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Common offset and refraction seismic data have been collected over four longwall panels at two different locations within West Virginia. Surveys prior, during and after mining were made in both areas. The velocity of seismic wave propagation in the overburden rocks is observed to decrease in response to panel emplacement. Variations in the magnitude of this reduction, apparent as an additional first arrival time delay in the after mining common offset surveys, do not appear to correlate directly to the amount of subsidence that has occurred. Velocity reduction and signal absorption zones are also produced in the cover beyond the panel face over the adjacent unmined areas. Signal absorption and velocity reduction zones have also been observed over room and pillar entry areas between mined panels. Velocity reduction zones have been- observed beyond the panel face in both shallow and deep mines, but the signal absorption zone has not been observed over the deeper mine. Changes in the compressional wave velocities of the cover rocks are associated with changes in their elastic moduli. Differences between borehole derived and surface seismic derived interval velocities, are related to differences in the properties of the bulk volume sampled by each method. |