Effects of mining subsidence observed by time-lapse seismic reflection profiling
Extracting coal from underground mineworkings causes the overlying rocks to subside with associated changes in the stress regime. The aim of the study reported here was to apply the surface seismic reflection method to study the effect of subsidence on seismic velocity. Two sets of time-lapse surveys were carried out over two longwall mining panels in the Selby Coalfield. Seismic lines were profiled parallel and perpendicular to adjacent panels H45 and H46, respectively. A total of twenty-one repeated surveys were carried out along the two lines over a period of three years. The effect monitored was due to mining in the Bamsley Seam, at 550 m depth. As mining progressed, the traveltime of a strong reflection event from an anhydrite bed at 150 m depth was measured after processing the data with standard techniques. An overall increase in traveltime of about 4 % was observed. The progressive increase in traveltime over panel H45 correlated well with empirical calculations of differential subsidence between the surface and the anhydrite. However, the magnitude of the change must principally be accounted for by a decrease in seismic velocity, associated with a reduction in the vertical effective stress. Although the traveltime over panel H46 was also found to increase, and to correlate quite well with die expected differential subsidence, the agreement was less good along this transverse profile. This is attributed to asymmetric subsidence effects because the ground on the SW side of the panel had already been worked by panel H45, but the ground on the NE side was unworked. At the time of each seismic survey across panel H46, the profile was also levelled, and it was found that surface subsidence values along the profile increased towards panel H45. As most of the subsidence caused by mining panel H45 would have been completed by the time the H46 profile was surveyed, the effect must be at least partly attributed to asymmetric subsidence due to panel H46. Where the ground had been weakened by subsidence due to mining H45, near-total subsidence from mining H46 took place rapidly; but in the previously unworked ground on the NE side of panel H46, the residual subsidence was presumably delayed by competent strata in the overburden. Further work is needed to confirm whether this explanation is correct.