216 research outputs found
Stability of Monitoring Weak Changes in Multiply Scattering Media with Ambient Noise Correlation: Laboratory Experiments
Previous studies have shown that small changes can be monitored in a
scattering medium by observing phase shifts in the coda. Passive monitoring of
weak changes through ambient noise correlation has already been applied to
seismology, acoustics and engineering. Usually, this is done under the
assumption that a properly reconstructed Green function as well as stable
background noise sources are necessary. In order to further develop this
monitoring technique, a laboratory experiment was performed in the 2.5MHz range
in a gel with scattering inclusions, comparing an active (pulse-echo) form of
monitoring to a passive (correlation) one. Present results show that
temperature changes in the medium can be observed even if the Green function
(GF) of the medium is not reconstructed. Moreover, this article establishes
that the GF reconstruction in the correlations is not a necessary condition:
the only condition to monitoring with correlation (passive experiment) is the
relative stability of the background noise structure
Joint inversion of P-wave velocity and density, application to La SoufriĂšre of Guadeloupe hydrothermal system
International audienceWe present the result of a 3-D gravity and P-wave traveltime joint inversion applied to the hydrothermal system of La Soufri'ere of Guadeloupe. The joint inversion process is used here to overcome the different resolution limitations attached to the two data sets. P-wave traveltimes were obtained from three active seismic surveys that were conducted from 2001 to 2007. Gravity data collected during a microgravity campaign is described in a companion paper. We use a joint inversion process based on a Bayesian formulation and a deterministic iterative approach. The coupling between slowness and density is introduced through a supplementary constraint in the misfit function that tries to minimize the distance between parameter values and a theoretical relationship. This relationship is derived from measurements on samples representative of Mt Pel'ee of Martinique and La Soufri'ere volcanoes. We chose a grid discretization that leads to an under-determined problem that we regularize using spatial exponential covariance between the nodes parameters. Our results are compared to geophysical electromagnetic results obtained using resistivity and VLF surveys. They confirm the presence of highly contrasted dense/fast and light/slow zones in La Soufri'ere dome and crater basement. Our images suggest however that some non-conductive zones may be massive andesite bodies rather than argilized zones, and that these bodies may have deeper roots than hypothesized
Modelling Seismic Wave Propagation for Geophysical Imaging
International audienceThe Earth is an heterogeneous complex media from the mineral composition scale (10â6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (GuÌnther et al., 2006; RuÌcker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields
The High-Resolution Imaging (HRI) Portable Array: A Seismic (and Internet) Network Dedicated to Kilometric-scale Seismic Imaging
International audienceWe have developed a network of portable seismic stations dedicated to the high resolution imaging of geological, potentially hazardous, targets. These targets - volcanoes, fault zones, landslide areas - are characterized by strong medium heterogeneities, rugged topography, rough field conditions, and require dedicated equipment in order to maximize the number of recording points. This new network is designed to a) operate experiments with a limited size crew, b) run on low power for possible use in remote areas and difficult conditions, c) record both active and passive seismic sources. The actual network consists of 30 clusters of 9 channels digital acquisition system (DAS), equipped with 6 vertical sensors plus 1 three-component sensor. Each DAS uses Ethernet and 802.11 (WiFi) connections that permit to a single operator to remotely control the entire network. We present the main characteristics of this new portable array, describe the calibration method developed for our sensors and show examples of configuration and recordings for two recent experiments
Unprecedented seismic swarm in the Maurienne valley (2017â2019) observed by the SISmalp Alpine seismic network: operational monitoring and management
Toward Forecasting Volcanic Eruptions using Seismic Noise
During inter-eruption periods, magma pressurization yields subtle changes of
the elastic properties of volcanic edifices. We use the reproducibility
properties of the ambient seismic noise recorded on the Piton de la Fournaise
volcano to measure relative seismic velocity variations of less than 0.1 % with
a temporal resolution of one day. Our results show that five studied volcanic
eruptions were preceded by clearly detectable seismic velocity decreases within
the zone of magma injection. These precursors reflect the edifice dilatation
induced by magma pressurization and can be useful indicators to improve the
forecasting of volcanic eruptions.Comment: Supplementary information:
http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary
video:
http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av
Th1 Disabled Function in Response to TLR4 Stimulation of Monocyte-Derived DC from Patients Chronically-Infected by Hepatitis C Virus
Background: Lack of protective antibodies and inefficient cytotoxic responses are characteristics of chronic hepatitis C infection. A defect in dendritic cell (DC) function has thus been suspected, but this remains a controversial issue. Methods and Findings: Here we show that monocyte-derived DC (MoDC) from chronically-infected patients can mature in response to TLR1/2, TLR2/6 or TLR3 ligands. In contrast, when stimulated with the TLR4 ligand LPS, MoDC from patients show a profound defect in inducing IFNc secretion by allogeneic T cells. This defect is not due to defective phenotypic maturation or to the presence of HCV-RNA in DC or monocytes but is correlated to reduced IL-12 secretion by DC. Restoration of DC ability to stimulate IFNc secretion can be obtained by blocking MEK activation in DC, indicating that MEK/ ERK pathway is involved in the Th1 defect of MoDC. Monocytes from HCV patients present increased spontaneous secretion of cytokines and chemokines, especially MIP-1b. Addition of MIP-1b on healthy monocytes during differentiation results in DC that have Th1 defect characteristic of MoDC from HCV patients, suggesting that MIP-1b secretion by HCV monocytes participates in the Th1 defect of DC. Conclusions: Our data indicate that monocytes from HCV patients are activated in vivo. This interferes with their differentiation into DC, leading to deficient TLR4 signaling in these cells that are enable to induce a Th1 response. Thi
Prognostic interaction between expression of p53 and estrogen receptor in patients with node-negative breast cancer: results from IBCSG Trials VIII and IX
Diffraction d'ondes elastiques par des milieux heterogenes : simulations numeriques et inversion de coda
SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Les enjeux des territoires de sante
International audienceLa RGPP (RĂ©vision GĂ©nĂ©rale des Politiques Publiques) est un programme en cours d'une grande portĂ©e pour la rĂ©forme de l'Ătat français. Cette rĂ©forme est formulĂ©e dans le langage du management en termes d'objectifs, d'indicateurs de performance. Un aspect clĂ© de la RGPP rĂ©side dans la refondation de l'Ătat « territorial ». Il existe un lien entre la RGPP et la rĂ©forme de la dĂ©centralisation territoriale issue de la loi du 16-12-2010. Sans doute pour la premiĂšre fois depuis les lois Defferre de 1982, la rĂ©forme des gouvernements locaux et rĂ©gionaux français est justifiĂ©e en termes de discours global sur la productivitĂ© (plutĂŽt que sur le registre plus usitĂ© de la proximitĂ© et de la dĂ©mocratie). En ce sens, aprĂšs l'entrĂ©e en vigueur progressive d'une nouvelle rĂ©forme des collectivitĂ©s territoriales en 2010, il s'agit d'analyser l'impact de la RGPP sur l'administration territoriale de l'Ătat et son incidence sur les collectivitĂ©s locales, oeuvre rarement tentĂ©e de croiser les influences croisĂ©es de deux rĂ©formes territoriales d'envergure. Le Centre de Recherche sur la DĂ©centralisation Territoriale (CRDT - EA 3312) de l'UniversitĂ© de Reims Champagne-Ardenne et le Groupement de Recherches sur l'Administration Locale en Europe (GIS GRALE CNRS) ont organisĂ© un colloque international sur la RGPP et la rĂ©forme des collectivitĂ©s territoriales Ă l'UniversitĂ© de Reims Champagne-Ardenne les 25 et 26 janvier 2012, dont cet ouvrage rend compte et prolonge les actes du colloque « quelle nouvelle rĂ©forme pour les collectivitĂ©s territoriales françaises ? » parus dans cette collection en 201
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