Subsurface cavity detection over Patherdih colliery, Jharia Coalfield, India using electrical resistivity tomography

The present study deals with delineation and mapping of subsurface voids which are very common around the Patherdih colliery of Jharia coal field, India. These are caused by coal fire and old unplanned/illegal mining activity. Initially, a conceptual model of void formation associated with coal fire has been proposed. The burning of coal leads to the void formation due to volume reduction because of transformation of coal to ashes. It is supposed to have significant resistivity contrasts between bedrock i.e. mainly sandstone/clay stone/shale etc. and void space, which are filled with ashes generated from burned coal seam. Field situations have been simulated through forward modeling and the data generated by gradient and dipole–dipole arrays have been corrupted by 5 % noise. The simulations have been carried out for two voids with varied resistivity. The inversions of the data for both the arrays bring up the dimension of the void within reasonable accuracy unlike its resistivity. In fact, the considered resistive void seems to be a conducting void. In the present study, data have been collected by a state-of-the-art FlashRES-Universal electrical resistivity tomography (ERT) instrument, which have 61-channel and 64 electrode data acquisition capacity. ERT sections have been generated over two profiles using Wenner, Schlumberger, gradient, dipole–dipole and joint inversion of all combine arrays. Different resistive anomalous features with resistivity of about 300–200 Ωm have been delineated in both profiles. These are associated with voids at the depths of about 23–25 and 45–54 m generated by burning of coal seams XIVA and XIV, respectively. The obtained results are in broad agreement with the available litholog and surface manifestation of coal fire. The results prove the efficacy of the ERT technique for detection of voids associated with coal fire.