Immediately north of Houghton-le-Spring, Co. Durham, is a low ridge of Permian Magnesian Limestone overlying the Carboniferous Coal Measures. Running along this ridge is a zone of surface fissures. These fissures are clearly associated with mining subsidence as they are of recent origin and are situated above a near-vertical fault which has acted as a barrier limiting mineworkings to the north and south in several underlying coal seams. Mining adjacent to the fault began in the last century and continued until 1960.
Seismic refraction profiles have been run across the fissured zone using shear and compressional waves. The fissured zone can be identified by its low shear-wave seismic velocity. Furthermore, the boundary of the fissured zone can be accurately located simply by observing the differences in amplitudes of compressional-wave arrivals received at geophones spanning the boundary from an explosive shot in solid ground. These observations could be useful in site investigation, and the seismic method is recommended for detecting similar fissures in brittle sedimentary rock overlying mineworkings.