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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