Transient electromagnetic measurements to late delay times over the Woodlawn ore body

Abstract

A traverse over the Woodlawn ore body was carried out by CSIRO in September 1974 to test a partially-completed multichannel portable instrument for transient electromagnetic (TEM) measurements. Measurements were made in areas where the Bureau of Mineral Resources (BMR) had reported noise interference with the Russian-made MPPO-1 instrument, and details are given of the investigation of the main sources of noise (of frequency 0.050, 22,44 and about 800 kHz in the area. It is deduced that the most likely source of interference with the MPPO-1 was the high level 800 kHz RF broadcast signal. Results of the traverse and an investigation of the effect of loop size on the decay curve are presented. High signal-to-noise ratios ranging from 4 (off the body) to 280 (directly over the body) are obtained for the latest delay time of 60?80 ms. The traverse shows a marked difference between the early- and late-time response, which falls by 2 orders of magnitude 90 m away from the ore body, while the early-time response falls by about 1 order of magnitude. The decay curves show the presence of at least three time constants. Decreasing the loop sides from 45 m to 11.3 m produces little effect on the shape of the decay curve, but when the loop sides are increased from 45 m to 90 m or beyond, the bulk of the response is caused by less conductive host rock overlying the deeper part of the ore body, and the decay curve obtained has shorter time constants than those of the decay curve measured with the smaller loops (of sides 45 m or less). A portable multichannel instrument incorporating the improvements established in these and previous investigations is at present being constructed.