have been generated to try to better explain the observed field results, as well as simulate the response that other AEM systems would produce over the deposit. While the Voisey?s Bay deposit undoubtedly contains mineralisation of very high conductivity, the processing and modelling shows the AEM technique to be an effective means to locate and discriminate targets of high conductance. Although such conductance discrimination may only be achievable on a relative scale, it is seen as generally adequate for most exploration situations.