Underground seismic mapping of coal seam discontinuities at West Wallsend No. 2 Colliery

Abstract

An experimental in-seam seismic survey was recently conducted at West Wallsend No. 2 Colliery in the northern coalfield of New South Wales. The objectives were to acquire field data on scattering from a known dyke and to determine the transmission characteristics of old workings. Shot timing errors, gas bubbling in the geophone holes, and the limited dynamic range of the recording instruments all affected data quality. The Borehole Seam supports leaky P, SH and P?SV modes. These propagate in the plane of the seam, with group velocities at 75 Hz of 3.7, 2.0 and 1.6 km s-1. Attenuation rates are high. The seam is lossy (Q » 10). Dispersion is influenced by the proximity of the Young Wallsend Seam. Two prominent structures were mapped between the geophone roadway and the dyke, by migrating SH arrivals. One is believed to be a feeder dyke, the other may be a sand channel. These obstructions prevented scattering analysis of the main dyke. The dyke was modelled as a specular, resonant reflector. The reflection coefficent is low for leaky modes with appreciable energy outside the seam. The water filled heading (old workings) acted as a bamer to SH waves. It was relatively transparent to P waves at 100 Hz. The P wave shadow cast at 300 Hz was back-projected by algebraic reconstruction tomography (ART). The map showed up a north?south corridor of competent rock between the mines between 40 m and 90 m wide ? compared to the actual figure of 75 m.