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Saint-Martin-Vésubie, France

Grandjean G.,Bureau de Recherches Geologiques et Minieres | Gourry J.C.,Bureau de Recherches Geologiques et Minieres | Sanchez O.,Bureau de Recherches Geologiques et Minieres | Bitri A.,Bureau de Recherches Geologiques et Minieres | Garambois S.,LGIT
Journal of Applied Geophysics | Year: 2011

This study of the Ballandaz landslide (Savoie, French Alps) was carried out as part of the ANR/PGCU-SIGMA research project. Its aim was to characterize the slope by different methods of geophysical imagery, and then use this characterization to provide a combined interpretation of the morpho-structures in order to simulate the geomechanical behavior of the sliding mass. First, electromagnetic mapping was used to identify the variations of the surface lithology and moisture from which one can more precisely locate the active zone of the slope. Then, electrical and seismic 2D imagery methods were used along several transverse and longitudinal profiles in order to produce electrical resistivity and seismic-velocity depth sections showing up the slope's structures. The H/V method was also tested locally to determine the depth to basement so as to complement the profiles: it reveals potential seismic site effects from the deeper structures. Helped by drilling and inclinometer surveys, the geophysical interpretations have revealed the various units structuring the landslide: (i) the active, very heterogeneous shallow level overlying (ii) a more rigid, less porous, and probably stable shallow bedrock, (iii) boulders, and (iv) the sound geological basement of quartzite and gypsiferous facies. The study has shown the usefulness of combining such different sounding techniques for studying complex environments like landslides; it has also revealed the limitations of each method when used for studying very heterogeneous environments. © 2011 Elsevier B.V.

Ducret G.,Ecole Normale Superieure de Paris | Doin M.P.,Ecole Normale Superieure de Paris | Grandin R.,Ecole Normale Superieure de Paris | Lasserre C.,LGIT | Guillaso S.,TU Berlin
International Geoscience and Remote Sensing Symposium (IGARSS) | Year: 2011

Synthetic Aperture Radar interferometry allows to measure spatio-temporal patterns of deformation. However this geodetic technique is limited by unwrapping difficulties linked with temporal decorrelation and topographic errors in partially incoherent and mountainous areas. This paper presents a new algorithm to correct and remove DEM errors in order to improve the phase unwrapping step. The method consists in a mix approach between Small Baseline and Permanent Scatterers strategy using a series of wrapped interferograms. First we develop our methodology and then we apply it to a series of wrapped ENVISAT interferograms on the Tibetan plateau. © 2011 IEEE.

Mordret A.,CNRS Paris Institute of Global Physics | Shapiro N.M.,CNRS Paris Institute of Global Physics | Singh S.,CNRS Paris Institute of Global Physics | Roux P.,LGIT | Barkved O.I.,British Petroleum
3rd Passive Seismic Workshop: Actively Passive! | Year: 2011

We used 6 hours of vertical component continuous data recorded from more than 2400 receivers of the Valhall LoFS for passive seismic interferometry. The Correlation Functions contain symmetrical Scholte-waves in the 0.1-2 Hz frequency range and we showed by beamforming that the sources of these waves were homogeneously distributed around the array. We constructed group velocity dispersion curves of the extracted Scholte-waves and inverted them to produce a group velocity map of the Valhall field subsurface. We found that seismic velocities were higher in the center of the array than at its edge. It is in good agreement with geomechanical models based on the sea-floor subsidence due to the reservoir production.

Etienne V.,Geoazur | Virieux J.,LGIT | Operto S.,Geoazur
72nd European Association of Geoscientists and Engineers Conference and Exhibition 2010: A New Spring for Geoscience. Incorporating SPE EUROPEC 2010 | Year: 2010

We present a discontinuous Galerkin finite-element method (DG-FEM) suitable to seismic modelling in large scale 3D elastic media. The method makes use of unstructured tetrahedral meshes locally refined to the medium properties (h-adaptivity) and of interpolation orders that can change from one element to another according to an adequate criterion (p-adaptivity). These two features allow us to reduce significantly the numerical cost of the simulations. While the efficiency of DG-FEM has been largely demonstrated with high interpolation orders, we favour the use of low orders more appropriate to the applications we are interested in. In particular, we address the issues of seismic modelling or seismic imaging in case of complex geological structures requiring a fine medium discretisation. © 2010, European Association of Geoscientists and Engineers.

Bretaudeau F.,Laboratoire Central des Ponts et Chaussee LCPC | Leparoux D.,Laboratoire Central des Ponts et Chaussee LCPC | Brossier R.,LGIT | Operto S.,Geoazur | Abraham O.,Laboratoire Central des Ponts et Chaussee LCPC
Near Surface 2010 - 16th European Meeting of Environmental and Engineering Geophysics | Year: 2010

Quantitative imaging of the first meters of the underground with seismic methods is an important challenge for many applications. This task is difficult because of heterogeneity, strong attenuation, low frequency content and large dominance of surface waves in the data. Tomography, migration, and dispersion analysis of surface wave are not efficient in all contexts. Elastic full waveform inversion was developed for crustal imaging but has a great potential for near surface applications as it could help to image very heterogeneous media by exploiting the whole complexity of the seismograms. We try here to (1) understand how behaves this method on near surface context, and (2) anticipate on the field application. We use for that a simultaneous approach by applying the waveform inversion on a simple three layer medium from synthetic data obtained by numerical modeling, and from experimental data obtained by small scale modeling. We present the small scale model, the acquisition of synthetic and experimental data, and we present an inversion result. Images of the Vp and Vs parameters are reconstructed. Both lateral and vertical variations are well determined. Images from synthetic and experimental data can be compared. The results are similar but we identify differences related to noise.

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