Electrical resistivity tomography survey for delineating uncharted mine galleries in West Bengal, India

The history of subsidence, fires, flooding and other kinds of environmental hazards related to shallow coal workings in India goes back to colonial times some 300 years ago. As coal production accelerated in modern times, so did the environmental and socio-economic drawbacks related to exploitation. In the mid-1980s, a hydropneumatic sand-stowing method was developed to fill in abandoned galleries but their exact location had to be known. Unfortunately, most of these old workings are uncharted and consequently large tracts of land cannot be stabilized. A research programme making use of integrated surface, borehole and cross-hole geophysical methods was undertaken over a five-year span to try to solve this problem. Surface geophysical methods, being cheaper and faster than their cross- and downhole counterparts, were used to cover larger areas on an exploratory basis, while cross-hole methods were employed to locate more accurately one or a network of galleries to be perforated by drillhole(s) and used as a conduit for sand stowing. We report the results of one of the cross-hole geophysical methods: electrical resistivity tomography (ERT). A pole-dipole configuration is used and both cross-hole and surface-borehole methodologies are tested. Forward modelling and inversion of synthetic data making use of downhole and surface physical and geometrical parameters are presented first. This phase is followed by the inversion of real data. It is concluded that ERT is not applicable for the detection of dry voids, but is effective in a waterlogged environment which is estimated to represent 85-90% of the cases. In waterlogged galleries, ERT is applicable in both cross-hole and surface-downhole modes, the latter allowing a larger surface coverage at low cost. ERT is thus a reliable geophysical tool to image water-filled voids and an adequate technique to address environmental and geotechnical problems.