Case History Illustrating Interpretation Problems in Transient Electromagnetic Surveys

During an exploration programme for volcanogenic massive sulphides in the Mt Windsor belt of north-east Queensland a variety of transient electromagnetic (TEM) profiles were measured over a particular prospect. On the basis of separated-loop SIROTEM and fixed-loop EM 37 profiles the most distinctive anomaly was initially interpreted to be due to a steeply dipping bedrock conductor, but drilling and down-hole SIROTEM could not adequately explain the anomaly. Subsequent coincident-loop SIROTEM and fixed-loop EM 37 and UTEM surveys utilizing a transmitter loop on each side of the interpreted conductor demonstrated that its source was not within bedrock but was actually the edge of a conductive surf icial layer. Comparison of the field data with analogue model profiles confirms this interpretation. As it is apparent that similar geological situations will commonly be encountered during TEM surveys in conductive environments, caution in interpretation is required. In a number of cases apparent bedrock conductors may need to be re-surveyed with a different field configuration to confirm their validity before drill testing.