Detection of illegal mine voids using electrical resistivity tomography: The case-study of Raniganj coalfield (India)

• Delineation and mapping of illegal coal mine voids/galleries
• 2D ERT section using joint inversion of all combined arrays
• A geoelectric model of four illegal coal mine galleries

Unauthorised coal mining activities may result in development of hidden hollows, rat holes, galleries, goafs, shafts etc., which pose great threats of land subsidence, fire, water flooding leading to severe environmental hazards, health problems and safety issues to the local people. Present study deals with delineation and mapping of unauthorised coal mine voids/galleries over an abandoned old mine around Khudia open cast mine, Nirsa, Raniganj coalfield, India. Electrical resistivity tomography (ERT) study comprising Wenner, Schlumberger, Dipole-Dipole, and Gradient arrays has been carried out along three parallel profiles over the affected area. Further joint inversion of all combined arrays has also been carried out using 2.5D resistivity inversion program, to combine the relative advantages of all the arrays, for producing superior results. 2D ERT sections have been generated for the filtered data sets with a constant quality factors and two different current thresholds. The best results have been obtained, from joint inversion of all combine arrays for the filtered data with higher current threshold. The observed resistivity anomalies are well correlated with the depth of coal seam occurrences as observed in the borehole litholog of nearby area. Finally, a geoelectric model of four unauthorised coal mine galleries has been established with their extension and orientation over the study area. The results demonstrate the suitability of the ERT technique through joint inversion of all combined arrays for characterization of illegal coal mine workings.

A geoelectric model of illegal mine voids/galleries has been established based on three parallel 2D ERT sections: (a) AA/ (b) BB/ and (c) CC/, which are generated using joint inversion of combined arrays. The galleries in central part (G2 & G3, Fig. 9c) are exposed through open (d) portal. The galleries of both sides (G1 & G4, Fig. 9c) may be interconnected with the portal (d) near profile AA/, as there is no signature of additional portal. (e) Borehole log from Fig. 1c. (f) Final proposed model of illegal mine galleries.Figure optionsDownload as PowerPoint slide