Multiconfiguration GPR measurements for geometric fracture characterization in limestone cliffs (Alps)

Until now, geophysical methods have been rarely used to investigate vertical limestone cliffs, mainly due to the extreme conditions for data acquisition. Nevertheless, these techniques are the only available methods which could provide information on the internal state or a rock mass in terms of discontinuities, which play a major role in rock-fall hazards. In this case study, detailed GPR measurements were carried out on a test site with different acquisition configurations deployed on vertical cliff faces. Conventional 2D profiles, common midpoints (CMP) and transmission data were acquired to evaluate the potential of radar waves to improve the characterization of the geometry and properties of the main discontinuities (fractures) within the massif. The results show that the 3D geometry of fractures, which is a crucial parameter for stability assessment, can be retrieved by combining vertical and horizontal profiles performed along the cliff. CMP profiles acquired along the cliff allow a velocity profile to be obtained as a function of depth. Finally, transmission experiments, which generate complex radargrams, have provided valuable and quantitative information on the rock mass, through the modelling of the waves generated. On the other hand, a velocity tomography obtained from the first arrivals travelling through the rock mass from the transmitters to the receivers, shows an image of the investigated zone with a poor resolution

Parametrization of reflectometry fluctuation frequency spectra for systematic study of tokamak fusion plasma turbulence

We describe a way to parameterize power spectra extracted from fixed-frequency reflectometry data, with a view to systematic studies of turbulence properties in tokamak plasmas. Analysis of typical frequency spectra obtained from a new database suggests decomposition in a set of four key components: the direct current component, low-frequency fluctuations, broadband (BB) turbulence, and the noise level. For the decomposition in the identified components, different kinds of functions are tested and their fitting performance is analyzed to determine the optimal spectrum parametrization. In particular, for the BB turbulence, three models are compared qualitatively based on a number of representative spectrum test cases, notably the generalized Gaussian, the Voigt, and the Taylor model. In addition, quantitative performance testing is accomplished using the weighted residual sum of squares and the Bayesian information criterion in a large database including 350 000 spectra obtained in Tore Supra. Next, parametrization by the Taylor model is applied to Ohmically heated plasmas, and a BB energy basin is systematically observed in the core plasma region, which shrinks with decreasing radial position of the q = 1 surface. This basin might be explained by a drop of the density fluctuation level inside the q = 1 surface

Multiscale oil slicks segmentation

This study focuses on the segmentation and characterization of oil slicks from Synthetic Aperture Radar (SAR) data. Viscosity notably reduces the roughness of the sea surface which takes a major part in the backscattering. Hence, an oil slick is characterized by a low-backscattered energy and appears as a dark area in images. This is the reason why most of detection algorithms are based on histogram thresholding, but they appear not satisfactory as the number of false alarms is generally high. Since oil slicks have specific impact on ocean wave spectra (from gravity-capillary waves to the swell), we propose to use a Markovian model adapted to a multiscale description of the original image. This unsupervised segmentation method allows to take into account the different states of the sea surface through its spectra. Thanks to the mixture estimation, it is possible to statistically characterize the detected areas and then to prevent from most of false alarms. Results of segmentation are shown with two types of scenarios. The first one concerns oil spill in the Mediterranean sea detected by the ERS SAR sensor at a resolution of 25 m. The second scenario is related to the Prestige’s wreck acquired by the Envisat ASAR sensor in a wide swath mode at a resolution of 150 m.Nous nous intéressons à la détection et à la caractérisation des nappes d'hydrocarbure à partir d'images Radar à Synthèse d'Ouverture (RSO). La viscosité de l'hydrocarbure atténue sensiblement la rugosité de surface qui participe majoritairement à la rétro-diffusion. En conséquence, un film visqueux est caractérisé par un déficit d'énergie rétro-diffusée et apparaît comme une zone sombre dans les images. Cependant, la plupart des techniques de détection, basée sur un seuillage d'histogramme, s'avère insatisfaisante, puisqu'elle engendre un nombre élevé de fausses alarmes. En considérant le fait qu'un film visqueux a un impact caractéristique sur la répartition de l'énergie des vagues selon les différentes longueurs d'onde (des vagues de gravité-capillarité jusqu'à la houle), nous avons développé une méthode de segmentation markovienne adaptée à une représentation multiéchelle de l'image originale. Cette méthode permet d'obtenir une classification qui tient compte des différents états du spectre de vagues. Grâce à l'estimation des lois intervenant dans le mélange, cette méthode de segmentation permet de caractériser statistiquement les zones détectées et ainsi de se prémunir contre de nombreuses fausses alarmes. Cette stratégie a été appliquée avec succès à des images RSO de différentes résolutions (ERS-SAR en mode PRI à 25 m et ENVISAT-ASAR en mode Wide Swath à 150 m de résolution) correspondant à des situations particulières comme le naufrage d'un navire avec un hydrocarbure lourd en Atlantique et le dégazage de cuves avec un hydrocarbure plus fluide en Méditerranée

L'usinage ultrasonic : cas d'application au bois

Malgré l'accroissement de l'utilisation des matériaux plastiques dans la fabrication des produits de consommation courante, le bois progresse toujours. Aussi, il faut en permanence faire évoluer les techniques de travail du bois, et en particulier, l'usinage, devenu un enjeu majeur dans cette industrie transformatrice du bois. La qualité des surfaces usinées est complexe à maîtriser car fonctionnellement précise et déterminante pour les opérations de finition. Par exemple, l'apparition de produits aqueux à faible impact environnemental (teinte) nécessite de mieux maîtriser les risques de soulèvement de fibres, non acceptables en terme d'aspect et de toucher. Ainsi de nombreuses recherches sont actuellement menées pour un meilleur compromis Qualité/Productivité. Nos travaux proposent une nouvelle piste, l'usinage ultrasonore rotatif. Le présent article nous permet tout d'abord de rappeler des notions nécessaires sur les ultrasons et sur leurs exploitations. Ensuite, nous allons décrire les conditions, les essais et les résultats de nos travaux sur l'usinage ultrasonore appliqués au bois

Tunable distributed sensing performance in Ca-based nanoparticle-doped optical fibers

Rayleigh scattering enhanced nanoparticle-doped optical fibers is a technology very promising for distributed sensing applications, however, it remains largely unexplored. This work demonstrates for the first time the possibility of tuning Rayleigh scattering and optical losses in Ca-based nanoparticle-doped silica optical fibers by controlling the kinetics of the re-nucleation process that nanoparticles undergo during fiber drawing by controlling preform feed, drawing speed and temperature. A 3D study by SEM, FIB-SEM and optical backscatter reflectometry (OBR) reveals an early-time kinetics at 1870 °C, with tunable Rayleigh scattering enhancement 43.2–47.4 dB, regarding a long-haul single mode fiber, SMF-28, and associated sensing lengths of 3–5.5 m. At 2065 °C, kinetics is slower and nanoparticle dissolution is favored. Consequently, enhanced scattering values of 24.9–26.9 dB/m and sensing lengths of 135–250 m are attained. Finally, thermal stability above 500 °C and tunable distributed temperature sensitivity are proved, from 18.6 pm/°C to 23.9 pm/°C, ∼1.9–2.4 times larger than in a SMF-28. These results show the promising future of Rayleigh scattering enhanced nanoparticle-doped optical fibers for distributed sensing

Lack of dietary polyunsaturated fatty acids causes synapse dysfunction in the drosophila visual system

Polyunsaturated fatty acids (PUFAs) are essential nutrients for animals and necessary for the normal functioning of the nervous system. A lack of PUFAs can result from the consumption of a deficient diet or genetic factors, which impact PUFA uptake and metabolism. Both can cause synaptic dysfunction, which is associated with numerous disorders. However, there is a knowledge gap linking these neuronal dysfunctions and their underlying molecular mechanisms. Because of its genetic manipulability and its easy, fast, and cheap breeding, Drosophila melanogaster has emerged as an excellent model organism for genetic screens, helping to identify the genetic bases of such events. As a first step towards the understanding of PUFA implications in Drosophila synaptic physiology we designed a breeding medium containing only very low amounts of PUFAs. We then used the fly’s visual system, a well-established model for studying signal transmission and neurological disorders, to measure the effects of a PUFA deficiency on synaptic function. Using both visual performance and eye electrophysiology, we found that PUFA deficiency strongly affected synaptic transmission in the fly’s visual system. These defects were rescued by diets containing omega-3 or omega-6 PUFAs alone or in combination. In summary, manipulating PUFA contents in the fly’s diet was powerful to investigate the role of these nutrients on the fly´s visual synaptic function. This study aims at showing how the first visual synapse of Drosophila can serve as a simple model to study the effects of PUFAs on synapse function. A similar approach could be further used to screen for genetic factors underlying the molecular mechanisms of synaptic dysfunctions associated with altered PUFA levels

Change detection in remote sensing observations

International audienceThe aim of this chapter is to draw an overview of change detection techniques in remote sensing earth observation. In remote sensing application, a change may be considered to as an alteration of the surface components. But temporal analysis of remote sensing image is facing several difficulties, among them the large amount of data to be processed and also the very few number of temporal observations. Despite the lack of temporal model, the natural evolution of landscape and the evolution induced by the sensors, many valuable techniques exist that perform change detection from two or more images acquired from the same or from different sensors. This overview has been drawn from the CACHANT initiative supported by the GdR ISIS in 2006 and the presentation that have been given durng the two meetings