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Quantitative aspects of mining induced seismicity in a part of the Welkom GoldfieldFerreira, Ricardo Isidro Loureiro January 1997 (has links)
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
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Quantitative aspects of mining induced seismicity in a part of the Welkom Goldfield /cRicardo Isidro Loureiro Ferreira.Ferreira, Ricardo Isidro Loureiro. January 1997 (has links)
A dissertation submitted to the Faculty of Science, University of the
Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of
Master of Science 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. / AC 2018
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A relative moment tensor inversion technique applied to seismicity induced by miningAndersen, Lindsay, Marguerite 18 July 2001 (has links)
Three hybrid moment tensor inversion methods were developed for seismic sources
originating from a small source region. These techniques attempt to compensate for
various types of systematic error (or noise) that influence seismograms recorded in the
underground environment in order to achieve an accurate and robust measure of the
seismic moment tensor. The term 'hybrid' was used to distinguish between the
relative method proposed by Dahm (1995) and the methods developed in this thesis.
The hybrid methods were essentially weighting schemes designed to enhance the
accuracy of the computed moment tensors by decreasing the influence of any low
quality observations, to damp (or amplify) any signals that have been overestimated
(or underestimated) due to local site effects, and to correct for raypath focussing or
defocussing that results from inhomogeneities in the rockmass.
The weighting or correction applied to a particular observation was derived from the
residuals determined when observed data were compared with corresponding
theoretical data (for a particular geophone site, sensor orientation and wave phase) and
were calculated using a cluster of events rather than a single event. The first and
second weighting schemes were indirectly related to the mean and the median of the
residuals where the residuals were defined as the ratio of the theoretical to observed
data. In the third scheme, the residuals were defined as the difference between the
observed and theoretical data and the weights were based on the distance of a data
point (measured in standard deviations) from the mean residual. In each of the
weighting schemes, the correction was applied iteratively until the standard error of
the least-squares solution (normalised to the scalar seismic moment) was a minimum.
The schemes were non-linear because new weights were calculated for each iteration.
A number of stability tests using synthetic data were carried out to quantify the source
resolving capabilities of the hybrid methods under various extreme conditions. The
synthetic events were pure double-couple sources having identical fault-plane
orientations, and differing only in rake. This similarity in the mechanisms was chosen
because the waveforms of tightly grouped events recorded underground often show
high degrees of similarity. For each test, the results computed using the three hybrid
methods were compared with one another and with those computed using the single
event, absolute method and two relative methods (with and without a reference
mechanism). In the noise-free situation, it was found that the relative method without
reference mechanism showed the highest resolution of mechanisms, provided that the
coverage of the focal sphere was not too sparse (> 3 stations). The hybrid method
using a median correction was found to be the most robust of all the methods tested in
the most extreme case of poor coverage (2 stations) of the focal sphere.
When increasing levels of pseudo-random noise were applied to the data, the absolute
moment tensor inversion method, the hybrid method using a median correction, and
the hybrid method using a weighted mean correction all showed similar robustness
and stability in extreme configurations concerning network coverage of the focal
sphere and noise level. When increasing levels of systematic noise were added to the
data, the hybrid methods using a median correction and weighted mean correction
were found to exhibit similar robustness and stability in extreme configurations
concerning network coverage of the focal sphere and systematic noise. In all
situations investigated, these two hybrid methods outperformed the relative and
absolute methods.
The hybrid moment tensor inversion methods using a median and weighted mean
correction were applied to a cluster of 14 events, having remarkably similar
waveforms, recorded at Oryx Gold Mine. For comparative purposes, the absolute
method was also applied. The inputs to the inversion methods consisted of the spectral
plateaus of both P- and S-waves at frequencies below the comer frequency of the
time-integrated displacement traces. The polarities of dominant motion were used as
an additional constraint and were determined from cross-correlation of observed with
synthetic P- or S-waves. The solutions computed using the hybrid moment tensor
inversion using a median correction displayed a distinct improvement after the
iterative residual correction procedure was applied. The radiation patterns and faultplane
solutions showed a high degree of similarity, and are probably more accurate
reflections of reality than those computed using the absolute moment tensor inversion
methods. These observations are very encouraging and point towards the method's
potential for use as a standard processing tool for mine seismicity.
The implications of this work are a better understanding of the focal mechanisms of
seismic events induced by mining activities, ultimately leading to improved safety
underground.
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