Seismic transmission and electrical resistivity tomography for the delineation of mine galleries in the Raniganj Coalfield,India

Subsidence, fire, flooding and other kinds of environmental hazards related to shallow coal workings in the Raniganj Coalfield necessitated stabilization of the abandoned galleries at North Searsole, Bansra and Dhandadih. The working coal-seams at these collieries are Kenda bottom, Purandip bottom and Jambad top, respectively. For an accurate location of the network of galleries in the coal-seam, seismic transmission and resistivity tomographic methods are applied and the results are reported here. In the 2D tomographic formulation for both seismic exploration and electrical resistivity measurements, the genetic algorithm is implemented for an optimal solution even starting with poor initial models. The residual traveltime and transfer-resistance 2D distributions are generated to analyse the quality of the data recorded in the field. In these images, each cell corresponds to one source–receiver pair for each different cross-hole combination. All the colliery data are tested for their quality, preliminary 1D model generation and the construction of final velocity and resistivity images, depicting the coal-seams with or without voids/galleries. The estimated tomograms show the lateral S- or P-wave velocity and resistivity variations at different depths in the North Searsole, Bansra and Dhandadih collieries. While seismic tomography could depict the dry voids in the coal-seam at North Searsole and Bansra, resistivity tomography failed to do so, due to small resistivity contrasts between the void and the coal-seam bed. At Dhandadih, however, where the galleries are waterlogged, both seismic and resistivity tomography imaged the voids successfully. Generally, in all the cases, the coal-seams are better delineated by electrical resistivity tomography than by seismic tomography.