Fault location by underground seismic survey

Seismic energy can be coupled into a coal seam waveguide from an underground explosion. Guided modes scattered by buried faults may illuminate an underground geophone array. The fault mapping procedures that form the subject of this paper are based on coherent digital recording of signals sensed by the elements of such an array. The inverse scattering problem tackled is unusual. Coal seams are dispersive; the dispersion characteristics are unknown at the outset; the arrays used for imaging are rarely plane; the fractional bandwidths available at the recorder are extreme; the coal seam may well be overmoded. These questions are addressed in the light of experience of processing data derived from a panel of coal in the Blackshale Seam of Pye Hill Colliery. Dispersed signals recovered from this seam are recompressed using a novel spectral-warping procedure closely related to frequency-domain migration. Seam maps are reconstructed, after recompression, by diffraction migration, or broadband holography, implemented in the space-time domain with a delay sum processor modified for operation with dispersive data. The formation of holographic images in the nearfield of an undersampled broadband array is both discussed analytically and demonstrated experimentally. It is shown, ii particular, that a 1 m vertical throw, cutting across the 3 m thick seam surveyed, can be mapped both in reflection and (if mode conversion occurs) in transmission, with a resolution that is adequate to current mine survey needs.