Land subsidence is a serious problem in Indian coalfields due to old underground mine workings. Unfortunately, most of these are uncharted as no mine plans are available. The hidden galleries, goafs, shafts etc. may pose great threat for future mine development as well as to the local environment. The mine workings should be charted to undertake an effective preventive action. In the present study, 2D electrical resistivity tomography (ERT) technique has been used to detect underground mine workings, mainly air or water filled galleries. Initially, the whole exercise has been executed through a synthetic model study. Gaussian random noise of 5mV/A has been added with synthetic data to demonstrate field condition which provides realistic results. ERT survey was conducted over a part of Jogidih coal mine of Jharia coal field in India for a first time. Four electrode configurations, Wenner, Schlumberger, dipole-dipole and gradient were considered for this study. The results indicate the presence of sub-surface water and air filled cavity due to high resistivity contrast with surroundings.

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Subsidence is a major environmental hazard associated with shallow abandoned coal workings in the Raniganj coalfield of eastern India. Most of the old abandoned coal workings are poorly documented. There is a need to detect and quantitatively infer these workings through some inexpensive geophysical techniques. High resolution reflection seismic is one such technique that suits this problem because of its sensitivity to the presence of voids through direct reflection and scattering properties of P waves. In this study, cavity identification and location remained uncertain on the basis of 2-D stacked sections in the presence of noise. To improve interpretation, a 2-D zero-offset finite-difference model was developed to characterize the seismic signature over the cavities. The model represents a rectangular cavity associated with air-filled and water-filled conditions in a multilayered interface. A seismic migration technique is also applied to the zero-offset sections to further improve the interpretation. The numerical results show the top and bottom of the cavities clearly with their shape properly delineated for air-filled cavities. The base of water-filled galleries are detected as strong positive reflections.

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Abstract:In hydrocarbon exploration, wave-equation migration techniques play an important role in imaging the complex geological structures. Usually, post-stack migration scheme is applied to the seismic data to improve the resolution with restoration of dipping reflectors to their true position. As a result, the migrated time sections are interpretable in terms of subsurface features. As a numerical study, three fault models are considered for the present study. First of all, synthetic time sections are generated corresponding to three models. Later, post stack migration schemes such as Gazdag(PS), Phase-shift with turning rays and reverse time migration (T-K) domain techniques are applied in order to judge the imaging accuracy, preservation of true amplitude and computational speed. All the three post stack time migrated sections delineate the structure with their throw. However, the reverse time migrations (T-K) clearly delineate the reflectors in restoring the throw properly with minimum computational time. In order to test the validity the numerical results, similar exercise has been undertaken using field seismic data of KG basin, India. The results indicates that the field migrated sections are imaged. But, the reverse time migration (T-K ) provides the best subsurface image with restoration of reflectors and collapse of diffracted events with least computational time. Gazdag (PS) and Phase-Shift with turning migrated section shows the reduction of amplitude whereas, the reverse time migration preserved the amplitude fully.