Propagation, Imagerie et Monitoring Acoustiques : Développements Méthodologiques et Expérimentaux pour des Systèmes Complexes en Géosciences

Les ondes acoustiques transportent de l'information relative aux propriétés physiques du milieu de propagation et peuvent ainsi contribuer à l'imagerie de la structure interne et au monitoring de la dynamique de systèmes complexes tels que les fonds marins, les massifs rocheux et les systèmes volcaniques hydrothermaux. La synthèse des travaux de recherche présentés dans ce mémoire s'inscrit dans cette approche globale : de nouvelles méthodes d'analyse sismique multiéchelle par réponse en ondelettes et de simulation de propagation sont mises au point pour l'étude de milieux granulaires immergés ; par déstabilisation gravitaire, les émissions acoustiques induites par réarrangements des grains peuvent être exploitées comme précurseurs et signatures du régime d'avalanche ; l'analyse sismique multiéchelle permet de proposer de nouveaux attributs sismiques tenant compte de sources large bande à support fréquentiel limité et d'aborder la fusion de données multicapteurs dans le formalisme des ondelettes ; des sondages sismiques associés à l'identification de microsismicité induite dans un massif argileux en cours d'excavation renseignent sur l'initiation de la zone endommagée par redistribution des contraintes géomécaniques. Ces travaux de recherche, dont le contexte et les publications scientifiques associées introduisent ce mémoire, donnent lieu à des perspectives de valorisation illustrées en dernière partie

Microseismicity Induced in the Opalinus Clay by a Gallery Excavation in the Mont Terri Underground Rock Laboratory

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High-resolution seismic imaging in deep sea from a joint deep-towed/OBH reflection experiment : application to a Mass Transport Complex offshore Nigeria

International audienceWe assess the feasibility of high-resolution seismic depth imaging in deep water based on a new geophysical approach involving the joint use of a deep-towed seismic device (SYSIF) and ocean bottom hydrophones (OBHs). Source signature measurement enables signature deconvolution to be used to improve the vertical resolution and signal-to-noise ratio. The source signature was also used to precisely determine direct traveltimes that were inverted to relocate source and receiver positions. The very high accuracy of the positioning that was obtained enabled depth imaging and a stack of the OBH data to be performed. The determination of the P-wave velocity distribution was realized by the adaptation of an iterative focusing approach to the specific acquisition geometry. This innovative experiment combined with advanced processing succeeded in reaching lateral and vertical resolution (2.5 and 1 m) in accordance with the objectives of imaging fine scale structures and correlation with in situ measurements. To illustrate the technological and processing advances of the approach, we present a first application performed during the ERIG3D cruise offshore Nigeria with the seismic data acquired over NG1, a buried Mass Transport Complex (MTC) interpreted as a debris flow by conventional data. Evidence for a slide nature of a part of the MTC was provided by the high resolution of the OBH depth images. Rigid behaviour may be inferred from movement of coherent material inside the MTC and thrust structures at the base of the MTC. Furthermore, a silt layer that was disrupted during emplacement but has maintained its stratigraphic position supports a short transport distance

Multiscale analysis of waves reflected by complex interfaces: Basic principles and experiments

International audience[1] This paper considers the reflection of waves by multiscale interfaces in the framework of the wavelet transform. First, we show how the wavelet transform is efficient to detect and characterize abrupt changes present in a signal. Locally homogeneous abrupt changes have conspicuous cone-like signatures in the wavelet transform from which their regularity may be obtained. Multiscale clusters of nearby singularities produce a hierarchical arrangement of conical patterns where the multiscale structure of the cluster may be identified. Second, the wavelet response is introduced as a natural extension of the wavelet transform when the signal to be analyzed (i.e., the velocity structure of the medium) can only be remotely probed by propagating wavelets into the medium instead of being directly convolved as in the wavelet transform. The reflected waves produced by the incident wavelets onto the reflectors present in the medium constitute the wavelet response. We show that both transforms are equivalent when multiple scattering is neglected and that cone-like features and ridge functions can be recognized in the wavelet response as well. Experimental applications of the acoustical wavelet response show how useful information can be obtained about remote multiscale reflectors. A first experiment implements the synthetic cases discussed before and concerns the characterization of planar reflectors with finite thicknesses. Another experiment concerns the multiscale characterization of a complex interface constituted by the surface of a layer of monodisperse glass beads immersed in water. Citation: Le Gonidec, Y., D. Gibert, and J.-N. Proust , Multiscale analysis of waves reflected by complex interfaces: Basic principles and experiments

Gaussian derivative wavelet propagation in a single scattering bubbly water beyond resonance frequency

Acoustic pulses transmitted across air bubbles in water are usually analyzed in the ν frequency domain to determine attenuation and phase velocity for comparison with effective models. In the present work, acoustic experiments performed beyond the bubble resonance frequency highlight an attenuation approximated to a(ν/ν0)^(-y) with y>0 and ν0 the wavelet source peak frequency, a significant shape variability of the waveform with the propagation distance x<0.74m, and a nearly constant normalized amplitude spectrum. The amplitude spectrum can be characterized by a fractional derivative order γ_x=κ_γ x, with κ_γ a constant defined by a numerical optimization method, and a time dilation factor δ_x. The phase spectrum is shifted by πγ_x/2 and explains the experimental waveform changes. The phase velocity can be approximated to ṽ0 /(1 − κ_γ ṽ0 /4ν), where ṽ0 is similar to the sound speed in water. It is also shown that the waveform shape, quantified by γ_x , is correlated to the waveform amplitude. With the assumption ayx<0.15, the range of applicability may be extended to the ocean water column around gas seep sites with potential interests in underwater communication

Multiscale seismic attributes: source-corrected wavelet response and application to high-resolution seismic data

International audienceA wavelet-based method was presented in a previous work to introduce multiscale seismic attributes for high-resolution seismic data. Because of the limited frequency bandwidth of the seismic source, we observed distortions in the seismic attributes based on the wavelet response of the subsurface discontinuities (Le Gonidec et al.). In this paper, we go further in the seismic source-correction by considering Lévy alpha-stable distributions introduced in the formalism of the continuous wavelet transform (CWT). The wavelets are Gaussian derivative functions (GDF), characterized by a derivative order. We show that a high-resolution seismic source, after a classical signature processing, can be taken into account with a GDF. We demonstrate that in the framework of the Born approximation, the CWT of a seismic trace involving such a finite frequency bandwidth can be made equivalent to the CWT of the impulse response of the subsurface and is defined for a reduced range of dilations. We apply the method for the SYSIF seismic device (Marsset et al.; Ker et al.) and show that the source-corrections allow to define seismic attributes for layer thicknesses in the range [24; 115 cm]. We present the analysis for two seismic reflectors identified on a SYSIF profile, and we show that the source-corrected multiscale analysis quantifies their complex geometries

Milieux granulaires: Simulations numériques et expérimentations en cuve acoustique

International audienceWe are interested in transmitted or refracted field by a granular medium, where wavelength λ and size of the heterogeneities are quasi equal. In this case, the wave regime is no longer propagative but becomes diffusive, with multiple scattering. Experimentally, if we measure on several points the field, the wave "sees" a medium differently organized. Nevertheless, a common information for all the measures should exist since the studied medium is globally the same. Acoustic experiments upon diluted media (grains inserted in a gel) are then carried out, together with numerical modeling based on Mie theory (at the local scale of the grain). The analysis of the refracted field by the different heterogeneities is done by extracting the modulation laws via the wavelet transform. The analysis brings out characteristic parameters which allow us, by playing on the ratio longueur d'onde λ / mean free path l, to tend towards a valid model for dense media. Experiments are carried out in reflection on sand. The numerical results are then validated.Nous nous intéressons au champ transmis ou réfracté par un milieu granulaire, où longueur d'onde λl et taille des hétérogénéités sont du même ordre de grandeur. Dans ce cas, le régime de l'onde n est plus propagatif mais devient diffusif avec présence de multiple diffuseurs. Expérimentalement, si nous mesurons en plusieurs points le champ résultant, l'onde "voit" un milieu organisé différemment. Cependant, une information commune à toutes les mesures doit pourtant exister car le milieu étude est globalement le même. Des expériences acoustiques sur des milieux dilués (grains insérés dans un gel) sont donc réalisées, conjointement à une modélisation numérique basée sur la théorie de Mie (à l’échelle locale du grain). L'analyse du champ réfracté par les différentes hétérogénéités se fait par extraction des lois de modulation via la transformée en ondelettes. L'analyse fait ressortir des paramètres caractéristiques qui nous permettent en jouant sur le rapport λ/ libre parcours moyen l, de tendre vers un modèle valable pour les milieux denses. Des expériences sont réalisées en réflexion sur du sable. Les résultats numériques correspondant sont alors validés