Search

Cavity due to underground old mine workings is one of the major threats to the coal mines and the overlying subsurface and surface properties, which need to be protected. The detection of old mine workings and stability assessment of overlying strata are common problems in most of the Indian coalfields. Several coal mines in India are loss‐making, mainly due to different types of mine hazards. Khandra mine is one such mine at Raniganj Coalfield, Eastern Coalfields Ltd., a subsidiary of Coal India Limited. In the present study, 2‐dimensional and 3‐dimensional electrical resistivity tomography were carried out for detailed subsurface characterization. It supports delineating underground workings, including the nature of voids/cavities (air or water‐filled). Excessive distortions were reported in electrical resistivity tomography application, especially at the near‐surface, owing to large resistivity variations. Refinement of the model by half‐unit electrode spacing was attempted here to reduce the distortions with minimum possible absolute errors. 3‐Dimensional resistivity volumetric model was also developed with the help of five electrical resistivity tomography parallel profiles for better apprehension of the subsurface. Analysis provided important inputs for stability analysis using 3‐dimensional numerical modelling. The physico‐mechanical properties of the overlying strata, pre‐excavation in situ stresses, boundary conditions and the mine geometry simulation were incorporated for understanding the stability analysis. Stability analysis was carried out using the finite difference technique. The analysis of 3‐dimensional numerical modelling indicated that two distinct layers comprising (i) laterite/part of the course to medium‐grained sandstone and (ii) developed galleries of R‐IX seam exhibited a very low safety factor below 1.0, indicating potholing/subsidence susceptibility. The other three layers comprising parts of fine‐grained sandstone exhibited a relatively higher safety factor of around 2.0, indicating moderately stable zones, but not on a long‐term basis. Parts of Siduli stream embankments need suitable retaining walls to avoid water inundation for the stability of the area. [ABSTRACT FROM AUTHOR]

The focus of the study was to evaluate the rock strata transformation above an underground mine panel during pre-and-post mining based on an electrical resistivity tomography (ERT) geophysical technique. The data were acquired using two electrode arrays (i.e., pole-dipole and Wenner-Schlumberger [WS]) with electrode spacing of 6 m up to 570 m in length. The resistivity data were digitally processed through RES2DINV software. The formation of fractured rock in the overburden caused by underground mining operations was clearly distinguished by resistivity anomalies observed using the poledipole array. Variation in the hydrogeological condition in the strata was also assessed during the pre-and-post mining conditions. The impact of compression and tension generated in the strata as a result of extraction of coal and subsidence in the overburden rock was observed through prominent resistivity variations observed in an inversion modelling of the WS true resistivity profile. The influence of surface topography in the near surface top weathered rock was also distinctly outlined by the resistivity measurements. ERT is a strong tool capable of providing valuable information regarding strata deformation caused by mining. [ABSTRACT FROM AUTHOR]

Pothole, an undesirable catastrophic consequence, is a common phenomenon in old coal mine workings, especially at shallow depth endangering habitats and surface structures. Precise identification of pothole potential zone is vital to prevent any major setbacks. Limited sub-surface information leads to branching out from the realistic ground conditions resulting in improper inferences. Resistivity imaging system is a favourable tool for gathering and strengthening technical information to demarcate the potential areas of pothole occurrence as it has proven ability to translate the characteristic of strata. Geological disturbance, one of the causes for pothole, has been located precisely by electrical resistivity survey at the study site. The overlying rock type has been correlated with resistivity range and validated with borehole lithology. Considering influencing parameters for pothole i.e. position of underground workings, geological disturbance and nature of near surface strata potential locations of the pothole likely to occur has been demarcated based on resistivity characteristics. Two potholes occurred in the past also helped in strengthening in interpretation. [ABSTRACT FROM AUTHOR]

Water management is a strategic factor in old coal mine working areas due to disturbance in the aquifer caused by subsidence resulting in drying of wells and streams. Cavity formation due to underground mining leads to roof fall, heaving of clay floors and collapse of coal pillars in soft clay floors. Logging of water in old mine workings is a common phenomenon especially during rainy season. Mine water is extensively used in various mining activities, and hence, a study has been conducted in part of Jamadoba 6 and 7 Pits, Jharia coalfield, India, to assess the hydrological condition during and post-monsoon. Initially, a conceptual model has been prepared to identify the possible sources of water including its path entering into old mine workings. Electrical resistivity tomography (ERT) i.e. Wenner-Schlumberger configuration with quality factor 5 has been adopted as it is effectual and reliable in detecting subsurface cavities at shallow depth and the possible path of transmission of subsurface water into the mine void with season variation. The study helped in identification of abandoned incline, porous and permeable strata conditions. The correlation of the resistivity variations is observed to be in close approximation with working plans and available borehole lithology. This technique proved to be suitable for adopting such studies. [ABSTRACT FROM AUTHOR]

Analysis of non-uniqueness model in resistivity imaging data is vital in inaugurating the consistency of models. Nevertheless, such analysis is moderately unusual in resistivity imaging data set. Electrical resistivity tomography (ERT) technique is being constantly used in many scientific areas including engineering, environmental and archaeological survey. Primarily, the inversion algorithm techniques are employed on synthetic model data set with and without some random Gaussian noise, and its validity is tested by filed data set. The study was conducted in the premises of Central Institute of Mining and Fuel Research (CIMFR), Dhanbad by laying an ERT profile of 480 m length with 5 m electrode spacing using Syscal Pro (Iris instrument) resistivity meter. Two standard arrays were used in this study namely Wenner-Schlumberger and dipole-dipole array. The data set was mixed to a single array to achieve better resolution and enhanced clarification. On processing data by Prosys-II software, it was exported in Res2Dinv software for inversion. In this context, data was inverted by different algorithm techniques i.e. least square (L2-norm) and robust inversion (L1-norm). Exemplary results related to the heterogeneity of the resistivity structure within the high and low resistivity anomaly were obtained by robust inversion method. The obtained results are in broad agreement with the simulation model. [ABSTRACT FROM AUTHOR]