A Transient Electromagnetic (TEM) geophysical survey was trialled in Bowen Basin Permian coal measures overprinted by Cretaceous intrusion. The trial’s intention was to compare whether TEM could assess residual coal more accurately and economically than drilling.
TEM results were promising but had limitations. TEM did not compare to drilling for defining coal thickness or relative level definition. However, TEM resolved conductive anomalies laterally; providing a qualitative picture of remnant coal.
Three material types and associated conductivities occur within the trial area: overburden (sandstone/siltstone), intrusive sills and heat affected coal.
Ground conductivity was analysed by measuring decay of residual current within the TEM loop. Fast decay related to resistive overburden and intrusion, attenuated decay related to conductive coal.
Generally speaking, carbon content governs coal resistivity where good quality, low ash, bright coal is more resistive relative to dull, high ash coal. Though in the trial area, relative to intrusion and overburden, coal was found to be the most conductive of the three materials.
By modelling conductive anomalies, a general picture emerged of remnant coal regions. Subsequent highwall exposure reconciled with conductive TEM anomalies.
Stedman, H J, 2017. Findings from a transient electromagnetic trial for heat-affected coal, in Proceedings Tenth International Mining Geology Conference 2017, pp 233–238 (The Australasian Institute of Mining and Metallurgy: Melbourne).