More than pretty images – Towards confidence bounds on segmentation thresholds

International audienceWe present an approach to assess the uncertainty associated to classifying voxels into different material phases. The approach consists in a spectral deconvolution of the grey-level histogram using a Gaussian mixture approach, followed by an iterative classification procedure based on the occurrence frequency. As phase attributions become increasingly more uncertain as iterations proceed, confidence bounds on the final segmentation are naturally obtained

Laboratory monitoring of P-waves in partially saturated sand

International audienceEnergy dissipation is observed on seismic data when a wave propagates through a porous medium, involving different frequency regimes depending on the nature of rock and fluid types. We focus here on the role of partial fluid saturation in unconsolidated porous media, looking in particular at P-wave phase velocity and attenuation. The study consists in running an experiment in a sand-filled tank partially saturated with water. Seismic propagation in the tank is generated in the kHz range by hitting a steel ball on a granite plate. Seismic data are recorded by buried accelerometers and injecting or extracting water controls the partial saturation. Several imbibition/drainage cycles were performed between the water and gas residual saturations. A Continuous Wavelet Transform applied on seismic records allowed us to extract the direct P-wave at each receiver.We observe an hysteresis in phase velocities and inverse quality factors between imbibition and drainage. Phase velocities and inverse quality factors are then jointly inverted to get a final poro-visco-elastic model of the partially saturated sand that satisfactorily reproduces the data. The model formulation consists in generalizing the Biot theory to effective properties of the fluid and medium (permeability and bulk modulus) in order to properly explain the phase velocity variation as a function of the saturation. The strong level of attenuation measured experimentally is further explained by an anelastic effect due to grain to grain sliding, adding to Biot's losses. This study shows that fluid distribution at microscopic scale has strong influence on the attenuation of direct P-waves at macroscopic scale and confirms that seismic prospection may be a powerful tool for the characterization of transport phenomena in porous media

Potential of X-ray tomography for the exploration of vegetal concretes’ porous structure

International audienceVegetal concretes offer promising perspectives as building materials thanks to their low environmental impact and interesting hygrothermal behavior. Their insulating properties and moisture buffering capacity are largely controlled by their microstructure. In this context, X-ray tomography is a promising technique as it enables inner inspection of the microstructure in three dimensions in a non-destructive fashion. Geometrical characteristics, such as the overall porosity and the aggregate volume ratio, can be quantified within the limits of the accuracy of the technique. The present study focuses on a vegetal concrete obtained by combining a metakaolin-based pozzolanic binder with sunflower bark chips. Our measurements show that the vegetal concrete exhibits an open and interconnected pore space, with pore sizes varying over multiple orders of magnitude. The arrangement of the particles is clearly affected by their elongated shape in combination with the compacting force applied during settlement. The material also exhibits shrinkage-induced cracks at the interface between bio-aggregates and binder paste. These observations demonstrate the potential and limitations of X-ray tomography applied to bio-based concretes. The three-dimensional datasets yield more insight compared to typical two-dimensional digital imaging methods such as SEM. Moreover, the non-destructive nature of the method could allow to monitor ageing mechanisms of bio-based concretes by scanning the same sample at different points in time. Similar to other techniques such as SEM, high-resolution tomographic scans can only be obtained on relatively small samples, which raises the question about the representativeness of the specimens in view of the high heterogeneity of vegetal concretes. Nevertheless, the non-destructive nature of the method enables performing multiple scans of different locations within the same sample and to combine the data. X-ray tomography is thus a powerful tool, which can easily be combined with other techniques and contribute to characterize the microstructure of vegetal concretes

Creating innovative composite materials to enhance the kinetics of CO 2 capture by hydroquinone clathrates

This study addresses both the preparation of a reactive medium composed of porous particles impregnated with hydroquinone (HQ), an organic compound capable of forming gas clathrates, and an evaluation of the kinetic performance of these composite materials for CO2 capture. Two types of porous silica particles of different sizes and pore diameters were tested. The porous particles were impregnated with HQ by a dry impregnation (DI) method in a fluidized bed, and by a wet impregnation (WI) method. The impregnation effectiveness of the two methods is discussed, and the reactivity of the composite materials formed in terms of CO2 capture and storage capacity is studied experimentally. The experimental results showed that the HQ adheres well on the silica without any chemical modification of the deposit’s structure. We demonstrated that the impregnation technique plays a very important role in the kinetics of CO2 capture. A series of experiments performed using a magnetic suspension balance at 3.0 MPa and 323 K showed that the silica-based impregnated particles reversibly capture and store CO2, and that the CO2 capture kinetics are significantly enhanced compared to the results obtained with pure powdered HQ. Finally, we demonstrated that CO2 capture is faster with dry-impregnated particles

La politique d'intégration et de lutte contre les discriminations en Bretagne

Le soutien pour l'intégration et la lutte contre les discriminations sont affaires d'État et le Fasild, né il y a cinquante ans, constitue l'une des pierres angulaires de cette politique actualisée en janvier 2005. Les dispositifs se renforcent donc et se diversifient. En Bretagne, grâce à différentes enquêtes de terrain, les actions et partenariats se multiplient, inscrits dans de nombreux programmes nationaux, régionaux, départementaux qui tous attestent d'une réelle volonté d'action.Petit-Sénéchal Pascale. La politique d'intégration et de lutte contre les discriminations en Bretagne. In: Hommes et Migrations, n°1260, Mars-avril 2006. Bretagne : Terre d’immigration. pp. 50-62

Une nouvelle approche de l'immigration bretonne

Etiemble Angélina, Petit-Sénéchal Pascale. Une nouvelle approche de l'immigration bretonne. In: Hommes et Migrations, n°1260, Mars-avril 2006. Bretagne : Terre d’immigration. pp. 6-7

Feasibility of acoustic imaging for in-situ characterization of subsurface soil injected with fresh mortar

International audienceThis paper investigates the potential of acoustic imaging technologies to determine the spatial geometry of the soil-fresh mortar interface (water-cement ratio > 2). The proposed study is first devoted to the characterization and monitoring of the acoustic properties of this evolving material. They were analysed in the laboratory using a specifically-designed experimental device for eight hours, in order to assess the evolution of the mixed material as a function of its maturity. The measured seismic dataset shows the very low P wave velocities and their evolution as a function of time (hardening), their large attenuation and low dispersion characteristics in the [0.5-6 kHz] frequency range. In particular, seismic attenuation remains the lowest when mortar mixture is younger than 90 min, which consequently constitutes the optimum time window for imagery purposes. A comparison between the measured properties and existing theoretical models shows that the material can be considered as a fluid mixture with solid particles in suspension, which exhibits poorly compressible elastic behaviour during the first 8 h. In a second part, numerical seismograms are generated to assess the reflected P wave properties as a function of offset for a mortar/soil interface. Such study permitted to establish the optimum source-receiver configuration, which results from a compromise between acquiring large amplitudes for signal to noise ratio issues (reflectivity, attenuation), while to correctly analyse the P wave velocity using the hyperbola of the reflected wave. A correct assessment of the in-situ velocity, which depends on both the mixture characteristics (i.e. the soil conditions) and the mortar maturity, is a necessary step towards a correct 2D image of the interface

First investigation of quartz and calcite shape fabrics in strained shales by means of X-ray tomography

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First investigation of quartz and calcite shape fabrics in strained shales by means of X-ray tomography

International audienceWe document the evolution of the 3D fabric of shale along a km-long strain gradient in the Jaca basin (Southern Pyrenees, Spain). With respect to the distance from a thrust, samples were collected in the cleavage-free domain, at the onset of the pencil-cleavage domain, within the pencil-cleavage domain and within the slaty-cleavage domain. By combining high resolution X-ray computed tomography (XCT) and energy dispersive X-ray spectroscopy (EDS), the morphology and the Shape Preferred Orientation (SPO) of thousands of quartz grains, calcite grains and pores was studied. In the least deformed samples, quartz and calcite display mean foliation parallel to bedding with comparable dispersion. In the pencil-cleavage domain, quartz foliation still follows the bedding while calcite foliation is mostly governed by cleavage. In the slaty-cleavage domain, calcite shows a much better organization, with foliation parallel to cleavage, mimicking closely the pore fabric. By contrast, the quartz shape fabric is much less defined, scattered between bedding and cleavage planes. This suggests that quartz grain act as rigid marker in the ductile matrix, while calcite grain orientation is governed by dissolution-precipitation processes