isabelle contrucci

SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

In the frame of LISA and CROP Mare II near vertical deep reflection projects in Northern Tyrrhenian area, between Tuscany, Latium and Corsica, integrative wide angle reflection/refraction seismic data were acquired. In this paper we present the results of processing and interpretation of the data collected on the Tuscan side. Structural and stratigraphical information about upper crust was obtained examining the seismic sections available, while regarding the static correction, at this moment we used to constrain and correct the data only the information derived from literature. In this area the presence of the peri-Tyrrhenian-Tuscan domain was recognized. This domain has a crustal thickness between 20-25 km with regional trend dipping toward Corsica Island. The result of the interpretation of these data allow us to draw a map of the crust-mantle boundary along Tuscan off-shore. In the off-shore (about 20 km far the coast) between Piombino-Orbetello we recognized a minimum Moho dept...

In 2004, in order to better understand processes involved in large-scale mine collapse, an instrumentation was settled in the surrounding of a salt cavern located at a depth of 180 m in NE France. The cavern was mined by solution mining until the large-scale ground failure occurred. A high resolution multi-parameter monitoring system was deployed in the framework of the GISOS (Scientific Interest Group on the Impact and Safety of Underground Structures formed by INERIS, BRGM, INPL and ENSG). Instrumentation, installed by INERIS, consisted of a microseismic network, coupled to automatic-measurement system for levelling (Tacheometer and RTK GPS). Quasi real time transmission of the data to INERIS, at Nancy, enabled rock mass activity of the site to be monitored on a few hours basis. Also, the various recorded observations, in the beginning of spring2008, led the operator to cause the collapse in February 2009. This was done by intensive extraction of the brine contained in the cavern,...

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Interpreting microseismicity induced by a collapsing solution mining cavity Jannes Kinscher, Isabelle Contrucci, Pascal Bernard, S. Cesca, Anne Mangeney, Jack-Pierre Piguet, Pascal Bigarre

Microseismic events (MSE) in underground mines are the result of stress perturbations and rock fractures due to mining excavations. In the last decades, the problem of mining-induced seismicity has become more and more important due to the increasing depth of underground mines. Indeed, in situ stresses on average increase with depth, resulting in seismic activity intensification, both in terms of events rate and radiated seismic energy. Understanding how and why MSE occur is, then, a fundamental purpose in order to control the rate of seismicity and guarantee mine infrastructures and mine workers safety. With the aim of better understanding interactions between stress modifications induced by mining and the generation of seismic activity, different factors need to be taken into account: geological structures and rock properties, initial state of stress and stress modifications, but also mining method, excavation geometry and blasting practice, are some of the principal aspects affec...

In order to improve our understanding in brutal large scale ground failure phenomenon, a salt solution mining cavern was instrumented in 2004 previously to its expected collapse as part of its mining scheme. A permanent early warning system was set up, including a high resolution microseismic monitoring network linked to a surface field displacement measurement system. The important amount of data collected during this 5 years experiment offered real-time insight of the evolution of the geological system. The complete data set recorded during the experiment made it possible to track with precision the main stages in the evolution of the cavern. The early signs of failure were detected by high resolution microseismic monitoring during spring 2008: a shift in microseismic background regime as well as recurrent microseismic episodes were undoubtedly associated to a general process of rock failure due to the salt cavern extending up to a critical size. This was accompanied by a few epis...

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. 3D acoustic and microseismic location of collapse events in complex, 3D geological structures Emmanuelle Klein, Anthony Lomax, Armand Lizeur, Frauke Klingelhoefer, Isabelle Contrucci, Pascal Bigarre

Abstract In this work we investigated seismic and aseismic rock mass behaviors in response to deep underground mining. For this purpose, an area under production of the metal mine of Garpenberg (Sweden) was instrumented with a geophysical and geotechnical monitoring network. In situ monitoring data were analyzed and interpreted considering mining operations and the local geological setting. In addition, a 3D elasto-plastic numerical model was built to better understand the interactions between quasi-static stress changes due to mining and the generation of the induced seismicity. Results of this multiparameter approach show a complex rock mass response. We observed two main types of seismic behaviors: one local and temporally short, directly induced by production, the other long-lasting over time and remote from excavations being mainly controlled by geological heterogeneities. In addition to seismicity, we also observed creep-like phenomena induced by mining. In turn, these time-dependent strains appear to be a third mechanism driving seismicity. All these findings underline the importance of considering both seismic and aseismic deformations when one wants to characterize the rock mass response to mining. This significantly enhances our understanding of the phenomena involved, as well as their interactions, for an improved hazard assessment in deep mining operations.

Recent studies have demonstrated the success of automatic full-waveform detection and location methods in analyzing and monitoring natural and induced seismicity. These approaches have been shown to provide a significant improvement in events detectability, increasing the significance of statistical analysis that permits to identify small changes of seismicity rates in space and time. Although currently nontrivial and by far nonstandard, application of such methods to seismic monitoring of active mines could significantly improve forecasting of potential destructive rockburst events. The main challenges of such applications are related to the presence of a wide range of seismic noise sources that have to do with mining activity and a high sampling rate of recorded data (several kHz), posing problems for real-time data transfer and processing. In this study, we propose an adapted full-waveform-based automatic method for the detection and location of microseismic events that makes use...

With the aim of understanding the nature of mining-induced seismicity, microseismic activity in the deep metal mine of Garpenberg (Sweden) has been recorded during 2 years of excavations. The studied area of the mine is operated using sublevel stoping method with backfilling, between depths of around 1000 and 1300 meters. Spatiotemporal analysis of microseismic activity is presented and correlated with the occurrence of mining blasts. A clear dependence is observed between blasts and seismic sequences, even if the rock mass response to mining appears to be very variable across space and time. Two main clusters are observed: one located in the major production area (Central Cluster), while the second (Right Cluster) is located at some distance from the excavations, in a zone characterized by a heterogeneous distribution of weak materials and stiff rock masses. By analyzing seismic source parameters, we demonstrate that the two clusters are characterized by different dynamics. In addition, we show how Right Cluster events are mainly controlled by geological heterogeneities, which impose high stress concentrations in the stiff rock masses surrounding weak lenses. High apparent stresses and corner frequencies associated with the Right Cluster events agree with our proposed model. This suggests elevated stresses in the seismic source region and small source dimensions; indeed, fractures cannot propagate along great distances due to the presence of weak lenses interbedded with the breaking stiff rock mass.

Accuracy of source locations of microseismic spatial swarms monitored in the vicinity of underground cavities by local seismic stations is a critical issue for the geotechnical expert in charge of assessing the local failure mechanism and its potential significance on the short-term overall stability. In order to quantify the gain in hypocenter resolution that earthquake relocation algorithms offer, multiplets were simulated using artificial sources performed in abandoned mines of the Lorraine iron-basin, region of north-eastern France. Sources consisted in small mining blasts shot in underground mine pillars at 200 meters deep, accurately controlled in terms of coordinates, orientation and energy. These ``multiplet blasts'' were configured to reproduce multiplet characteristics that may be defined as a group of spatially close events with similar waveforms, source mechanisms, and different origin times and magnitudes. With such a recorded dataset, first, the true orientatio...

A la fin des annees 1990, lors de la cessation de pres de deux siecles d'activite miniere intense en France, une serie d'effondrements et d'affaissement de terrain catastrophiques a eu lieu dans des zones urbanisees situees a l'aplomb d'anciennes exploitations souterraines partielles desaffectees. L'ampleur des exploitations conduites dans des conditions comparables et donc potentiellement dangereuses a declenche l'instauration d'une veritable politique de l'apres-mine et la mise en oeuvre d'une strategie de prevention a l'echelle nationale. Des etudes d'alea ont ainsi mis en evidence de nombreuses zones urbanisees classees a risque. Neanmoins, la multitude de ces zones d'une part, et la rarete des evenements de subsidence, a la fois imprevisibles et susceptibles de survenir plusieurs decennies apres la fin de l'exploitation d'autre part, a conduit les instances decisionnelles a deployer l'utilisation de la surveillance permanente comme une composante majeure de la strategie de prevention. Des experiences de terrain ont ete conduites pour qualifier, valider et optimiser des methodes de surveillance dans differents contextes geologiques et miniers, y compris une experience d'effondrement controle. Le retour d'experience est ici decrit et discute, une decennie apres la mise en oeuvre du premier systeme d'alerte micro sismique precoce.

To improve our understanding in large scale ground failure phenomenon induced by old underground mining works, a field experiment was undertaken by the partners of GISOS1in 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, at Cerville-Buissoncourt southwards from Nancy, France. To monitor the cavern collapse, a permanent monitoring system was set up. This comprises a SYTGEM multi-parameter system featuring high resolution microseismic (5 1D geophones, 4 3D geophones, 40-1000 Hz) linked to ground surface monitoring (tacheometer and GPS high resolution measurements). The important amount of data transmitted for on-line processing offered daily insight of the evolution of the geological system. The early signs of unstable evolution were detected by high resolution microseismic monitoring during spring 2008 : a shift in microseismic background regime as well as recurrent microseismic episodes were associated to a general process of rock failure due to the cavern growing upwards. This was accompanied by a few episodes of massive roof falls, with no ground surface movement detected. During a second and last stage of evolution that lasted three days, the cavern collapse was triggered 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. The first peak of activity, due again to massive roof falls below the Beaumont dolomite bed, also initiated the brittle rupture of this bed. The second and major peak of activity marked its rapid and energetic failure; it also corresponded to an acceleration in the subsidence rate. The last peak of activity, a few hours later, weaker in terms of number of events and energy released, signed the final collapse. The complete dataset confirms the major role of the Beaumont dolomite bed in the site stability. Besides, if the preliminary analysis of the microseismicity patterns right before the general collapse, confirms that the acceleration in the microseismic activity follows a power law, it also shows that the b-value is a complex marker in such context. Even though local falls of b-value well correlate, as expected, to local increases in microseismic energy release, it seems inappropriate to use it for forecasting purposes: over a short period of time, several b-value decreases are observed while a continuous pumping regime is applied to the system. So, apparent b-value variations don't show clear precursory pattern, as observed in other case-study. Nevertheless, further investigations over the different stages of evolution of the cavern, including the collapse period, are needed to confirm this preliminary result.

The study area stretches over the northern edge of the Storegga slides, from the axial region of the slides, in 2200 m water depth, to the upper slope, north of the slides, in 700 m water depth. High resolution seismic lines point to intense small-scale fracturing and diffuse degassing through the near seafloor sediments in the slides. In contrast, the

The Atlantic margin of Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia Margin of North America. The SISMAR marine seismic survey acquired over 1000 km of wide-angle seismic profiles in two different zones in order to image the deep structure of the margin, characterize the nature of

The role of fluid flow and gas hydrate dissociation in the triggering of the Storegga slides south of the Voring Plateau, off Norway has been extensively investigated in the past few years. On the northern flank of the Storegga slides, a gas hydrate related Bottom Simulating Reflector (BSR) marks the base of the hydrate stability zone. Intensive sediment faulting, gas

In France, besides the room-and-pillar method, solution mining has also been carried out largely, creating numerous abandoned cavities, presently collapsed or not. Nowadays, controlled collapse of the huge cavities created by intensive solution mining has been introduced as a full part of the exploitation scheme, avoiding then a projected permanent hazard. Then, opportunity to monitor early warning signs, precursory events and ground collapse mechanisms was seized through a relevant research operation in 2005. This consisted in the field monitoring experiment of the controlled collapse of an approximately 1.5 million cubic meters solution mined cavern lying between 190 and 280 meters deep. The experiment resulted in the recording of a considerable scientific dataset, including tens of thousands microseismic events along with numerous strain, displacement and pressure time series.

Differentes methodes sont mises en oeuvre pour caracteriser les vitesses des ondes sismiques et l'etendue de l'EDZ (Excavation Damaged Zone) autour des ouvrages souterrains. Ce travail presente une synthese de resultats obtenus dans deux galeries de la station experimentale IRSN de Tournemire

A salt cavern worked by dissolution in north-eastern France (54), was instrumented in 2004 prior its collapse. The multi-parameter high resolution monitoring system consisted of a microseismic network, coupled to automatic levelling measurement system. This enabled the monitoring of the overburden response to changes in the cavern, on a daily basis, up until its collapse in February 2009. In particular, all the data highlights the major role of a competent layer, vertically above the cavern at a depth of 120 meters. The analysis of the microseismicity shows that from the spring of 2008, this layer alone provided the stability of the site, while no significant sign of instability was detected by the levelling measurements. Its general failure, nearly a year later, was followed a few hours afterwards by the formation of a crater with abrupt edges, nearly 150m in diameter. If these preliminary results clearly highlight the exceptional character of this experiment and of the information drawn from it from an operational point of view, they also show the necessity to determine accurately the various failure mechanisms in play. In particular the microseismic ‘burst’ events mechanism, detected during the main peaks of the microseismic activity, must be identified, in order to understand and analyse the data in their entirety.

La premiere partie de cette these a ete consacree a l'etude de la nature de la croute dans le bassin nord ligure. Les profils de sismiques multi-traces ont mis en evidence un accident majeur dans le socle sur la marge provencale. Cet accident a ete associe a un changement de nature de la croute. La limite du domaine oceanique a ete placee le long de cet accident. Sur la marge corse, il n'existe pas d'accident similaire a celui observe sur la marge provencale. La limite du domaine oceanique a ete fixee par le magnetisme, en identifiant cote corse des anomalies similaires a celles qui sont observees cote provencal a l'aplomb de l'accident. L'enregistrement a terre sur la cote corse, du profil sismique permet d'imager le moho sous le sel messinien, car il est eclaire avec des rais d'incidence oblique. Le retraitement des esp de la region a permis de mettre en evidence l'existence d'une croute oceanique au centre du bassin et une croute de type transitionnelle au large des marges provencale et corse. Cette croute transitionnelle est caracterisee par la presence, dans la croute inferieure, d'une couche d'une vitesse de 7. 2 km/s. Une limite du domaine oceanique a ete proposee au nord du bassin ligure. La deuxieme partie de cette these porte sur le bassin nord tyrrhenien. L'analyse combinee de la sismique reflexion multi-traces, de l'inversion gravimetrique tridimensionnelle et des enregistrements a terre de sismique reflexion grand-angle/refraction, a permis d'obtenir une image tridimensionnelle du bassin nord tyrrhenien. Le bassin corse est probablement d'age burdigalien, voir d'age oligocene superieur. Sa direction structurale et son age indiquent que ce bassin est contemporain de l'ouverture liguro-provencale. Le flux de chaleur corrige de l'effet des sediments est faible, malgre la remontee du moho sous le bassin. Les anomalies gravimetriques a l'air libre et les anomalies magnetique sont negatives, confirmant la profondeur importante du bassin. Les enregistrements de sismique refraction confirment la faible profondeur du moho sous le bassin. Une arrivee tardive sur ces enregistrements a ete associee a la base de la lithosphere. Le bassin corse a subi des reactivations recentes au tortonien, liees au rebond isostatique de la corse hercynienne du a l'ecroulement gravitaire de la corse alpine. La structure des sediments sur cette bordure du bassin corse suggere une geometrie en paliers et rampes pour la faille de detachement majeure. Cette faille de detachement se prolongerait sous l'ile d'elbe, ou une zone de cisaillement ductile a ete observee.

The CAMI-AFT experiment was carried out with a view to calibrating real-time microseismic Systems installed on sites prone to post-mining collapse, in the Lorraine Iron Basin (France) by improving automatic routines applied to microseismic waveform processing and reliable source parameter estimâtes. To this intent, mine blasts, with well-known location and energy source, were carried out on a few monitored sites still accessible underground. In-depth analysis of ail recorded seismograms first allowed us to define a relationship for accurate source energy calculation derived from recorded seismograms. Data were used to test and calibrate waveform polarization routines as well as numerical multilayered velocity models. Finally, automatic 3D localization of low-magnitude microseismic events (M < 0) based on a small number of stations was thus improved.

ABSTRACT The deep structure of the west African continental margin, offshore Angola, was investigated during the ZaiAngo Joint project, conducted in march 2000 by IFREMER et Total Fina Elf Production. Seismic data generated by a " single-bubble " air gun array (Avedik et al., 1993) and simultaneously recorded on a 4.5 km long streamer and on Ocean Bottom Seismometers (OBSs) provide an image of the deep structures located below the Aptian salt layer that were unrevealed so far using conventional seismics. The combination of vertical reflection and wide angle reflexion / refraction seismics show that : 1) the crust thins very abruptly, from more than 30 km to about 6 km, over a lateral distance of less than 50 km ; only one tilted block is visible on the MCS sections only in the area where the crustal thinning occurs, on the shallower part of the continental slope ; 2) the transitional domain, 160 km long, is characterized by a sub-salt sediment basin and a thin crust (6 km in average), which is underlain, on the continent side, by a anomalous velocity zone (7.2 7.8 km/s). Sub-salt sediment layers are flat, suggesting that the sediment depostion occured in a phase characterized by subsidence unaffected by active tectonism ; 3) the oceanic domain is characterized by the presence of a anomalous velocity zone (7.6 km/s) at its base. If present, tilted blocks at the slope foot or in the deepest part of the basin are necessarily of limited size, too small to be visible on the MCS sections. This suggests that the extension that created the tilted blocks (wherever present or not) is not significant enough to be entirely held responsible for the observed crustal thinning. In addition, the sub-salt sediment infill, supposedly syn-rift, is flatly layered, suggesting that the basin was mainly formed by subsidence. If crustal thinning had occured by horizontal stretching, the sediment infill would show a fan-shaped layering. These observations cannot be readily explained by commonly used models of crustal thinning. Then, if extension alone cannot account for all the observed amount of crustal thinning, alternative models are needed.