Les placers à silice ultrapure français : étude gîtologique comparative

The comparative study of two French high-purity silica placers of European class suitable for the electrometallurgy of silicon and ferrosilicon, located at Boudeau and Thédirac, in the Dordogne and Lot departments respectively, indicates strong similarities as regards the sources and the transportation of quartz, as well as a same process of undercover karstification as a determining factor in the formation of the deposits. The main difference lies in the number of the stages necessary to the formation of the trap. This comparative study allows the constitution of dynamic models for the genesis of these economic quartz concentrations. To cite this article: L. Désindes et al., C. R. Geoscience 337 (2005).     

Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France

The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a (σ₁–σ₃) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean (σ₁–σ₃) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings.     

Experimental and numerical investigations of the behaviour of a heat exchanger pile

The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid‐filled pipes cast in concrete and then to building environments. To improve the knowledge in the field of geothermal structures, the behaviour of a pile subjected to thermo‐mechanical loads is studied in situ. The aim is to study the increased loads on pile due to thermal effects. The maximum thermal increment applied to the pile is on the order of 21°C and the mechanical load reached 1300 kN. Coupled multi‐physical finite element modelling is carried out to simulate the observed experimental results. It is shown that the numerical model is able to reproduce the most significant thermo‐mechanical effects. Copyright © 2006 John Wiley & Sons, Ltd.     

L'inclinométrie,un nouvel outil pour le suivi temporel des aquifères ?

This study explores the sensitivity of tilt measurements to hydrogeologic phenomena in order to use them to characterise aquifers and explain the signals observed on the instruments. The challenge is to separate the hydrogeological contribution from other phenomena that affect this type of measurements. First, we applied the theory propounded by Farell (1972) and Pagiatakis (1990) to simple hydrogeological cases. Then a simulation of the tiltmeter signal obtained in the Seine valley reveals that the order of magnitude is significant: the signal of a load with hydrologic origin therefore represents a significant part of any tiltmeter signal. Moreover, it can be interpreted in hydrogeological terms: it depends directly on the load gradient through a convolution formalism. Consequently, it is possible to imagine a use of the tiltmeter in hydrogeology to characterise, to a certain extent, mass transfer of water in the ground. To cite this article: T. Rerolle et al., C. R. Geoscience 338 (2006).     

Trends and climate evolution: Statistical approach for very high temperatures in France

The existence of an increasing trend in average temperatures during the last 50 years is widely acknowledged. Furthermore, there is compelling evidence of the variability of extremes, and rapid strides are made in studies of these events. Indeed, by extending the results of the “extreme value theory” (EVT) to the non-stationary case, analyses can examine the presence of trends in extreme values of stochastic processes. Definition of extreme events, their statistical significance as well as their interpretations have to be handled with great care when used for environmental concerns and public safety. Thus, we will discuss the validity of the hypothesis allowing the use of mathematical theories for these problems. To answer safety requirements, respect installation norms and reduce public risk, return levels are a major operational goal, obtained with the EVT. In this paper, we give quantitative results for observations of high temperatures over the 1950–2003 period in 47 stations in France. We examined the validity of the non-stationary EVT and introduced the notion of return levels (RL) in a time-varying context. Our analysis puts particular accent on the difference between methods used to describe extremes, to perform advanced fits and tests (climatic science), and those estimating the probability of rare future events (security problems in an evolving climate). After enouncing the method used for trend identification of extremes in term of easily interpretable parameters of distribution laws, we apply the procedure to long series of temperature measurements and check the influence of data length on trend estimation. We also address the problem of choosing the part of observations allowing appropriate extrapolation. In our analysis, we determined the influence of the 2003 heat wave on trend and return-level estimation comparing it to the RL in a stationary context. The application of the procedure to 47 stations spread over France is a first step for a refined spatial analysis. Working on the behavior of distribution parameters while assessing trend identification is a primary tool in order to classify climatic change with respect to the location of the station and open a systematic work using the same methodology for other variables and multivariate studies.     

Noninvasive determination of the location and distribution of DNAPL using advanced seismic reflection techniques

Recent advances in seismic reflection amplitude analysis (e.g., amplitude versus offset-AVO, bright spot mapping) technology to directly detect the presence of subsurface DNAPL (e.g., CCl4) were applied to 216-Z-9 crib, 200 West Area, DOE Hanford Site, Washington. Modeling to determine what type of anomaly might be present was performed. Model results were incorporated in the interpretation of the seismic data to determine the location of any seismic amplitude anomalies associated with the presence of high concentrations of CCl4. Seismic reflection profiles were collected and analyzed for the presence of DNAPL. Structure contour maps of the contact between the Hanford fine unit and the Plio/Pleistocene unit and between the Plio/Pleistocene unit and the caliche layer were interpreted to determine potential DNAPL flow direction. Models indicate that the contact between the Plio/Pleistocene unit and the caliche should have a positive reflection coefficient. When high concentrations of CCl4 are present, the reflection coefficient of this interface displays a noticeable positive increase in the seismic amplitude (i.e., bright spot). Amplitude data contoured on the Plio/Pleistocene-caliche boundary display high values indicating the presence of DNAPL to the north and east of the crib area. The seismic data agree well with the well control in areas of high concentrations of CCl4.     

Standard candles from the Gaia perspective

The ESA Gaia mission will bring a new era to the domain of standard candles. Progresses in this domain will be achieved thanks to unprecedented astrometric precision, whole-sky coverage and the combination of photometric, spectrophotometric and spectroscopic measurements. The fundamental outcome of the mission will be the Gaia catalogue produced by the Gaia Data Analysis and Processing Consortium (DPAC), which will contain a variable source classification and specific properties for stars of specific variability types. We review what will be produced for Cepheids, RR Lyrae, Long Period Variable stars and eclipsing binaries.     

Structure,composition, and location of organic matter in the enstatite chondrite Sahara 97096 (EH3)

Abstract– The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine‐grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar δ¹⁵N and δ¹³C; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium‐heterogeneity and having mean H‐ and N‐isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer‐sized areas highly enriched in ¹⁵N (δ¹⁵N up to 1600‰). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.     

Finite Difference Implicit Structural Modeling of Geological Structures

We introduce a new method for implicit structural modeling. The main developments in this paper are the new regularization operators we propose by extending inherent properties of the classic one-dimensional discrete second derivative operator to higher dimensions. The proposed regularization operators discretize naturally on the Cartesian grid using finite differences, owing to the highly symmetric nature of the Cartesian grid. Furthermore, the proposed regularization operators do not require any special treatment on boundary nodes, and their generalization to higher dimensions is straightforward. As a result, the proposed method has the advantage of being simple to implement. Numerical examples show that the proposed method is robust and numerically efficient.

     

Unbracketing the multiplicity of trauma in North Kivu,Democratic Republic of Congo

As international health organizations have increasingly acknowledged the global burden of psychological trauma, global health experts have sought to appraise and organize the treatment of trauma through objective, neutral forms of classification and calculation. Rather than see trauma as a singular thing whose biological, social and psychological formation is bracketed by expert perspectives, this paper focuses on how psychological trauma is enacted—that is, brought into being and sustained—in particular contexts and practices. If trauma can be made and unmade in practices, rather than assumed to be a stable thing, then these practices become a matter of concern rather than fact for geographers. We take as our empirical focus the province of North Kivu in the eastern Democratic Republic of Congo, where trauma has become a matter of growing local, national and international concern. Working with and beyond the conceptual work of Annemarie Mol, we demonstrate how different ‘versions’ of trauma uneasily co-exist in the region. Interrogating these versions, we explore the tensions that arise from attempting to explain and summate the incidence of trauma that is not singular or stable but is, instead, emergent and enacted in a variety of practices.