Electrical Resistivity Tomography for the detection of subsurface cavities and related hazards caused by underground coal mining in Coahuila

Abstract

This study used Electrical Resistivity Tomography (ERT) to identify the subsurface cavities in the carboniferous region on the State of Coahuila caused by underground coal mining and the hazards they induce. Measurements were taken next to roads and bridges support in three sectors with mining activity. Authors used roads to run the ERT profiles in order to avoid private properties, with some of these profiles including areas with subsidence. It is considered that subsidence is an anthropogenic related to both shallow and relatively deep underground mining activity. The electrical resistivity cross-sections obtained from the geophysical inversion process identified several cavities with a variety of dimensions ranging upwards from a depth of 7 m. Most of these structures cause deformations on surface in the form of subsidence or fractures. In general, the cavities identified in two of the sectors analyzed are filled with water or sediments. In a third sector analyzed in this study, all cavities were either empty or partially filled with collapsed material from their walls and ceiling. Geoelectrical sections reveal other characteristics of the geological environment, such as zones in which aquifer levels decrease, strata dipping (indicating prior subsidence), and faults and fractures that affect the stratigraphic sequence. The decrease in aquifer levels indicates changes in the hydrodynamic groundwater regime that have been caused by the voids generated by mining. These changes cause the ground to sink due to lithostatic pressure and the weight of the vehicles passing above. As a final result, the areas at the greatest potential risk of hazards related to ground subsidence are located within the three analyzed sectors.