SEA-FLOOR MINING SUBSIDENCE IN THE SYDNEY COALFIELD

The operation oi any mine under large bodies of water brings with it t he need for careful design of extraction operations. This is especiall y true oi coal mines, that are characterized by extensive areas of ext raction beneath weak, stratified rock and overburden. Longwall coal pr oduction in the Sydney Coalfield, Nova Scotia has, for many years, bee n concentrated in longwall mine workings under the Atlantic Ocean. Gro und movement above the caved area behind the longwalls is typically ch aracterized on the overlying ocean floor by shallow trough-shaped subs idence depressions. Precise knowledge of the magnitude and extent of s uch subsidence is critical to the optimization of mine design to ensur e safe and successful operations. Since 1983, the Cape Breton Coal Res earch laboratory (CBCRL) and the Cape Breton Development Corporation ( CBDC) have jointly researched the measurement of this seafloor subside nce. By observing seafloor topography before and after mining, compari sons are made to reveal the subsidence troughs. Many profiles have now been identified first using a precision echosounder, but more recentl y, using swath and sweep bathymetry together with the Geological Surve y of Canada-Atlantic (GSCA) and the Canadian Hydrographic Survey (CHS) . Despite the relatively poor accuracy (+/- 30%) oi these approaches c ompared to the more usual geodetic land surveying methods (where land access is possible), much progress has been made in this key area of i mproving mine design. The paper summarizes the findings over both Phal en and Prince Collieries, giving the principal characteristics of obse rved transverse subsidence troughs. For the first lime, exciting new d evelopments obtained by swath bathymetry in 1994 over Prince Colliery are presented. These indicate that in certain circumstances and using the latest imaging techniques, post-mining surveys alone may suffice t o identify seafloor subsidence.