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

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

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.