The application of seismic surveys to the evaluation of shallow coal deposits
The viability of using surface seismic techniques in the exploration of shallow coal deposits in the UK is investigated. The geological factors that affect the economic viability of a prospective site are the overburden ratio and the presence of faulting, drift channels and previous mine workings. The seismic refraction technique using both congressional (P) waves and shear (S) waves is used and also the P wave reflection technique. The P wave source used is explosive and the shear wave source is a horizontal hammer. Seven site surveys are described, six in North East England, and one in the North West. The refraction technique using the Plus-Minus and Generalized Reciprocal Methods of interpretation is used to locate faults, estimate drift cover thicknesses and pinpoint areas of previous opencast excavation. Faults are located by a change in refractor velocity, or by a sudden change in refractor depth. Shear waves display a greater ability to image sharp depth anomalies than P waves, due to the greater velocity contrast between consolidated and unconsolidated rocks that S waves exhibit. S waves are better lithological indicators than P waves as there exist distinct ranges of S wave velocities for unconsolidated (150 to 400m/sec) and consolidated (650 to 1400m/sec) rocks. The fundamental problem with shallow reflection surveys on land is that the shallow reflections occur at the same time as surface waves and ground roll. The reflection technique is used to locate faults and detect the boundaries of old mine workings. No reflections shallower than 70m are recorded, and in areas of thick glacial drift cover no frequencies higher than l00Hz are detected. The results of the survey to locate positions of old mine workings are inconclusive, although some features seen on the final sections could be related to boundaries between unworked and worked strata.