43 research outputs found

    New robust observables on Rayleigh waves affected by an underground cavity: from numerical to experimental modelling

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    International audienceThe investigation and monitoring of shallow hazards due to the presence of underground cavities remain a challenge for geophysical approaches. Thus, seismic surface waves have been tested in several recent research projects in order to detect and localize voids as well as to determine their geometries. Among these works, numerous numerical studies have proved the feasibility of Rayleigh waves to detect cavities. However, most imagery processes adapted to R waves are faced with difficulties when applying them to real data. This limitation points to a major problem: the interactions between Rayleigh waves and a cavity are complex, particularly in the case of dispersing and attenuating surrounding media. Here, a combined approach based on numerical and experimental data obtained in a reduced-scale measurement bench is conducted to better understand the seismic wave propagation phenomena involved in the presence of a cavity and define robust observables that can be used in field measurements. The observables bearing the cavity signature are studied qualitatively and quantitatively on numerical and experimental recordings. The latter take into account all the propagation phenomena involved. The observations are carried out on the vertical and horizontal component of the Rayleigh wave displacement. The selected observables are studied depending on non-dimensional cavity's parameters versus the frequency, that is the wavelength-to-size ratio and the wavelength-to-depth ratio. The effects of the cavity's parameters on the observables show particularities as a function of these components, such as a higher rate of the amplitude on the horizontal component as well as a perturbation of the direct seismic surface wave amplitude above the cavity, also higher on the horizontal component. This latter feature is particularly visible on the variation of the elliptical particle motion recorded at the surface. It can be linked to the mode conversions that occur in the vicinity of the cavity and which predominate on the horizontal component when the signal is normalized

    Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/GĂ©ofrance 3D Program) Imagerie sismique de la zone interne de la chaĂźne hercynienne sud-armoricaine (projet Armor 2/programme GĂ©ofrance 3D)

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    International audienceWe present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning. Nous prĂ©sentons les rĂ©sultats et l'interprĂ©tation d'un profil de sismique rĂ©flexion en Ă©coute longue, de 72 km de long Ă  travers les zones internes de la chaĂźne hercynienne sud-armoricaine. Le profil est d'excellente qualitĂ©, avec une forte rĂ©flectivitĂ© Ă  travers toute la croĂ»te. Le profil, « ARMOR 2 Sud », est corrĂ©lĂ© avec le profil « ARMOR 2 Nord » publiĂ© en 2003. Les corrĂ©lations des rĂ©flexions les plus superficielles avec les donnĂ©es gĂ©ologiques et structurales de surface fournissent d'importantes informations sur la structure crustale qui rĂ©sulte d'un Ă©paississement dĂ©butant au DĂ©vonien supĂ©rieur et d'une extension rĂ©gionale au CarbonifĂšre supĂ©rieur. En particulier, la sismique met en Ă©vidence une zone sub-horizontale Ă  trĂšs forte rĂ©flectivitĂ©, de plus de 40 km de long Ă  environ 10–12 km de profondeur. Cette zone est interprĂ©tĂ©e comme une zone d'amincissement crustal majeur

    Design, Construction and In Situ Testing of a Muon Camera for Earth Science and Civil Engineering Applications

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    The MUST2 (MUon Survey Tomography based on Micromegas detectors for Unreachable Sites Technology) camera is based on a thin Time Projection Chamber read by a resistive Micromegas. This innovative combination presents interesting distinctive features compared to existing muon detection technologies. It allows a wide angular acceptance of the detector with a low weight and compact volume, well adapted for confined spaces or underground operation. The current work presents the results obtained during the calibration measurements at the reference site, the Low Background Noise Laboratory (LBNL). Preliminary results from field measurement campaign carried out at the dam overlooking the village of Saint-Saturnin-les-Apt (South-East of France) are presented and discussed

    Détail d'une structure régionale majeure par méthodes géophysiques combinées - cas de la faille de Fécamp-Lillebonne (Seine-Maritime, France)

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    International audienceLa faille de Lillebonne-FĂ©camp est une faille normale-dĂ©crochante dextre (Hauchard et Laignel, 2008 ; Wazi, 1988), orientĂ©e N150° (orientation armoricaine) Ă  regard vers l'ouest, Ă  rejet vertical variable (150 m dans le secteur de Bolbec, 40 m dans le secteur de Lillebonne). Il s'agit d'une structure gĂ©ologique rĂ©gionale majeure qui traverse le Pays de Caux sur plus de 25 km et se prolonge en Manche au large de FĂ©camp en prenant progressivement une orientation E-O. Au sud, cette structure n'atteindrait pas la Seine et viendrait buter contre l'accident de Notre-Dame-de-Gravenchon-Villequier, orientĂ© N060° (orientation varisque). A l'Ă©chelle du Bassin de Paris, cette structure s'inscrit dans la continuitĂ© des failles de la Seine, de Rambouillet et d'Etampes, et prĂ©sente la mĂȘme orientation que la structure du Bray. Dans le paysage structural du Pays de Caux, la faille de Lillebonne-FĂ©camp est Ă©galement associĂ©e Ă  de larges structures plicatives Ă  faible pendage, globalement parallĂšles Ă  la faille (Ragot, 1988). En 2017, des investigations gĂ©ophysiques ont Ă©tĂ© rĂ©alisĂ©es par le BRGM dans le secteur de Bolbec pour amĂ©liorer les connaissances gĂ©ologiques et structurales dans la zone. L'interprĂ©tation et la corrĂ©lation conjointe des profils gĂ©o-Ă©lectriques, sismiques et de la carte gravimĂ©trique a permis de confirmer la thĂšse de Ragot sur la gĂ©omĂ©trie en faisceau de la faille, et de proposer une nouvelle carte structurale pour le secteur de Bolbec

    Determination des structures et des vitesses dans la croute par migrations des donnees de sismique reflexion profonde ECORS

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    SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : TD 83471 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

    Karst investigations using microgravity and MASW; Application to Orléans, France

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    International audienceAn integrated geophysical approach to detecting and characterizing karst structures in an urban environment was applied experimentally to partially explored karst conduits located in OrlĂ©ans, France. Microgravity was performed to detect voids, in conjunction with multichannel analysis of surface waves (MASW) for the purpose of identifying areas of mechanical weakness.Microgravity detected negative anomalies corresponding to known conduits and succeeded in identifying the probable extensions of this network in unexplored areas. Control boreholes located on these extensions encountered several levels of water‐saturated voids, probably belonging to the shallowest part of the karst system, overlying the main conduits. Buried urban networks, accurately located by ground‐penetrating radar (GPR), were shown to have no significant gravity effect. Simulations using the compact inversion approach to characterize the size and density of environmental disturbances confirmed this conclusion. In this context, the gravity method has been shown to be suitable for detecting near‐surface (<25 m deep) karst features.The MASW method, which analyses Rayleigh‐wave propagation, can determine the mechanical behaviour of superficial formations and serve as an indicator for subsurface heterogeneities such as voids or fractures. At the OrlĂ©ans site, MASW provided evidence of perturbed zones superimposed on gravity anomalies, characterized by the appearance of several dispersion modes, velocity inversions and the attenuation of seismic markers. One of these features was characterized by low velocities and was interpreted as an area of mechanical weakness, confirmed by pressure measurements in the boreholes.Repeated gravity measurements, or time‐lapse microgravity, were conducted on the anomalous areas to ascertain gravity reproducibility and detect possible temporal variations due to subsurface mass redistribution that may indicate site instability. A two‐year experiment revealed low‐amplitude gravity changes that were recorded in the two sensitive zones. However, their significance is still unclear and these changes need to be validated by further repeat experiments

    A new structural cooperative inversion approach applied to the combined tomography of electrical resistivity and seismic refraction travel times.

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    International audienceWe present a new cooperative inversion approach that allows for a combined inversion of independent physical parameters by exchanging structural information. The technique is based on an iterative cluster analysis step using a Fuzzy c-Mean technique (FCM). Cluster analysis aims at identifying groups of similar objects, and helps to discover distribution of patterns and interesting correlations in data sets. An occurring specific pattern of one parameter facilitates the development of coherent structure in the other using a reference model term in the least square solution associated to the linearized minimization of the cost function problem. In the presence of structures that can be seen by both methods it leads to sharp contrasted models. The technique is applied to the inversion of electrical resistivity and seismic refraction travel times. Two synthetic co-located data sets show how different structures are resolved with and without structural cooperative inversion. It is discussed how the quality of the inversion results is improved by the new approach
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