Quantitative aspects of mining induced seismicity in a part of the Welkom Goldfield /cRicardo Isidro Loureiro Ferreira.

[摘要] Rockbursts continue to be one of the more high profile and problematic workerhazards in the South African gold mining industry. Recent advances in the technologyof seismic monitoring systems and seismic data analysis and interpretation methodshold considerable promise towards improving the success rate of rockburst controlmeasures. This study tests different methods for the evaluation of the response ofgeological structures to mining induced stress changes.A small part of Western Holdings Gold Mine in the Welkom goldfield -- the PostmaArea -- offers a challenge because of its geological complexity, accessibility and highincidence of seismicity. The sensitivity of the local network to ground motions in thisarea of interest and the expected spatial location accuracy is established and deemedadequate for a detailed investigation of seismic activity. The local mining geometry,geology and methods of mining are discussed. The fractured state of the rock massobserved in situ, close to the stope faces, is in agreement with the results of numericalelastic modelling and the high stresses inferred seismically. Almost immediately afterthe incidence of a large event (ML 3.7) which occurred close to one end of a dyke, anincreased rate of seismic activity became apparent at another part of the same dyke,some 250 m to the east. A change in the state of seismic stress, before and after thelarge event, points to a transfer of stress along this geological discontinuity.A quantitative analysis of recorded seismicity indicates spatial and temporal variationsin the state of stress and strain throughout the rock mass surrounding Miningexcavations. The elastic stress modelling performed routinely by rock mechanicsengineers in the deep gold mines is, by itself, incapable of catering for the rheologicalnature of the rock mass, but taken together with independent seismic evaluations of afault orthogonal to a highly stressed dyke it is shown that both methods are mutuallycomplementary and can enhance the assessment of the seismic instability of thestructures. A back-analysis is conducted on ten large seismic events (ML 2.5) toidentify precursors. These show that the timely recognition of high gradients inphysical seismic parameters pertaining to strain rate and stress in time and spaceimmediately prior to major seismic events is a real and practical possibility, as suchconstituting an early warning mechanism. The fore-warning of a large event is bestserved by an analysis of seismicity over the short term (weeks or days) throughtime-history variations and/or contouring of various seismic parameters, althoughlong-term seismic responses (months or weeks) characterise specific patterns andtrends which are useful in the forecast.

[发布机构] University of the Witwatersrand