Geophysical and Image Processing Methods for Detection of Fireholes in Brown Coal, Latrobe Valley

Fireholes at the top of the thick Latrobe Valley brown coal seams pose a geotechnical hazard to overburden dredges and reduce coal reserves. Overburden thickness (typically 10 to 15 m) is up to 50 m in the fireholes, which are from 20 to hundreds of metres in diameter and are infilled with baked clays, soft lacustrine clays and alluvial deposits.

Given the complexity of firehole geometry and overburden geology, firehole definition prior to overburden stripping, by drilling alone, is expensive and is not definitive. To improve firehole exploration, geophysical methods were tried in a test area with good borehole control (115 holes), near Morwell open cut.

Grid geophysics (20m.x20m., 2805 grid stations) using gravity, EM34 20 m loop conductivity and high resolution magnetics gave very good results. Shallow seismic reflection methods were not successful.

Residual gravity defined overburden thickness variations best with gravity highs of up to 6.5 micrometres/sec² over the fireholes. EM conductivity showed reasonable correlation with overburden thickness, with EM conductivity highs over fireholes infilled with lower resistivity lacustrine clays and silts. High resolution magnetics using a TM-3 caesium vapour magnetometer, despite high cultural interference, showed broad, low amplitude highs over fireholes where higher susceptibility baked clays are thickest.

The three geophysical data sets and overburden data were gridded (5m.x5m.) and the grids dumped to a MicroBrian image processing system. Conventional image processing analysis was carried out to compare, enhance, filter, display and classify the complementary data sets. A classification scheme for overburden type based on geophysical responses plus a routine firehole exploration methodology using residual gravity, EM, magnetics, progressive drilling data and the image processor was devised to reduce drilling costs and increase exploration confidence. The case history presents the results of the grid geophysics and image processing approach.