A dissertation submitted to the Faculty of Science, University of the
Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of
Master of Scieuce in Geophysics . / Rockbursts continue to be one of the more high profile and problematic worker
hazards in the South African gold mining industry. Recent advances in the technology
of seismic monitoring systems and seismic data analysis and interpretation methods
hold considerable promise towards improving the success rate of rockburst control
measures. This study tests different methods for the evaluation of the response of
geological structures to mining induced stress changes.
A small part of Western Holdings Gold Mine in the Welkom goldfield -- the Postma
Area -- offers a challenge because of its geological complexity, accessibility and high
incidence of seismicity. The sensitivity of the local network to ground motions in this
area of interest and the expected spatial location accuracy is established and deemed
adequate for a detailed investigation of seismic activity. The local mining geometry,
geology and methods of mining are discussed. The fractured state of the rock mass
observed in situ, close to the stope faces, is in agreement with the results of numerical
elastic modelling and the high stresses inferred seismically. Almost immediately after
the incidence of a large event (ML 3.7) which occurred close to one end of a dyke, an
increased 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 the
large event, points to a transfer of stress along this geological discontinuity.
A quantitative analysis of recorded seismicity indicates spatial and temporal variations
in the state of stress and strain throughout the rock mass surrounding Mining
excavations. The elastic stress modelling performed routinely by rock mechanics
engineers in the deep gold mines is, by itself, incapable of catering for the rheological
nature of the rock mass, but taken together with independent seismic evaluations of a
fault orthogonal to a highly stressed dyke it is shown that both methods are mutually
complementary and can enhance the assessment of the seismic instability of the
structures. A back-analysis is conducted on ten large seismic events (ML> 2.5) to
identify precursors. These show that the timely recognition of high gradients in
physical seismic parameters pertaining to strain rate and stress in time and space
immediately prior to major seismic events is a real and practical possibility, as such
constituting an early warning mechanism. The fore-warning of a large event is best
served by an analysis of seismicity over the short term (weeks or days) through
time-history variations and/or contouring of various seismic parameters, although
long-term seismic responses (months or weeks) characterise specific patterns and
trends which are useful in the forecast. / AC2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/24502 |
| Date | January 1997 |
| Creators | Ferreira, Ricardo Isidro Loureiro |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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