Follow-up drill hole surveying to determine unidentified EM targets

Abstract

In these two case histories, drill hole surveying using down-hole electromagnetic surveys and wireline conductivity probes are used to determine the source of geophysical targets that remained unidentified after initial drill testing. In the first example, after drilling the identification of the geophysical target remained uncertain, despite surface EM surveys determining it had a high conductance. Subsequent DHEM and conductivity surveys were clearly able to locate and define the targeted conductor. In the second example, deep AMT targets could not be identified after drill testing. Using data from an AEM survey over the same area, and after subsequent DHEM surveying, it appears that the targets are probably artefacts of complex (frequency dependent) conductivity in the near surface soils and regolith. Targeting errors are very costly. These examples emphasise how critical follow-up drill hole surveying can be to resolving unidentified geophysical targets and ensuring that exploration practices are sound and efficient.