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Imaging tectonic faults with small vertical offsets in argilittes (clay-rock) using geophysical methods is challenging. In the context of deep radioactive waste disposals, the presence of such faults has to be assessed since they can modify the rock confining properties. In the Tournemire Experimental Platform (TEP, France), fault zones with small vertical offsets and complex shape have been identified from underground works. However, 3D high-resolution surface seismic methods have shown limitations in this context that led us to consider the detection and characterization of the faults directly from underground works. We investigate here the potential of seismic full waveform inversion (FWI) applied in a transmission configuration to image the clay-rock medium in a horizontal plane between galleries, and compare it with first-arrival traveltime tomography (FATT). Our objective is to characterize seismic velocities of a block of argilittes crossed by a subvertical fault zone with a small vertical offset. The specific measurement configuration allows us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2D isotropic horizontal imaging domain. Our FWI scheme relies on a robust adaptation of early-arrival waveform tomography. The results obtained with FATT and FWI are in accordance and both correlate with the geological observations from the gallery walls and boreholes. We show that even though various simplifications are done in the inversion scheme and only a part of the data is used, FWI allows to get higher resolution images than FATT, and is especially less sensitive to the incomplete illumination as it uses also diffracted energy. The results provided in this study highlight the complexity of the fault zone, showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water.

Recent advances in seismic inversion methods allow us to get high-resolution quantitative representations of mechanical parameters of the Earth. One of the most promising inversion method is the Full-waveform inversion (FWI) method which basically consists of an iterative data fitting process based on a local optimisation (Tarantola, 1984) and unlike conventional methods (e.g. first arrival times tomography) all the information available in the data can be handled. This method has been first applied to depth exploration (Virieux and Operto, 2009), but some authors already applied it to quantitatively image the near-surface. For example, G'elis (2005) succeeded to image cavities using synthetic data including surface waves arrivals. However one of the most important remaining challenges is the validation of the method with real field data that presents many difficulties: field data are often measured on not well-known media composed of high heterogeneity degree, important intrinsic attenuation and with prominent three dimensional effects. Considering all these difficulties, the necessity to have an experimental intermediate validation step becomes obvious. The aim is to reproduce the field experiments in a well-controlled environment at a smaller scale using a well-defined spacio-temporal scalling. The physical reduced scale modeling has several advantages compared to field-measurements: first the model physical parameters are well controlled, thus numerically repeatable; secondly, the great flexibility of the setup enables to experiment easily novel measurement configurations. This validation approach has already been first applied for geophysical imaging purposes by French (1974). Recently Bretaudeau (2010) designed an ultrasonic measurement bench called MUSC with several features that enable an excellent reproducibility of field measurements from the surface. For instance, a special attention has been paid to select a source that can be assimilated to a point source and the sensors displacements are controlled with an accuracy of 10 m. Using this experimental setup, Bretaudeau (2010) showed promising results on the inversion using the vertical component. Due to recent technological improvements (Blum et al., 2010), it is now possible to record the vertical and the horizontal components simultaneously using a new laser interferometer device which has been implemented in the MUSC measurement bench. In this context, the aim of the study presented here is to investigate numerically the benefits of the each component in the frame of the FWI method and to present the recent advances on measurements of both components on reduced physical scale models. First, the physical reduced scale model and its numerical representation will be described, then a numerical study will present the benefits provided by each component for velocities reconstructions and finally measurements of the vertical and horizontal components on the reduced physical scale model will be presented and compared to numerical simulations.

A new tomographical technique is introduced : a Pseudo-Ray Iterative Averaging Method. It is a geometrical method. During the inversion process, influence zones are defined. Their size is chosen a priori, under physical considerations. This technique transforms the process into an inversion method without blocks. For each zone,weighting factors are defined in order to discriminate between the rays as well as between the zones. The ray tracing is analytical. The combination of these techniques produces a very small computing time consuming method. It's possible to add to the process, evaluations of the resolution and the error as it is made for the matricial case. For civil engineering, several applications are described. Celerity or Q- factor tomographies are computed from measurements of travel times, rise- times or amplitudes. Special devices have been designed in arder to minimize the field data acquisition lime., Une nouvelle méthode de tomographie est décrite (PRIAM~2D). C'est une méthode géométrique itérative. Au cours de l'inversion, des zones d'influence sont définies pour chacune des cases prises en compte. La taille de ces zones est choisie a priori (à partir de considérations physiques). Leur existence transforme la méthode géométrique en une technique d'inversion sans blocs. Le calcul au sein de chacune des zones comporte des pondérations visant à discriminer les rais entre eux. Celles- ci permettent également de discriminer certaines parties de l'image les autorisant ainsi à varier plus librement que d'autres. Les rais utilisés sont analytiques. Des évaluations de la résolution et des erreurs sont introduites. Ces techniques permettent d'obtenir des résultats comparables à ceux fournis par des méthodes matricielles tout en conservant des temps de calcul nettement inférieurs. Dans le domaine du Génie Civil plusieurs applications sont envisagées. Des sols, ainsi que certaines parties d'ouvrages d'Art, peuvent être auscultés. Il est possible d'obtenir des cartes de la vitesse des ondes de compression ou du facteur de qualité, à partir de mesures de durées de propagation, de temps de montée ou d'amplitudes. En outre, Une chaine de mesure spéciale a été mise au point de manière à minimiser le temps de la mesure.

Les récentes applications des théories de l'inversion à la physique du Globe ont été adaptées à l'échelle du Génie-Civil. Des mesures de durées de propagation entre puits permettent d'obtenir la répartition des vitesses sismiques dans le plan contenant les deux forages. Le problème de l'inversion en facteur de quaiité peut se poser dans les mêmes termes que ceux de l'inversion en vitesse. Des mesures d'amplitude~ permettent d'obtenir la répartition des facteurs de qualité. La mise au point des programmes d'inversion a nécessité une élaboration préalable de programmes de simulation. Les données synthétiques obtenues ont permis de juger de l'influence des divers paramètres sur les résultats des inversions ainsi que de tester les critères de contrôle des résultats. Par la suite, un matériel spécifique a été mis au point afin d'obtenir des données réelles. Les géométries des premières Images calculées, en vitesse et en facteur de qualité sont identiques. Ceci apporte une crédibilité importante aux résultats de l'inversion en facteur de qualité. L'application particulière décrite ici est le contrôle des injections. Des mesures identiques avant et après injection ont permis de déterminer clairement les conséquences diverses de l'injection dans les sous-sols étudiés., pas de résumé