Apport de la méthode microsismique à la compréhension de l'impact de l'ennoyage sur la stabilité des ouvrages miniers abandonnées

National audienceL'objectif de la présente recherche est de déterminer, à partir des expérimentations in situ, l'apport de la méthode microsismique dans l'identification et la compréhension des mécanismes de déformation des ouvrages miniers sous l'influence de l'eau (ennoyage)

Auscultation microsismique appliquée à la détection des éboulements des falaises crayeuses

National audienceThe microseismic monitoring carried out on an experimental site showed that the chalk rock ruptures are recordable by a seismic network composed of geophones and accelerometers. The microseismic method also showed the capacity to detect the precursory signs of a cliff collapse. This study enabled to dimension the essential parameters for the implementation of an operational microseismic system to monitor unstable cliffs zones and predict rock-falls hazard.L'auscultation microsismique menée sur une falaise maritime en tant que site expérimental a démontré que les microruptures d'un massif de craie sont détectables par un dispositif de capteurs de type géophone et accéléromètre. Elle a aussi montré les capacités de la méthode microsismique à détecter les signes précurseurs d'un éboulement. L'étude a permis d'établir les paramètres pour le dimensionnement d'un système de surveillance micro-sismique opérationnel d'un risque d'éboulement des falaises crayeuses

Real-time Microseismic Monitoring : Automatic Wave Processing and Multilayered Velocity Model for Accurate Event Location

International audienceMine-induced seismicity is a well-known phenomena wherever mining activities take place at great depths. At Provence colliery, France, longwall mining at 1100 meters deep induces numerous, daily tremors, triggered by the caving proeess above the tabular onebody. Many factors play an important part in this seismic activit

Detection and monitoring of high stress concentration zones induced by coal mining using numerical and microseismic method

Zones of high stress concentration induced by coal mining at a depth of 1250 meters in the Lorraine Collieries are detected and monitored using a combination of numerical and microseismic methods. Changes in the stress state induced by coal mining are estimated by means of numerical simulations. The areas of high stress concentration are located and monitored by local microseismic network. The study of microseismic activity recorded during mining made it possible to localise the zones of high stress concentration and validate the calculations done by numerical modelling. The results of this study hold out interesting prospects for using a combination of numerical modelling and microseismic monitoring for detecting, locating and monitoring of the zones prone to rockbursts

Seismic precursory patterns before a cliff collapse and critical-point phenomena

We analyse the statistical pattern of seismicity before a 1-2 103 m3 chalk cliff collapse on the Normandie ocean shore, Western France. We show that a power law acceleration of seismicity rate and energy in both 40 Hz-1.5 kHz and 2 Hz-10kHz frequency range, is defined on 3 order of magnitude, within 2 hours from the collapse time. Simultaneously, the average size of the seismic events increases toward the time to failure. These in-situ results are derived from the only station located within one rupture length distance from the rock fall rupture plane. They mimic the "critical point" like behavior recovered from physical and numerical experiments before brittle failures and tertiary creep failures. Our analysis of this first seismic monitoring data of a cliff collapse suggests that the thermodynamic phase transition models for failure may apply for cliff collapse

Local amplification of deep mining induced vibrations - Part.2: Simulation of the ground motion in a coal basin

This work investigates the impact of deep coal mining induced vibrations on surface constructions using numerical tools. An experimental study of the geological site amplification and of its influence on mining induced vibrations has already been published in a previous paper (Part 1: Experimental evidence for site effects in a coal basin). Measurements have shown the existence of an amplification area in the southern part of the basin where drilling data have shown the presence of particularly fractured and soft stratigraphic units. The present study, using the Boundary Element Method (BEM) in the frequency domain, first investigates canonical geological structures in order to get general results for various sites. The amplification level at the surface is given as a function of the shape of the basin and of the velocity contrast with the bedrock. Next, the particular coal basin previously studied experimentally (Driad-Lebeau et al., 2009) is modeled numerically by BEM. The amplification phenomena characterized numerically for the induced vibrations are found to be compatible with the experimental findings: amplification level, frequency range, location. Finally, the whole work was necessary to fully assess the propagation and amplification of mine induced vibrations. The numerical results quantifying amplification can also be used to study other coal basins or various types of alluvial sites

Microseismicity of an unstable rock mass:from field monitoring to laboratory testing

The field‐scale microseismic (MS) activity of an unstable rock mass is known to be an important tool to assess damage and cracking processes eventually leading to macroscopic failures. However, MS‐event rates alone may not be enough for a complete understanding of the trigger mechanisms of mechanical instabilities. Acoustic Emission (AE) techniques at the laboratory scale can be used to provide complementary information. In this study, we report a MS/AE comparison to assess the stability of a granitic rock mass in the northwestern Italian Alps (Madonna del Sasso). An attempt to bridge the gap between the two different scales of observation, and the different site and laboratory conditions, is undertaken to gain insights on the rock mass behavior as a function of external governing factors. Time‐ and frequency‐domain parameters of the MS/AE waveforms are compared and discussed with this aim. At the field scale, special attention is devoted to the correlation of the MS‐event rate with meteorological parameters (air temperature and rainfalls). At the laboratory scale, AE rates, waveforms, and spectral content, recorded under controlled temperature and fluid conditions, are analyzed in order to better constrain the physical mechanisms responsible for the observed field patterns. The factors potentially governing the mechanical instability at the site were retrieved from the integration of the results. Abrupt thermal variations were identified as the main cause of the site microsesimicity, without highlighting irreversible acceleration in the MS‐event rate potentially anticipating the rock mass collapse

Seismic energy analysis as generated by impact and fragmentation of single-block experimental rockfalls

The analysis of seismic signals obtained from near-source triaxial accelerometer recordings of two sets of single-block rockfall experiments is presented. The tests were carried out under controlled conditions in two quarries in northeastern Spain; in the first test (Foj limestone quarry, Barcelona), 30 blocks were released with masses ranging between 475 and 11,480 kg. The second test (Ponderosa andesite quarry, Tarragona) consisted of the release of 44 blocks with masses from 466 to 13,581 kg. An accelerometer and three high-speed video cameras were deployed, so that the trajectories, velocities, and block fragmentation could be tracked precisely. These data were used to explore the relationship between seismic energy and rockfall kinetics (the latter obtained from video analysis). We determined absolute and relative values of seismic energy and used them to estimate rockfall volumes. Finally, the seismic signature of block fragmentation was assessed in both the frequency and time domains. The ratios of seismic energy after impact to kinetic energy before impact ranged between 10-7 and 10-4. These variables were weakly correlated. The use of seismic energy relative to impacting kinetic energy was preferred for the estimation of volumes. Block fragmentation impacts were dominated by higher acceleration spectrum centroid frequencies than those of nonfragmentation impacts: 56.62 ± 2.88 and 48.46 ± 4.39 Hz at Foj and 52.84 ± 12.73 and 38.14 ± 4.73 Hz at Ponderosa.Peer ReviewedPostprint (published version

Etude des critères microsismiques d'évaluation d'un risque de coups de terrains

National audienceSi tous les coups de terrains constituent des événements sismiques, tous les événements sismiques ne dégénèrent pas en coups de terrains. Les causes intrinsèques de ces événements sismiques particuliers, s'il en existe, sont mal connues et leur prévision à court terme soulève de grandes difficultés