Estimating Cover Thickness in the Southern Thomson Orogen – A Comparison of Applied Geophysics Estimates with Borehole Results

Abstract

Potential sites for up to sixteen stratigraphic boreholes were selected to intersect the basement geology throughout the southern Thomson Orogen as part of the Southern Thomson Project. At each of these sites high resolution estimates of cover thickness were derived by applying refraction seismic, Audio-MagnetoTelluric, Targeted Magnetic Inversion and passive seismic geophysical techniques. Estimating cover thickness in this way reduced the technical risk associated with drilling and allows for the various geophysical techniques to be compared at each site. A comparison of the estimates derived from the applied geophysical techniques with the actual cover thicknesses determined from borehole geological and geophysical logs, together with an analysis of the uncertainties for each method, has highlighted the effectiveness of each geophysical technique. These new data and interpretations contribute to an Explorers’ Toolkit of techniques to help reduce the technical risk to the mineral exploration industry in searching for new mineral deposits in covered terrains in general, and in particular in the underexplored terrain of the southern Thomson Orogen. The various geophysical estimates highlight that the basement-cover interface throughout the southern Thomson Orogen can be recognised by its seismic velocity, electrical conductivity and magnetic susceptibility contrasts. However, the cover thickness estimates determined by the geophysical techniques shown here provide non-coincident estimates in some cases and, as such, it is important to take into account their unique limitations and uncertainties. When comparing results from the GSQ Eulo 1 and GSQ Eulo 2 boreholes it is clear that the refraction technique produced the most accurate estimates due to a strong contrast between the velocity structure of the Eromanga Basin sediments and the basement geology throughout the study area.