Seismic imaging of stressed pillars in abandoned coal mines: Case studies fromAustralia

摘要

Systematic underground coal mining began in easternAustralia in 1802. Coal is nowAustralia's largest commodity export.With continuing expansion of this industry and population growth over the last 209 years many abandoned mines now lie within the precincts of large Australian cities, towns or high priority growth areas and are now adversely impacting surface developments due to fear of mine subsidence. Many of these old mines were operated with the room-and-pillar mining method and represent a major subsidence risk where standing and/or partially collapsed workings lie within 100m of the ground surface. Among the key issues hampering urban and infrastructure development are the lack of reliable data on the location and likely stability of these old workings and the costs of traditional investigative geotechnical technologies, principally drilling, that attempt to provide this information. It is widely accepted that P-wave borehole seismic imaging can greatly increase the cost-effectiveness of drilling by expanding the effective radius of investigation of a borehole and can locate regions of lower seismic velocity associated with collapsed workings and voids. However, it is less well recognised that regions of increased stress within the roof-pillar system, indicative of high risk standing workings, can also be seismically imaged as regions of higher seismic velocity. Demonstrations of this approach are provided by two case studies from Australian coalfields. The first study applies direct underground seismic imaging of a coal pillar in the Eastern Coalfields before and after it was split. This shows that approximately doubling the stress on the load-bearing regions of the split pillar increases P-wave seismic velocities by about forty percent. The second study, near the margins of old workings in theWestern Coalfields, uses crosshole and surface-to-borehole seismic imaging to identify unexpected mine voids that were not intersected in the boreholes and observes the high velocity signature typical of a high risk, standing pillar. It is concluded that using P-wave borehole seismic imaging technologies with increased focus on the regions of increased stress can greatly improve the assessment of high risk mine subsidence areas.