Microseismic monitoring is an integral part of scientific experiments or industrial operations associated with the hydraulic fracturing of bedrock, which is an effort to increase the permeability of rocks in the area and improve the exploitation, whether of heat, oil or natural gas. Monitoring of such newly formed or growing original fractures, usually followed by seismic events, is very important for observation and describing the hydraulic fracturing itself and its progress over time. Since the number of such events recorded is usually very high, classical methods of earthquake processing, where the arrival times of seismic waves are determined manually, seem to be very inappropriate. For this reason, automatic methods are used to process such datasets, which do not require manual determination of the arrival times of the seismic waves. In our case, the data were recorded by a dense network of surface receivers arranged in the shape of a ten-arm star. It was a total of more than nine hundred groups of vertical geophones, of which more than a quarter were also supplemented by a three-component seismic sensor. We then created an automatic procedure for the described network, the result of which is the determination of the location and the source mechanism for each seismic event. The recorded...

As hard rock mining progresses into higher stress mining conditions through either late stage extraction or mining at depth, the rock mass is driven not just to the peak strength, but often well into the post-peak until complete ‘failure’ occurs and easier mining conditions become evident. Limited research has been accomplished in identifying the transition of the rock mass and its behaviour into the post-peak and this research investigates this behaviour in detail. As the rock mass progressively fails, fractures are initiated through intact rock and extension and shear failure of these and pre-existing features occurs. Associated with this failure are microseismic events, which can be used to give an indication of the strength state of the rock mass. Based on an analogy to laboratory testing of intact rock and measurement of acoustic emissions, the microseismicity can be used to identify, fracture initiation, coalescence of fractures (yield), localization (peak-strength), accumulation of damage (post-peak) and ultimate failure (residual strength) leading to aseismic behaviour. The case studies presented in this thesis provide an opportunity to examine and analyse rock mass failure into the post-peak, through the regional and confined failures at the Williams and the Golden Giant mines, both in the Hemlo camp in Northern Ontario, Canada. At the Williams mine, the progressive failure of a sill pillar region into the post-peak was analysed; relating the seismic event density, combined with numerical modelling and a spatial and temporal examination of the principal components analysis (PCA), to characterize the extent, trend and state of the yielding zone, which formed a macrofracture shear structure. Observations of conventional displacement instrumentation, indicates regional dilation or shear of the rock mass occurs at or prior to the point of ‘disassociation’ (breakdown of stable PCA trends) when approaching the residual strength. At the Golden Giant mine, the complete process from initiation to aseismic behaviour is monitored in a highly stressed and confined pendent pillar. The PCA technique, numerical modelling and focal mechanism studies are used to define significant stages of the failure process, in which a similar macrofracture structure was formed. Temporal observations of key source parameters show significant changes prior to and at the point of coalescence and localization.

As hard rock mining progresses into higher stress mining conditions through either late stage extraction or mining at depth, the rock mass is driven not just to the peak strength, but often well into the post-peak until complete ‘failure’ occurs and easier mining conditions become evident. Limited research has been accomplished in identifying the transition of the rock mass and its behaviour into the post-peak and this research investigates this behaviour in detail. As the rock mass progressively fails, fractures are initiated through intact rock and extension and shear failure of these and pre-existing features occurs. Associated with this failure are microseismic events, which can be used to give an indication of the strength state of the rock mass. Based on an analogy to laboratory testing of intact rock and measurement of acoustic emissions, the microseismicity can be used to identify, fracture initiation, coalescence of fractures (yield), localization (peak-strength), accumulation of damage (post-peak) and ultimate failure (residual strength) leading to aseismic behaviour. The case studies presented in this thesis provide an opportunity to examine and analyse rock mass failure into the post-peak, through the regional and confined failures at the Williams and the Golden Giant mines, both in the Hemlo camp in Northern Ontario, Canada. At the Williams mine, the progressive failure of a sill pillar region into the post-peak was analysed; relating the seismic event density, combined with numerical modelling and a spatial and temporal examination of the principal components analysis (PCA), to characterize the extent, trend and state of the yielding zone, which formed a macrofracture shear structure. Observations of conventional displacement instrumentation, indicates regional dilation or shear of the rock mass occurs at or prior to the point of ‘disassociation’ (breakdown of stable PCA trends) when approaching the residual strength. At the Golden Giant mine, the complete process from initiation to aseismic behaviour is monitored in a highly stressed and confined pendent pillar. The PCA technique, numerical modelling and focal mechanism studies are used to define significant stages of the failure process, in which a similar macrofracture structure was formed. Temporal observations of key source parameters show significant changes prior to and at the point of coalescence and localization.

Pour progresser dans la compréhension des mécanismes liés aux instabilités des cavités souterraines à partir de la réponse microsismique associée, l'effondrement provoqué d'une cavité saline (~ 200 m en diamètre), créée par dissolution, a été instrumentée sur un site d’exploitation de SOLVAY à Cerville-Buissoncourt (Lorraine, France). Pendant l’expérimentation un vaste ensemble des données a été enregistré (~ 50,000 fichiers d'événements) dont la majorité (80%) est constitué d’essaims microsismiques singuliers. Cette thèse présente une analyse et une interprétation détaillée de cette base de données microsismiques grâce à l’adaptation de méthodologies de traitement originales, dont les résultats améliorent notre compréhension sur la nature de la microsismicité liée aux processus de création et d’effondrement des cavités souterraines, ainsi que sur l’évaluation de l’aléa associé. Les résultats principaux obtenus sont les suivants : les événements microsismiques sont comparables à des petits séismes tectoniques ayant des magnitudes de moment variant entre -3 et 1. (ii) L’ensemble des événements microsismiques montre un mécanisme en cisaillement (double-couple) remarquablement stable et est associé à un régime en faille inverse d’orientation NO - SE, plongeant à environ 35°– 55°. Ce phénomène est probablement lié à la présence de fractures préexistantes sur le site. (iii) L'origine des essaims microsismiques est certainement due à l'incapacité du système à créer des fractures de grandes dimensions capables de libérer des contraintes très importantes. Cela est probablement lié aux propriétés mécaniques du toit de la cavité. (iv) Les périodes d’effondrements du toit de la cavité sont associées à une dynamique de forçage systématique et montrent une réponse microsismique particulière, qui peut-être décrite par des lois statistiques. Les travaux de recherche de cette thèse confirment également, que la surveillance microsismique peut être un outil puissant pour étudier les processus d’instabilité des cavités souterraines, même avec un nombre réduit de capteurs si des outils d’analyse adaptés sont utilisés / In order to improve our understanding of hazardous underground cavities and its microseismic response, the development and collapse of a ~ 200 m wide salt solution mining cavity was monitored at Cerville-Buissoncourt in the Lorraine basin in NE France. The majority of the obtained dataset (~80%) was constituted of numerous unusual microseismic swarming events (~50.000 event files). This thesis presents innovative methods able to treat this specific microseismic data set, whose results provide new and fundamental insights into the principal characteristics of caving and collapsing related microseismicity and hazard assessment of excavated underground formations. The principal results are as follows: (i) the individual microseismic events are comparable to small natural tectonic earthquakes with moment magnitudes Mw ranging from around -3 to 1. (ii) Source mechanisms for most microseismic events are remarkable stable and demonstrate a predominant thrust faulting (double-couple) regime with faults similarly oriented NW-SE, dipping 35°-55° , what might be related to the presence of systematically arranged pre-existing fractures. (iii) The origin of microseismic swarming is suggested in the incapacity to sustain larger strains and to release larger stresses, what seems to be related to the mechanical constitution of the rock strata overlying the cavity (i.e. low strength materials). (iv) Caving and collapsing periods at the cavity roof are associated with systematic, self- reinforcing dynamics and have a distinct microseismic response, clearly observable from statistical analysis, which can be precisely described by empirical laws. The performed analysis and interpretation of the microseismicity at Cerville-Buissoncourt has shown that microseismic monitoring is a useful tool to constrain the mechanical and dynamical characteristics of an evolving and collapsing hazardous underground cavity

This research is aimed to develop a migration-based earthquake location algorithm and a passive seismic imaging approach to investigate microseismicity and image steep crustal structures in the mining area Schlema-Alberoda. The dataset includes single-component records of several microseismic events which occurred between 1998 and 2012 in the area. Through some tests and comparisons, the accuracy of the localization algorithm is proved. An expected extension of pre-existing faults within the granitic body and the connection between some of the structures is comfirmed by the location results. The passive imaging procedure is conducted using only P-wave secondary arrivals from the relocated events and by applying 3-D coherency migration. The reliability of the passive imaging results is verified by comparing the final image with the results of a 3-D active reflection seismic survey in the same area. In addition, the passive image complements the 3-D active image and reveals new structures that have not been imaged previously.