Activité microsismique et caractérisation de la détectabilité des réseaux de surveillance du bassin houiller de Gardanne

National audienceMining in Provence have left many underground voids that may cause subsidence or collapse in areas of high population density. Microseismic monitoring networks have been deployed to prevent this risk. Here we present the post-mining microseismic records between 2008 and 2012 and the characterization of networks detectability at different scales.L'activité minière en Provence a laissé de nombreux vides souterrains qui peuvent provoquer la subsidence des terrains ou des effondrements dans des zones à forte densité de population. Pour prévenir ces risques, une surveillance microsismique a été mise en place dans les zones à fort enjeux. Ainsi depuis l'arrêt des concessions minières de Provence, l'INERIS a installé 5 réseaux de surveillance microsismique sur le bassin houiller de Gardanne qui était exploité par Charbonnages de France. Puis, le BRGM-DPSM en tant que Maître d'ouvrage délégué de l'Etat, a demandé à l'INERIS la poursuite de la surveillance microsismique

Management of post-mining large-scale ground failures : blast swarms field experiment for calibration of permanent microseismic early-warning systems

International audienceIn France, decades of coal and iron-ore mining have left extensive underground cavities beneath or in the vicinity of urban areas. This poses an environmental challenge for society. To ensure post-mining risk management and public safety, wherever remediation is not possible, numerous real-time microseismic monitoring systems are being installed. The objective is to detect remote rock mass fracturing processes, precursory events and acceleration phases for appropriate and timely action. Although no consistent collapse has occurred in any of the monitored areas yet, single 3-D probes record many microseismic events of very low amplitude which create difficulties in the quantitative data analysis. The development of specific quantitative processing has therefore become a major issue in our research work. For that purpose, a field experiment was carried out on six of the instrumented sites. It consisted of sequences of small blasts in mine pillars which were accurately controlled in terms of the location, orientation and energy of the explosive source. The data analysis was used to calibrate parameters (velocity model, 3-D sensor orientation, etc.) for reliable 3-D localization and to develop an empirical law to estimate the source energy from the sensor energy. This work now enables us to analyze real microseismic events with a considerably better level of accuracy and to obtain enough information and confidence to discuss these data in terms of site stability

Semi-automatic detection and localization of microseismicity induced by a "salt dissolution provoked" cavity collapse

Natural underground cavities, active or abandoned mine workings, particularly when they are shallow, can provoke large scale land subsidence and collapses attended by catastrophic social-economic impacts. The potential of passive microseismic monitoring to prevent such disasters was already indicated by several studies. Nonetheless, to further improve monitoring reliability a better understanding of associated seismicity is inevitable. In this context, within a large multi-parameter research project at Cerville-Buissoncourt in Lorraine, France, the growth of a single, shallow, about 150 m diameter salt cavity created by salt dissolution mining was surveyed from 2004 until 2009 when the cavity reached its critical size and a 'controlled' collapse was initiated. During the experiment, a large microseismic data set was recorded by a triggered, high resolution geophone monitoring system. Initial processing and data inspection reveal very unusual seismic signals mainly appearing in complex swarming sequences (Mercerat et al., 2010; Contrucci et al., 2011). To resolve spatio-temporal characteristics of associated seismicity we developed an semi-automatic seismic event detection and localization algorithm adressing these abnormal signal characteritics. The detector design is based on a spectral envelope function calculated for each seismogram. By this function, coherent signals are distinguished from signals comprising rather randomly distributed frequency proportions as noise or CODA waves. First application tests demonstrated highly improved event detection results when analysing seismic events of highly varying size and duration occurring in a swarming sequence. In addition, we localized the detected seismic events using inter-station amplitude ratios as introduced by Battaglia and Aki (2003) and Taisne et al. (2011). Within this approach, hypocenter source inversion relies on the decay of seismic wave amplitudes along the source-receiver path. As a result, no troublesome a priori phase segmentation is needed and the entire data set can be processed. To calibrate the local seismic attenuation law we used ~700 seismic events with known hypocenter locations found by previous studies (Klein et al., 2011). The final optimized localization algorithm sufficienty constrained the tendency of actual hypocenter source location in the cavity region. Taken all together, our detection and localization strategy provides an appropriate first order approximation to study spatio-temporal attributes of seismicity from huge data sets associated with seismic signals of unknown or complex signature as observed for Cerville-Buissoncourt

Multi-parameter monitoring of a solution mining cavity collapse : first insight of precursors

International audienceIn order to improve our understanding of the large-scale ground failure phenomena caused by old underground mining works, a solution mine was instrumented in 2004 prior to its collapse as part of the mining scheme. A permanent monitoring system was set up, including a high-resolution microseismic network linked to a surface field-displacement measurement system. The large amount of data transmitted for on-line processing provided daily insight into the evolution of the geological system. First, microseismic activity showed upward progressive failure migration throughout 2008 without any significant surface movement. Second, after two days of intensive brine extraction, a high microseismicity and energy release rate marked the failure of a thin and very rigid bed at a depth of 120 m. This failure occurred 24 hours before the final collapse; it was followed by transient brine pressure signals, and by acceleration of the surface subsidence rate, reaching 1 milt in the final phas

Evolution monitoring of a solution-mining cavern in salt : identifying and analysing early-warning signals prior to collapse

International audienceRisk management of underground cavities requires a good working knowledge of accidental phenomena like subsidence or large-scale collapse. This was the context when the opportunity was taken to instrument a large size in use saline cavern, so as to test various auscultation techniques available under controlled conditions. A microseismic monitoring network coupled to a surface measurement system was installed to improve our knowledge of the mechanisms that initiate and govern the evolution of the cavern up to its collapse. After a stationary period combined with partial depressurization tests conducted in 2005 and 2007, the cavern appears to have entered into its final evolution phase, and this probably since early 2008. This results in continuous and highly sustained microseismic activity as well as the occurrence of a number of microseismic episodes localized around the cavern roof. The localization of the microseismic events, for some of these episodes, is closely correlated to the quasi-dynamic brine pressure variations and to the evolutions of the roof depth measured at observation boreholes. The microseismic activity turns out to be more precise when it comes to the evolution affecting the mine cavern than the movement measurements taken on the surface or sub-surface

Mining induced seismicity : monitoring of a large scale salt cavern collapse

International audienceTo improve our understanding in large scale ground failure phenomenon induced by old underground mining works, a field experiment was undertaken in collaboration with the SOLVAY mining company: a solution mine was instrumented in 2004 previously to its collapse which was triggered in February 2009, as part of the mining scheme. This solution mine is located in the Lorraine salt basin (France). To monitor the cavern collapse, a multi-parameter system featuring high resolution microseismic linked to ground surface leveling (tacheometer and GPS-RTK) was used. The data transmitted for on-line processing offered daily insight of the evolution of the underground cavity. The early signs of unstable evolution were detected during spring 2008: shifts in microseismic background regime end recurrent microseismic episodes were associated to a general upwards process of rock failure of the roof cavern, with no ground surface movement detected. The high microseismic regime of the cavern has convinced the operator to trigger the collapse by brine pumping. Three main microseismic regimes were then observed, each being well correlated with changes in both the surface subsidence rate and the brine level in the cavern

Accurate 3D location of mine induced seismicity in complex near-field underground conditions

International audienceThe monitoring of mine induced seismicity and rockburst hazard can be carried out at different scales of resolution. The strategy usually adopted aims to get a global coverage of the mine layout, based on a large-scale array of low-frequency seismic sensors which may extend up to tens of squared kilometers, in order to monitor the numerous disseminated mining works and enable a periodical rating of seismic hazard. However, some major drawbacks must be dealt with. First, the microseismic detection limits for each mining work considered as a stand-alone rockburst prone area induces the loss of all those small magnitude events occurring in the vicinity of the working faces. Moreover, quality of the seismic source location reveals insufficient to pinpoint unambiguously its very origin in the surrounding geology and mining voids. These two limits affect strongly the capabilities of ground engineers to detect for sure significant change in the seismic regime of any specific seismogenic zone. Refining the monitoring may be reached by the deployment of mobile local-scale arrays made of short period sensors around those mining works rated at higher risk. These permit to record a much more complete microseismic catalogue while enhancing drastically the accuracy and reliability of the expertise. However, the processing of small magnitude events recorded locally calls for some precautions due to some nearfield conditions adverse to accuracy. Besides the increasing complexity of the rock mass when dealing with higher frequency waves, the presence of important ancient backfilled works more or less interlaced with multilevel fast advancing mining works may be a very issue. This is often the case for highly productive methods based on retreat mining, cut-and-fill or sub-level mining. Because installing a socalled high resolution microseismic network rises new expectations in the understanding of the seismic activity versus the local geology and the mining process, the authors have undertaken the development of an evolving 3D velocity model taking into account not only the underground geological aspects encountered, but also and above all the dynamic mining process itself. Besides synthetic numerical tests that have been run to assess the relevance of this issue, we applied this new numerical procedure to a true microseismic dataset recorded during the controlled collapse of a large solution mined cavern lying 220 meters deep. Tens of thousands of microseismic events were recorded with the critical enlargement of the cavern towards the surface. 3D surveys of the cavern itself together with geophysical loggings of its geological overburden have been used to dimension the presence of this volume full brine and calibrate this innovative location procedure applied to selected microseismic swarms. Detailed comparisons with conventional 3D location studies have been established emphasizing the considerable gain brought by a more realistic three dimensional velocity model including the pre-existent and evolving voids before and during seismic crisis. This gain includes a much better separation of the fracturing processes in the cap rock and on the sidewalls of the cavern, discerning unambiguously the impact of the roof falls at the bottom of the cavity, delineating geological rock strata and discontinuities playing clearly a role in the physics of the collapse phenomena. Such a numerical procedure is currently being improved and automated for operational implementation in a deep mine

3D acoustic and microseismic location of collapse events in complex, 3D geological structures

International audiencePassive microseismics is a well developed technique that has gained importance in petroleum exploration operations as well as in geohazard assessment. When applied in complex geological environments, it requires advanced processing capabilities to ensure useful accuracy in the source location and characterization. Here we investigate a fast marching method to determine the travel-time field, rays and ray take-off angles in complex 3D media, for application with a direct-search event location method. We then illustrate and discuss the potential of the chosen methodology in the mining context. This methodology allows improvements in acoustic monitoring of large-scale underground mines by taking into account the intrinsic characteristics of propagation of the acoustic waves. Ongoing work on a dataset collected during the monitoring of a large-scale salt cavern collapse is also discussed. We expect that the use of an evolving 3D model will help to reduce the location errors and improve the dataset analysis, improving risk management for time-varying collapse events