High resolution quantitative seismic imaging of a strike-slip fault with small vertical o set in clay-rocks from underground galleries. Experimental Platform of Tournemire, France.

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

The potential of seismic methods for detecting cavities and buried objects: experimentation at a test site

International audienceOne of the recurring problems in civil engineering and landscape management is the detection of natural and man-made cavities in order to mitigate the problems of collapse and subsurface subsidence. In general, the position of the cavities is not known, either because they are not recorded in a database or because location maps are not available. In such cases, geophysical methods can provide an effective alternative for cavity detection, particularly ground-penetrating radar (GPR) and seismic methods, for which pertinent results have been recently obtained. Many studies carried out under real conditions have revealed that the signatures derived from interaction between seismic signals and voids are affected by complex geology, thus making them difficult to interpret. We decided to analyze this interaction under physical conditions as simple as possible, i.e., at a test site built specifically for that purpose. The test site was constructed of a homogeneous material and a void-equivalent body so that the ratio between wavelength and heterogeneity size was compatible with that encountered in reality. Numerical modeling was initially used to understand wave interaction with the body, prior to the design of various data-processing protocols. P-wave imagery and surface-wave sections were then acquired and processed. The work involved in this experiment and the associated results are presented, followed by a discussion concerning the reliability of such a study, and its consequences for future seismic projects

Time-lapse global inversion for surface-waves: A differential approach using a linear approximation of the Rayleigh wave phase velocity

(IF 2.17: Q2)International audienceSurface-wave methods have a wide range of possible applications in the monitoring of near-surface media. In classical monitoring methods, data measured at different times are inverted separately and independently, and the difference in the inversion results reflects the temporal changes in the medium. However, the results obtained in this way can be affected by the reproducibility of experimental measurements, measurement and inversion uncertainties, etc. In this study, we introduce Differential Inversion in surface-wave methods, which uses the difference between measured data as inversion input data, instead of the measured data themselves. More precisely, a linear approximation of Rayleigh wave phase velocity associated with its sensitivity kernel is used to relate the data difference with the model variations. This differential inversion approach is firstly tested with numerical data generated in a series of two-layer models in order to estimate the limitations of the linear approximation with a global inversion approach. It is shown that when the shear-wave velocity variation is more than 5%, the linear assumption is no longer relevant. With respect to the 5% of variation, we apply the differential inversion approach on laboratory data, obtained from three reduced-scale epoxy-resin two-layer models. The results show the feasibility of the differential inversion to estimate the shear-wave velocity differences between epoxy-resin models. The short calculation time is one of its advantages. However, a good estimation on the baseline is required in order to calculate the sensitivity kernel. Finally, the robustness of the proposed approach is verified by numerical data, using the Spectral Element Method to simulate wave propagation in the presence of an under-surface cavity. Keywords: Surface-wave methods, Differential inversion, Small scale models, Rayleigh wave sensitivity kernel, Spectral Element Method

Inversion des formes d'ondes électromagnétiques de données radar multioffsets

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Modélisation physique à échelle réduite pour l'adaptation de l'inversion des formes d'ondes sismiques au génie civil et à la subsurface

L'inversion des formes d'ondes (FWI) est une méthode d'imagerie sismique quantitative multi-paramètre actuellement en plein essor. La prise en compte de l'ensemble des phénomènes de propagation enregistrés, en particulier des ondes de surface, en font une technique prometteuse pour des applications de la subsurface sur des problématiques géotechniques ou environnementales. Cependant, l'adaptation au contexte de la subsurface d'une méthode sismique validée numériquement ou sur des applications d'imagerie profonde est une tâche difficile. Nous proposons ici une approche par modélisation physique à échelle réduite à l'aide d'un laboratoire de Mesures laser-Ultrasonore Sans Contact (MUSC) mis en place dans le cadre de cette thèse. Les différents éléments du laboratoire ont été caractérisés pour la modélisation sismique à échelle réduite et une attention particulière a été portée à la simulation de la source. Les données acquises ont été validées par confrontation avec des données obtenues par modélisation numérique à l'aide des codes fournis par Géoazur dans le cadre du projet ANR SEISCOPE. Dans un deuxième temps, le potentiel de la FWI pour des applications de la subsurface a été évalué par l'approche conjointe de la modélisation numérique et de la modélisation expérimentale. Nous avons ainsi pu montrer par cette double approche que la FWI permet d'obtenir une information sur la morphologie de cavités souterraines grâce à la prise en compte des ondes de surface. Par ailleurs, sur les différents modèles multicouches étudiés, l'information sur les zones les plus superficielles fournie par les ondes de surface dispersives a pu être exploitée par la FWIFull Wave Inversion (FWI) is a multiparameter quantitative method of seismic imaging that is currently booming. Taking into account all propagation phenomena recorded, especially the surface waves, makes it a promising technique for near surface applications on geotechnical and environmental issues. However, adapting a seismic method validated on synthetic data to the context of near surface or on deep imaging applications remains a difficult task. What we are proposing here is a reduced scale physical modelling approach using a laboratory of non contact laser-ultrasonic measurement, which will have been set up as part of this thesis. The various elements of the laboratory were validated for reduced scale seismic modelling, and particular attention has been paid to the simulation of the source. The acquired data has been validated by comparison with data obtained by numerical modelling using algorithmes provided by Géoazur within the context of the ANR SEISCOPE project. In a second step, we evaluated the potential of the use of FWI for near surface applications by using a joint approach of numerical modelling and experimental modelling. By this dual approach, we have thus shown that FWI allows to obtain information on the morphology of underground cavities by taking into account surface waves. Furthermore, on the various multilayer models we studied, the FWI was able to exploit the information on the most superficial areas supplied by dispersive surface wavesNANTES-BU Sciences (441092104) / SudocSudocFranceF

Numerical and Experimental Characterization of the Seismic Parameters of a Soft Soil Reinforced with Rigid Inclusions

International audienceThe characterization of the properties of soft soils reinforced with periodically placed cylindrical rigid inclusions is essential to improve the lifetime of railway tracks submitted to the critical speed issue. This work is about the use of numerical and experimental resources to characterize the velocity of surface waves of these reinforced media. The numerical approach is presented, using the spectral elements method (SEM3D) coupled with non-periodic homogenization. The homogenized medium is used to modify a discontinuous media into a medium with smoother contrasts of properties. Into the context of numerical simulations, it allows to suppress the sharp discontinuities, and thus reduces the cost of the computation linked to the mesh refinement. A cross-validation is made with a reduced scale experimental model, performed with the high-quality measurement bench MUSC. Two different configurations are studied, the first one with three inclusions is used to cross-validate the use of the homogenization method with the experimental measurements and the second is made on more complex case containing a high number of inclusions on the same type of resins

Benefits of the horizontal component in quantitative imaging of near-surface interfaces with lateral variations: synthetic model inversion and reduced scale modeling

International audienceIn near-surface quantitative seismic imaging, the mechanical properties of an heterogeneous medium are usually inferred from the measure of the normal velocity component at different locations. In this study, it is proposed to investigate the benefits of measuring also the tangential velocity component. For that purpose, a realistic synthetic model is defined and the benefits of each component are analyzed in the framework of seismic imaging by Full Waveform inversion. The model is a two-layer medium including lateral variations close to the surface and the synthetic data are generated using a visco-elastic finite elements code. An analysis of the information contained for the different components is carried out and the behavior of the inversion algorithm is studied for each component. To conclude this part, inversion results from inversion strategies integrating the two components are presented. The last part concerns the experimental modeling facility developed in order to experimentally validate the imaging methods. This measurement bench reproduces seismic measurement configurations at a reduced scale using an ultrasonic source and a laser interferometer. This facility has already been validated for the case of the measurement of the vertical component, and first experimental results of the horizontal component are presented

Suivi participatif de l'érosion côtière en 3D : Demain tous SENTINELLES grâce au protocole SELPHCoAST

International audienceDans le cadre du projet ODySéYeu, une réflexion collective (citoyens, associations, scientifiques, municipaux, privés) a été menée pour surveiller et suivre l'évolution de l'érosion littorale sur l'île d'Yeu (Vendée, France). Un cahier des charges basé sur les attentes des uns et des autres a été établi et les sociétés Studio-Matavai et DIGISCAN3D ont été mandatées pour mettre en place, avec l'équipe scientifique, une solution intégrée permettant de lever l'ensemble des verrous et mettre en oeuvre le suivi de 9 sites sur l'île. Basés sur l'utilisation et l'optimisation de la technique appelée Structure from Motion by Smart devices photographies (SFM-S), le protocole SELPhCoAST et l'application pour smartphones/tablettes SENTINELLES sont le fruit de ce travail collectif. Cet article présente les résultats des 20 premiers mois de suivi sur le site de la plage de Ker Châlon. Il illustre l'efficacité de cette solution intégrée qui permet à l'ensemble des citoyens de devenir acteurs de la surveillance de l'érosion côtière et d'être mieux informés des enjeux et des risques. Cette solution offre aux scientifiques la possibilité de suivre l'évolution des sites en 3D, à haute fréquence temporelle et à haute résolution spatiale. Elle permet aux décideurs de mettre en place des solutions de gestion du littoral parfaitement adaptées, en accord avec les citoyens-acteurs et "fondées sur la nature"

EDZ characterization with surface wave analysis: Experimental and numerical study for defining feasibility in the context of the Tournemire Platform (France)

The assessment of an Excavating Damaged Zone (EDZ) due to underground works is important in the context of radioactive waste disposal within a geological formation. Seismic surveys can pro- vide, in a non-destructive manner, information directly related to the mechanical properties of this damaged zone. However, the classical refraction method is unusable for galleries where a concrete layer caps the top surface. Moreover, other possible seismic methods like surface wave analysis can become complex to implement in such a context since guided waves are present and pass through a high-velocity concrete layer. For these reasons, the present paper will focus on an experimental feasibility study of the MASW (Multi Channel Analysis of Surface Waves) method for assessing an EDZ below a concrete layer in the specific context of the Tournemire experimental platform, which was drilled into an argillaceous formation in the Aveyron department of central France. This facil- ity is operated by IRSN (French institute for Radiological Protection and Nuclear Safety). This experimental study has been conducted in conjunction with a modelling results analysis. During an initial stage, the measurements carried out in situ are analysed and it is demonstrated that a receiv- ing profile longer than 8 m is required to distinguish the various modes. Moreover, the comparison of a hammer source and a sweep signal generated by a vibrator source shows that the latter is more appropriate when building the dispersion diagram, particularly for fundamental and first higher mode curves. A second stage shows that according to the analysis of theoretical curves for profiles with similar seismic velocities of the Tournemire platform medium, the dispersion curves are sensi- tive to EDZ characteristics. In the paper's final part, the dispersion curves obtained from the experimental seismic data are inverted using a global approach. Results of the inversion of the fundamental mode only and of both the fundamental mode and the first higher mode are com- mented upon, evidencing an improved quality for this last approach

Reconstructiong the internal architecture of giant medieval shell middens with the ground penetrating Radar

International audienc