Evaluation de l'endommagement dans le recouvrement d'une cavité saline par une approche numérique couplée continue-discrète

National audienceWith the objective to better understand the evolution of overburden damage on top of an underground solution mining, an in-situ experiment is undertaken over a salt cavity in the Lorraine region. The overburden overlying the salt cavity is characterized by a competent layer where most fragile damage, with the associated microseismicity, is expected. This work presents a coupled continuum-discrete modelling approach to simulate the mechanics of fracture initiation and propagation in the rock mass, a continuum approach FLAC for the marls and salt layers, and a discrete approach PFC for the competent layer. Numerous calibration experiments of the discrete model microproperties are firstly performed to reproduce the mechanical response of the competent layer samples. The numerical validation of the coupled approach is carried out by comparing the results with those from the fully continuum approach at the site scale. First modelling results suggest that the mechanisms of fracture in the competent layers is predominantly tensil. Moreover, microcrack simulation opens interesting perspectives for comparison with the observed microseismicity of the study area.Dans l'objectif de mieux comprendre la progression de l'endommagement dans le recouvrement d'une cavité saline créée par dissolution, une expérimentation in-situ a été entreprise sur une cavité de la région Lorraine. Le recouvrement est caractérisé par la présence d'un banc raide où la plupart de l'endommagement fragile (et de la microsismicité associée) est attendu lors de la reprise de l'exploitation. Ce travail présente une simulation numérique basée sur une approche couplant un modèle continu FLAC (pour les couches marneuses et le sel), et discret PFC (pour le banc raide), dont l'objectif est d'évaluer les mécanismes de développement des fractures dans les terrains du recouvrement. Une calibration exhaustive des microparamètres du modèle discret a été réalisée pour reproduire les propriétés macroscopiques d'échantillons provenant du site. La validation numérique du modèle est faite sur la base d'une comparaison à l'échelle du site entre les résultats d'une simulation purement continue (FLAC) avec ceux de la simulation couplée (FLAC-PFC). Les résultats montrent que le mécanisme de fracturation est fondamentalement en traction. Par ailleurs, l'analyse des microfissures calculées et de leur extension en fonction des sollicitations s'appliquant sur la cavité, ouvre des perspectives très intéressantes pour la confrontation avec des mesures qui seraient enregistrées lors de l'effondrement provoqué

A large scale continuum-discrete numerical modelling : application to overburden damage of a salt cavern

International audienceWith the objective to better understand the evolution of overburden damage on top of an underground solution mining, an in-situ experiment is undertaken above a salt cavity in the Lorraine region (NE of France). The overburden overlying the salt cavity is characterized by a competent layer where most brittle damage, with the associated microseismicity, is expected. This work presents a coupled continuum-discrete modelling approach to simulate the mechanics of fracture initiation and propagation in the rock mass, a continuum approach for the marls and salt layers, and a discrete micromechanical approach for the competent layer. A methodology of a large scale coupled continuum-discrete modelling is proposed and applied to the site of Cerville-Buissoncourt. The numerical verification of the hybrid approach response is achieved by comparing the results with those from the fully continuum model at the site scale when the competent layers behave elastically. The first results of a large scale modelling based on the hybrid approach suggest that the fracture mechanisms in the competent layers are predominantly tensile. Finally, these explicit microcracking simulations open interesting perspectives for comparison with the observed microseismicity of the study area (transient pressure experiment or future final collapse).Dans l'objectif de mieux comprendre la microsismicité induite dans des mines de sel, une expérimentation in situ a été réalisée sur une cavité saline de la région Lorraine (Nord-Est de France). Le recouvrement est caractérisé par la présence d'un banc raide dans lequel l'essentiel de l'activité microsismique est attendue lors de la reprise de l'exploitation. Ce papier présente une modélisation basée sur une approche couplée : continue (pour les marnes et le sel) et discrète (pour les bancs raides) pour évaluer les mécanismes de développement des fractures dans le recouvrement. Une méthodologie de modélisation à grande échelle couplant les approches continue et discrète a d'abord été proposée, puis appliquée au site de Cerville-Buissoncourt. La validation numérique de cette approche hybride est réalisée par comparaison avec la réponse d'un calcul continu à l'échelle du site. Les premiers résultats de cette modélisation hybride à grande échelle montrent que les mécanismes de fracturation prédominants dans les bancs raides restent les tractions. Ceci ouvre des perspectives intéressantes pour des confrontations avec les mesures qui seront enregistrées lors de l'effondrement provoqué (essai de pression, future effondrement après la reprise de l'exploitation)

Induced seismicity monitoring of an underground salt cavity under a transient pressure experiment

International audienceWithin the framework of a research project launched to assess the feasibility of seismic monitoring of underground growing cavities, this specific work focus on the analysis of the induced microseismicity generated by a controlled pressure experiment recently carried out in a salt mine environment. A local seismic array has been installed over a stable underground cavity within a salt layer located in the Lorraine basin (north-east France). The array includes four 3D components and three 1D component geophones deployed at depths between 30 m to 125 m in cemented boreholes drilled in the vicinity of the study area. The underground cavity under monitoring is located within a salt layer at 180 m depth and it presents a rather irregular shape that can be approximated by a cylindrical volume of 50 m height and 180 m diameter. Presently, the cavity fully saturated with brine inducing a significant pressure on its walls (~2.5 MPa) to keep the overburden mechanically stable. Nevertheless some small microsesimic events were recorded by the array and analysed (~500 events in two years of recording). In October 2005, a controlled pressure transient experiment has been carried out in the cavity in order to analyse the mechanical response of the overburden by tracking the induced microseismicity. The recorded events are mainly grouped in clusters of 3 to 30 seconds of signal duration with non-emergent first arrivals and rather low frequency content (between 20 to 120 Hz). Some of these events have been spatially located by travel-time picking close to the actual cavity and its immediate roof. Preliminary spectral analysis suggests sources possibly related to resonant fluid-filled cracks, and/or cavitations due to the dynamic excitation of the brine-filled cavity. Rock-debris falling into the cavity from delamination of clayey marls in the immediate roof is possibly another source of seismic excitation. No clear evidence of classical brittle ruptures in the competent layers of the overburden has been observed up to now. Current work is focused on the discrimination of all these possible mechanisms to better understand the damage processes within the rock mass

Induced seismicity in a salt mine environment evaluated by a coupled continuum-discrete modelling

International audienceWith the objective to better understand the induced microseismicity in a salt mine environment due to an underground solution mining, an in situ experiment is undertaken by GISOS in the Lorraine salt basin. The overburden overlying the salt cavity is characterized by the presence of two competent layers where most microseismic events are expected. This paper presents a coupled continuum-discrete modelling to simulate the mechanics of fracture initiation and propagation in the rock mass overlying the cavity: discrete approach for the competent layers and continuum approach for marls, salt and other rocks and soils. For the competent layers, numerous calibrations of the model microparameters based on the laboratory results are firstly performed. The first coupled modelling results suggest that the mechanism of fracturing in the competent layers is predominantly tensile as it could be expected. The results also show that the microseismic events associated to the progressive damage in the competent layers through microcraks development can be modelled. This opens interesting perspectives to assess the feasibility of seismic monitoring of underground cavities by comparing, in the future, the numerical modelling results with the recorded seismicity of the study area.Dans l'objectif de mieux comprendre la microsismicité induite dans des mines de sel, une expérimentation in situ a été entreprise par le GISOS sur une cavité saline de la région Lorraine. Le recouvrement est caractérisé par la présence de deux bancs raides dans lesquels l'essentiel de l'activité microsismique est attendue lors de la reprise de l'exploitation. Ce papier présente une modélisation basée sur une approche couplée : continue (pour les marnes et le sel) et discrète (pour les bancs raides) pour évaluer les mécanismes de développement des fractures dans le recouvrement. De nombreuses calibrations des microparamètres du modèle discret à partir des paramètres macroscopiques ont été nécessaires. Les résultats de la modélisation montrent que le mécanisme de fracturation dans les bancs compétents résulte essentiellement d'efforts en traction. Les premiers résultats montrent aussi que les évènements microsismiques peuvent très bien être mesurés à travers la formation des microfissures. Ceci ouvre des perspectives intéressantes pour des confrontations avec les mesures qui seront enregistrées lors de l'effondrement provoqué

A nodal high-order discontinuous Galerkin method for elastic wave propagation in arbitrary heterogeneous media

International audienceWe present an extension of the nodal discontinuous Galerkin method for elastic wave propagation to high interpolation orders and arbitrary heterogeneous media. The high-order lagrangian interpolation is based on a set of nodes with excellent interpolation properties in the standard triangular element. In order to take into account highly variable geological media, another set of suitable quadrature points is used where the physical and mechanical properties of the medium are defined. We implement the methodology in a 2-D discontinuous Galerkin solver. First, a convergence study confirms the hp-convergence of the method in a smoothly varying elastic medium. Then, we show the advantages of the present methodology, compared to the classical one with constant properties within the elements, in terms of the complexity of the mesh generation process by analysing the seismic amplification of a soft layer over an elastic half-space. Finally, to verify the proposed methodology in a more complex and realistic configuration , we compare the simulation results with the ones obtained by the spectral element method for a sedimentary basin with a realistic gradient velocity profile. Satisfactory results are obtained even for the case where the computational mesh does not honour the strong impedance contrast between the basin bottom and the bedrock

Étude de la réponse structurelle d’un bâtiment de grande hauteur à partir d’enregistrements accéléromètriques et de la modélisation par éléments finis

International audienceThe Nice prefecture is equipped by 24 accelerometric sensors, located at different levels, which continuously record the building response. A finite element model of the building is created based in design plans and information about materials provided during the conception project. Such model is used to obtain the natural frequencies of the structure, the modal shapes and the seismic response acceleration time histories. Modal frequencies of the building obtained from the data analysis are used to calibrate the numerical model. The response of the building to earthquakes, in the form of acceleration time histories at different storeys, is evaluated using the finite element model. Signals recorded during the recent Barcelonnette earthquake (April 2014, Mw = 4.9) are used as excitation at the base of the building. The imposition of a single or multiple signals at the base of the structure shows the importance of taking into account the rocking effects in the building response modelling. The numerical results are in agreement, in amplitude and phase, with accelerometric records filtered at different frequency bands.La préfecture de Nice est équipée d’un ensemble de 24 capteurs accélérométriques, répartis sur différents étages, qui enregistrent en continu les vibrations du bâtiment. Un modèle par éléments finis du bâtiment est créé à partir des plans de construction et des données sur les matériaux, issues du projet de dimensionnement d’origine. Ce modèle est utilisé pour obtenir les fréquences propres de vibration de la structure, les déformées modales et la réponse sismique dans le temps. Les fréquences modales du bâtiment obtenues par l’analyse des enregistrements sont utilisées pour calibrer le modèle numérique. La réponse du bâtiment aux séismes, en terme de séries temporelles d’accélération aux étages, est évaluée par le modèle d’éléments finis. Les signaux enregistrés lors du récent séisme de Barcelonnette (avril 2014, Mw = 4.9) sont utilisés comme sollicitations appliquées à la base du bâtiment. L’imposition d’une seule ou plusieurs excitations à la base de la structure mon- tre l’importance de la prise en compte des effets de «rocking» du bâtiment pour mieux comprendre sa réponse aux séismes. Les signaux numériques sont en accord dans différentes bandes de fréquence, en amplitude et en phase, avec les enregistrements

Modelling Seismic Wave Propagation for Geophysical Imaging

International audienceThe Earth is an heterogeneous complex media from the mineral composition scale (10−6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (Günther et al., 2006; Rücker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields

Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.Published2367-24205T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

PRENOLIN project. Results of the validation phase at sendai site

One of the objectives of the PRENOLIN project is the assessment of uncertainties associated with non-linear simulation of 1D site effects. An international benchmark is underway to test several numerical codes, including various non-linear soil constitutive models, to compute the non-linear seismic site response. The preliminary verification phase (i.e. comparison between numerical codes on simple, idealistic cases) is now followed by the validation phase, which compares predictions of such numerical estimations with actual strong motion data recorded from well-known sites. The benchmark presently involves 21 teams and 21 different non-linear computations. Extensive site characterization was performed at three sites of the Japanese KiK-net and PARI networks. This paper focuses on SENDAI site. The first results indicate that a careful analysis of the data for the lab measurement is required. The linear site response is overestimated while the non-linear effects are underestimated in the first iteration. According to these observations, a first set of recommendations for defining the non-linear soil parameters from lab measurements is proposed. PRENOLIN is part of two larger projects: SINAPS@, funded by the ANR (French National Research Agency) and SIGMA, funded by a consortium of nuclear operators (EDF, CEA, AREVA, ENL)