Connectivity evolution of 2D random distributions of disks and ellipses: application to polyphasic crystal rock distribution

International audienceThe ultimate goal of this work is to characterise the arrangement of mineral phases using 2D images. Three simple indices are defined in order to quantify their 2D connectivity and spatial distribution. Ultimate eroded construction is used to model grain clusters. First, Monte-Carlo simulations of 2D random images permit the testing of index sensitivity to the surface area fraction, the shape and the distribution of particles in terms of the percolation threshold. This allows us to define three normalised indices for rock phase characterisation. The results are in good agreement with the morphological characteristics of the studied granite phases. They permit a comparison and a quantification of the 2D spatial arrangement of the phases which are under the percolation threshold

Primary mineral connectivity of polyphasic igneous rocks by high-quality digitisation and 2D image analysis

International audienceIn order to produce accurate images for geological studies, a Canon colour laser copier CLC 300 was connected to a Silicon Graphic workstation. This connection turns the copier into a high-quality colour scanner and printer, while keeping its copier abilities. The digitised images can be processed and analysed with specific programs, written using the C language under the X-Window standard graphic environment. The basic versions of these programs can be easily modified for further developments. These tools were applied to the analyses of primary mineral connectivities in two igneous rocks: a medium-grained granite from Soultz-sous-Forêts and a fine-grained tonalite from Charroux-Civray. Results are presented and discussed in terms of fluid pathway location

Decreased burrowing activity of endogeic earthworms and effects on water infiltration in response to an increase in soil bulk density

International audienceEndogeic earthworms live and burrow in the soil to find their food. They burrow by pushing the soil aside or ingesting it and are thus sensitive to soil compaction. However there is a scarcity of data regarding the effects of soil bulk density on the burrowing behavior and activity of endogeic earthworms. We carried out laboratory experiments using repacked soil cores with various levels of bulk density (from 1.18 to 1.38 g cm-3) in which individuals of Aporrectodea caliginosa or Allolobophora chlorotica were incubated for six weeks. The burrow systems inside the soil cores and the compaction around the burrows were then analyzed using X-ray tomography. Soil water infiltration measurements were also carried out. The increase in bulk density had a negative impact on all burrow system characteristics (length, volume, diameter, continuity, number of burrows). When bulk density increased from 1.18 to 1.38 g cm-3, volume, diameter, continuity and the number of burrows decreased on average by 77 %, 21 %, 81 % and 58 %, respectively. The increase in density due to compaction around the burrows was similar whatever the species and the bulk density. Increasing soil bulk density from 1.18 to 1.38 g cm-3 also greatly decreased water infiltration (-89 % for both species) and increased breakthrough time (10 and 25-fold for A. chlorotica and A. caliginosa respectively). However, compared to a control without earthworms, water infiltration in cores incubated with endogeic species was only increased significantly at 1.18 and 1.23 g cm-3. This illustrates that burrows made by endogeic earthworms moderately increase water infiltration and only when the soil bulk density is low. Data provided in this study could be used to refine simulation models of earthworm burrowing behavior where burrowing is assumed to be mainly governed by soil water content, temperature and soil bulk density

Magnetic resonance imaging and relaxometry as tools to investigate water distribution in soils

International audienceRelaxation times and two imaging sequences (spin echo and single point imaging) were performed onto repacked soil samples to study respectively water distribution within the porosity and to measure water content profiles, distinguishing water contained in large pores from water contained in the whole porosity. These methods were applied to 25 samples of the same soil that was prepared to obtain aggregates of three different size, then repacked to five bulk densities. Samples were then equilibrated with water at five matric potentials. We found that T1 and T2 measurements present similar time distributions with essentially four peaks. We attributed the two shortest times to textural pores, and the two longest times to structural pores. The water profile measured with spin echo sequence was attributed to water contained in structural pore

Advanced characterization of macropore flow in undisturbed soil cores using time resolved three-dimensional CT images

Macropore flow refers to rapid by-pass flow generally occurring in natural macropores (e.g. earthworm burrows, spaced left by decayed roots, desiccation cracks and some open inter-aggregates spaces) [1]. During water infiltration in unsaturated soils, macropore flow may occur, controlled by the balance between vertical and lateral flow rates at macropore surfaces and by the connectivity and geometrical properties of the macroporous network. Although soil macroporosity has been widely imaged and characterized, the dynamic of water infiltration in undisturbed macroporous soils is still poorly understood and modeled. Some fundamental questions still remain i) the location of preferential flow paths or the identification of the active part of the macroporous network (few % of the soil porosity) and ii) the determination of the flow regimes in the active macropores [2]. We propose to characterize macropore flow and preferential flow paths geometry in an undisturbed soil core initially at field capacity using time resolved localization of water infiltration during a simulated rainfall event. Our results are based on the processing and analyzing of the 3D images recently obtained thanks to a novel methodological approach developed using the serial images acquired with a multi-slice helical CT during a simulated rainfall event. Water flow in macropores will be characterized by 1) the occurrence and detection of “water voxels” in the macroporous network and 2) the localization and water filling of the active macropores as a function of time. Finally, results will be discussed taking into acco unt for the temporal and spatial resolutions (voxel size) of the 3D images [3]

Metal soil pollution differentially affects both the behaviour and exposure of A. caliginosa and L. terrestris: a mesocosm study

International audienceThe effects on two earthworm species of a gradient of metal contamination in soil collected close to a 50-year-old lead recycling factory were investigated in mesocosms filled with soil sampled at three distances from the factory (10, 30 and 60 m). After 5 weeks of exposure, earthworm litter consumption and weight change were measured. Burrow systems were analysed using Xray tomography, and water infiltration was measured. No significant differences in earthworm weight or activity were observed between mesocosms filled with soil from 30 and 60 m. In contrast, both earthworm species significantly lost weight and burrowed less in the soil sampled at 10 m. In the cores filled with the soil collected at 10-m distance, Aporrectodea caliginosa avoided the highly contaminated first layer (0–5 cm) and burrowed deeper whereas Lumbricus terrestris burrowed relatively more in this layer. We assume that these different reactions are associated with their ecological types. Epi-anecic earthworms forage litter at the soil surface, whereas endogeic earthworms are geophagous and thus are able to forage deeper. This was further corroborated by the bioaccumulation factors measured for each species: for L. terrestris, BAF values for Pb and Cd only decreased slightly in the 10-m soil correlating with their overall reduced activity. However, BAF values for A. caliginosa were 20-fold lower compared to those observed in soil from 30 and 60 m. These modifications in burrowing behaviour in the 10-m mesocosms resulted in a significant and marked decrease in water infiltration rates but only for L. terrestris

Mineralogy, texture and porosity of Callovo-Oxfordian argillites of the Meuse/Haute-Marne region (eastern Paris Basin)

International audienceThe texture and porosity of Callovo-Oxfordian argillites, coming from the Meuse/Haute-Marne region, have been studied at different scales using borehole and laboratory measurements to more spot-like qualitative and quantita- tive analyses and descriptions of minerals and of the porous space. Due to their limitations and to their resolution, each of the experimental methods used explores differently and often complementarily the porous environment of argillites. The texture is characterised by two-dimensional imaging techniques (optical microscopy, scanning electron microscopy and autoradiography) that provide a view of the material from 0.1 micrometres to a few millimetres or even centimetres. Since borehole measurements are more integrative and representative of the rock in its natural state, they were used to analyse the spatial variability of the porosity. The measurements taken on laboratory samples served to characterise more finely the porous network by providing relevant geometric parameters in order to improve the understanding of the transfer and reaction processes occurring at the interface between fluids and minerals. In the end, a conceptual model for the spatial organisation of the porosity and of the minerals was developed for argillites on the basis of the overall in- formation collected at each step. The reference values for the connected porosity of argillites are obtained by combining the mercury porosity and the calculated porosity in the domain still free from mercury as determined by gas-adsorption techniques. The total connected porosity then varies between 14% for carbonated levels and 19.5% for more argilla- ceous levels. Those values match those measured in boreholes. In percentage of the total porosity, the macroporosity characterised by mercury porosimetry ranges from 20 to 40%, whereas the mesoporosity resulting from the coupling of mercury porosimetry with nitrogen adsorption oscillates between 60 and 80%. The microporosity determined by nitro- gen adsorption is lower than 2%. The relationship between the structure of the porosity and the mineralogy is described by considering 1) the prevailing calcic pole in the upper part of the series around the facies containing argilla- ceous-limestone beds (lithofacies C2d and C2c, and base of C2b) and 2) the argillaceous pole (lithofacies C2b and C2a). The properties of the porous network are discussed afterwards by reviewing water-saturation effects

Evolution of fluid pathways of Charroux-Civray tonalite (part 1) : alteration effects - an analytical approach

International audienceVarious experimental methods and image analysis procedures have been used to underscore the microporosity evolution of the Charroux-Civray tonalite. Throat pore effect is the dominant geometrical parameter, which controls permeability and determines the transport process. Indeed, in unaltered tonalite, flowing is not possible in spite of the fact that diffusion is made possible by a connected random network. Fluid pathways are located along the feldspar clusters joining quartz grains. Alteration effects are seen on the increased mean aperture of microcracks (feldspar dissolution). This paper raises the problem of the experimental limits in the study of very low permeable rocks

Porosity distribution in a clay gouge by image processing of 14C-PolyMethylMethAcrylate (14C-PMMA) autoradiographs

International audienceClay-rich fault gouge is a highly heterogeneous material composed of clasts of host rocks surrounded by a clay matrix. Moreover, this fault rock is characterised by different layers with varying properties: clast content, quantity of shear planes and lithology of rocks. Consequently, the accurate porosity characterisation inside this heterogeneous rock is not possible using usual techniques such as microscopy or bulk physical measurements. A specimen representative of the centimetric thickness of Saint Julien fault gouge has been impregnated by a 14C marked MethylMethAcrylate resin. From the micrometric to the centimetric scale, the spatial distribution of porosity (micro to macropores) is revealed on autoradiographs of different sample sections: clasts (sandstone or pelite) and illite-rich matrix composing the gouge are distinguishable according to their porosity contrast, and the layering is highlighted through the entire gouge thickness. Quantification of porosity by a specific image analysis software shows that clast porosity (ranging from 2.7% to 8.1%) is always lower than the porosity of the clay matrix (ranging from 10.5% to 15.3%). The porosity of these two gouge components always presents the same difference in all gouge layers. A detailed observation of porosity maps reveals the spatial relations linking clast to clay matrix porosities: porosity gradients around the clasts indicate that dissolution of carbonate cements by a recent fluid circulation into the clay part of the gouge is responsible of the actual porous network

Identifying the Functional Macropore Network Related to Preferential Flow in Structured Soils

International audienceUnderstanding the processes and mechanisms that control preferential flow in soils in relation to the properties of their structures is still challenging since fast flow and transport occur in a small fraction of the porosity, that is, the functional macropore network, making it difficult to image and characterize these processes at decimeter scales. The aim of the paper was therefore to propose a new image acquisition and analysis methodology to characterize preferential flow at the core scale and identify the resulting active macropore network. Water infiltration was monitored by a sequence of three-dimensional images (taken at 5-, 10-, or 15-min intervals) with an X-ray scanner that allows very fast acquisitions (10 s for a 135-mm diameter). A simultaneous dye tracer experiment was also conducted. Water infiltration was then imaged at each acquisition time by the voxels impacted by water during infiltration, named the water voxels. The number of times a voxel was impacted by water during the experiment was converted into data reflecting the water detection frequency at the given position in the soil column, named the local detection frequency. Compared with dye staining, the active macropore network was defined by macropores in which water voxels were the most frequently detected during the experiment (local detection frequency above 65%). The geometric properties of this active network, such as the connectivity, were significantly different from those of the total structure. This image processing methodology coupled to dynamic acquisitions can be used to improve the analysis of preferential flow processes related to soil structures at the core scale