Algebraic reconstruction of a two‐dimensional velocity inhomogeneity in the High Hazles seam of Thoresby colliery

Abstract

A high proportion of faces started in mechanized coal mines run into underground faults. The faults take many forms, from the splitting of a seam through a hidden stress pattern caused by subsidence or folding, to a washout or a vertical throw. All faults reduce face output. A throw of only 1.5 m can lead to a face being abandoned. Faults of this order cannot be mapped reliably from the surface. They may be mapped in an underground seismic survey. Roadways of a mine may give access to fast refracting horizons above and below a coal seam. Waveguiding in the plane of the seam, if it occurs, simplifies migration of wave trains reflected from discrete faults. The reduction problem involved in a fault mapping is only two‐dimensional. Given guiding, whether leaky or not, it is possible to map distributed faults of low reflectivity by shooting in transmission across a panel of coal. Algebraic reconstruction techniques are used here to reduce first break times‐of‐flight through a 425 × 950 m rectangular block of coal into the profile of a velocity inhomogeneity. Input data are derived by static correction from hand‐picked arrival times. The reduction itself is effected using an algorithm which accommodates underground site access restrictions. In back projecting first break velocities, a truncated cosine is used to weight the relative contributions of rays passing at different distances from any given mapping point. The reconstructed velocity field suggests that the coal panel is bisected by a ridge of higher velocity. The suspicion of a ridge is reinforced by results of an aberration test based on a standard Huygens‐Kirchhoff migration. The ridge is found to follow the general line of a system of pillars left in place during the mining of a lower horizon. It is concluded that channel waves may be used to map subsidence into old workings underground. Coal seams apparently share, with other sedimentary rocks, the property of a pressure‐sensitive seismic velocity.