Can electrical resistivity tomography describe soil structure evolution?

Variations of soil structure is significant for the understanding of water and gas transfer in soil profiles. In the context of arable land, soil structure can be compacted due to either agriculture operation (wheel tracks), or hardsetting and crusting processes. As a consequence, soil porosity is reduced which may lead to decrease water infiltration and to anoxic conditions. Porosity can be increased by cracks formation due to swelling and shrinking phenomenon. We present here a laboratory experiment based on soil electrical characteristics. Electrical resistivity allows a non destructive three dimensional and dynamical analysis of the soil structure. Our main objective is to detect cracks in the soil. Cracks form an electrical resistant object and the contrast of resistivity between air and soil is large enough to be detected. Our sample is an undisturbed soil block 240mm*170mm*160mm with an initial structure compacted by wheel traffic. Successive artificial cracks are generated. Electrodes built with 2 mm ceramic cups permit a good electrical contact at the soil surface whatever its water content. They are installed 15 mm apart and the electrical resistivity is monitored using a dipole-dipole and wenner multi-electrodes 2D imaging method which gives a picture of the subsurface resistivity. The interpreted resistivity sections show the major soil structure. The electrical response changes with the cracks formation. The structure information extracted from the electrical map is in good agreement with the artificially man-made cracks. These first results demonstrate the relevance of high resolution electrical imaging of the soil profile. Further experiments need to be carried out in order to monitor natural soil structure evolution during wetting-drying cycles

Impact of Redox Cycles on Manganese, Iron, Cobalt, and Lead in Nodules

International audienceRedox processes are responsible for Fe and Mn segregation as Fe–Mn oxide coatings or nodules. These nodules are also trace element scavengers in soils. Redox processes are of particular importance in seasonally saturated soil containing naturally high concentrations of trace metals. We investigated the dynamics of Fe–Mn nodules and two associated trace elements, Co and Pb, under controlled redox conditions in a column experiment, including five columns fed with mimicked topsoil solution that was elevated in Fe and Mn. The results show that the redox conditions reached 100 mV, which was sufficient to dissolve Mn oxides and release the associated Co, while Pb was readsorbed onto nodule surfaces. The amounts of Mn and Co released into the water were small compared with the quantities stored in the nodules (<0.1% of the initial stock stored in the nodules). The redox conditions were insufficient, however, to allow Fe oxide dissolution. On the contrary, 70 to 90% of the Fe entering the column was fixed onto the nodules. In terms of an environmental threat, these results showed that Pb would not be released from soil during nodule dissolution, whereas Co, which is less toxic, would be released

Soil cracks detection by 3D electrical resistivity.

Soil cracks, whose formation are associated to natural climate phenomena such as swelling and shrinking, play an important role in water and gas transfers. Up to now, their 3D structure was characterised either by serial sections (Cousin, 1996) which is a destructive technique or X-ray tomography (Macedo et al., 1998) which is applicable on limited size sample. Three-dimensional electrical resistivity prospecting enables now to monitor crack development and to characterise their geometry without any destruction of the medium under study. Three-dimensional electrical resistivity surveys are commonly gathered by a network of in-line survey arrays, such as Wenner, Schlummberg, or dipole-dipole (Xu and Noel, 1993; Zhou et al., 2002). As emphasized by Meheni et al. (1996) the resulting apparent resistivity maps are often different depending on the array orientation related to an electrical discontinuity. Chambers et al. (2002) underline that in heterogeneous medium 3D electrical resistivity model resolution was sensitive to electrode configuration orientation. Indeed asymmetric bodies or anisotropic material exhibit different behaviours depending on whether the current passes through them in one direction or in another (Scollar et al., 1990). It would be all the more true for medium having very contrasted resistivities like cracking soil. In that case the electrical current does not encounter the same resistance when it passes perpendicular or parallel to the resistant bodies. Measurements of apparent resistivity depend then on the location and orientation of the current source relative to the body under study (Bibby, 1986). Studies conducted by Habberjam and Watkins (1967) emphasized that the square array provide a measurement of resistivity less orientationally dependent than that given by a in-line array investigation. Intending to lead a more 3D accurate inversion, we have chosen to focus our attention on a 3D electrical resistivity data acquisition. We present here a three-dimensional electrical survey carried out by a square array quadripole for characterising the soil cracks network developing during a desiccation period

Electrical resistivity survey in soil science: a review .

Electrical resistivity of the soil can be considered as a proxy for the spatial and temporal variability of many other soil physical properties (i.e. structure, water content, or fluid composition). Because the method is non-destructive and very sensitive, it offers a very attractive tool for describing the subsurface properties without digging. It has been already applied in various contexts like: groundwater exploration, landfill and solute transfer delineation, agronomical management by identifying areas of excessive compaction or soil horizon thickness and bedrock depth, and at least assessing the soil hydrological properties. The surveys, depending on the areas heterogeneities can be performed in one-, two- or three-dimensions and also at different scales resolution from the centimetric scale to the regional scale. In this review, based on many electrical resistivity surveys, we expose the theory and the basic principles of the method, we overview the variation of electrical resistivity as a function of soil properties, we listed the main electrical device to performed one-, two- or three-dimensional surveys, and explain the basic principles of the data interpretation. At least, we discuss the main advantages and limits of the method

Establishement of crack indexes by electrical apparent resistivity data.

Soil cracks, whose formation are associated to natural climate phenomena, play an important role in water and gas transfer. Detecting cracks by non-destructive geophysical methods permits a 3D temporal monitoring of the cracking patterns. Electrical resistivity is indeed well adapted to recognize the electrical resistant signature of crack filled by air during a dessiccation period

Analyse tridimensionnelle non destructive de la fissuration d'un sol cultivé à partir de mesures de résistivité électrique

210 ref. *INRA Orléans, Documentation, Ardon 45166 Olivet Diffusion du document : INRA Orléans, Documentation, Ardon 45166 Olivet Diplôme : Dr. d'UniversitéOur objective was to characterize by experimentation, the soil cracks occurring during a desiccation period. Our approach was based on the experimentation on soil blocks at the laboratory and simulation of the current lines in a fractured medium by modelling. We chose and adapted the electrical resistivity method with a square array configuration at the centimetric scale, which permitted three-dimensional and non destructive measurements. We adapted specific electrodes Cu/CuSO4. With 3D simulations we calculated and estimated the sensitivity of an anisotropic index AAI. This index is correlated with the crack orientation and also to other geometrical crack parameters such as: width, depth and pitch of the crack. This index gives quantitative results when applied to experimental data on crack detection and temporal follow-up of cracking pattern. This work introduces a useful method for the accurate characterisation of a 3D experimental cracking pattern. In turn, this may provide preliminary information for the modelling of pore deformation.L'objectif de ce travail de thèse est la mise au point d'une méthode expérimentale de description tri-dimensionnelle de la fissuration d'un sol au cours de la dessiccation. Notre démarche repose sur un couplage entre expérimentation et modélisation de la répartition des lignes de courant en milieu fissuré. Nous avons adapté à notre échelle d'étude la méthode de la résistivité électrique en configuration carré qui permet une acquisition 3D non destructive. Cette partie métrologique a été rendue possible grâce à l'élaboration d'électrodes impolarisables Cu/CuSO4. A l'aide d'un modèle 3D adapté à la description de la résistivité en milieu fissuré, nous avons défini un indice d'anisotropie (AAI) et analysé sa sensibilité à différents paramètres géométriques d'une fissure. Cet indice est sensible à l'orientation, l'épaisseur, la profondeur et le pendage de la fissure. L'utilisation de cet indice, validée par l'expérimentation, permet une description qualitative de l'état de fissuration et de son évolution. Ce travail ouvre ainsi des perspectives sur la modélisation des milieux poreux déformables

Analyse tridimensionnelle non destructive de la fissuration d'un sol cultivé à partir de mesures de résistivité électrique

L'objectif de ce travail de thèse est la mise au point d'une méthode expérimentale de description tri-dimensionnelle de la fissuration d'un sol à la dessiccation. Nous avons adapté à notre échelle d'étude la méthode de la résistivité électrique (configuration carré), permettant une acquisition 3D et non destructive. Notre démarche repose sur un couplage entre expérimentation et modélisation de la répartition des lignes de courant en milieu fissuré. A l'aide de la modélisation 3D nous avons pu évaluer numériquement les effets de bords présents lors d'expérimentation en milieu fini. Nous avons également défini et évalué la sensibilité de deux indices d'anisotropie AAI et amax. L'indice amax permet de retrouver l'orientation de la fissure. L'indice AAI est également sensible à des paramètres géométriques tels que : épaisseur, profondeur, pendage. Ces indices validés par l'expérimentation permettent une description non redondante de l'état de fissuration et de son évolution. Ce travail ouvre ainsi des perspectives sur la modélisation des milieux poreux déformables.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Quantification of lessivage in soils: an experimental approach

Lessivage, understood as the vertical transfer of fine particles from a horizon, called eluviated, to another horizon, called illuviated, was never measured to our knowledge due to obvious technical difficulties. Two laboratory experiments focusing respectively on eluviation and on illuviation were designed. Eluviation was experienced on an undisturbed loamy soil column (called L1). Illuviation was experienced on a column made of undisturbed loamy horizon (L1) lying on a second undistrurbed monolith of a loamy horizon (called L2) having contrasting mineralogical and physicochemical properties. For both experiments, rain events of 30 mm were applied with two different intensities: 20 mm/h and 6mm/h. The lower boundary was free drainage during the rain event, and constant pressure head during the infiltration phase. Between two rain events, the soil columns were allowed to dry up in order to recover their initial water content. The water content in the column, the chemistry of the soil solution and the release of particles into the drainage water were monitored. These experiments allowed quantifying eluviation and relating it to preferential drainage, or to the succion applied at the base of the columns and to chemical composition of the soil water. The contribution of these different processes was most probably related to the structure of the soil column. We also determined that 20 to 80 % of the particles released by the L1 horizon were trapped into the L2 horizon allowing for the first time to our knowledge to quantify the intensity of particles fixation in natural soils

DONESOL pour ré-analyser le processus du lessivage : méta-analyse de données et modélisation

National audienc

Estimation of soil hydraulic properties evolution in an E&BT horizon

To predict soil evolution induced by human activities at the century scale, we need to be able to model this evolution on such a time scale. For that purpose, coupled water-geochemistry modelling approaches are one possibility. If it is well recognized that soil hydraulic properties evolve in space, their temporal change are rarely taken into account. On the one hand, models hardly support the temporal variability of soil hydraulic properties that must be constant parameters all along the simulation and, on the other hand, the temporal evolution of soil hydraulic properties is often not known. Few experimental data concern the temporal variability of soil properties in the surface horizon at the annual scale, due to agricultural practices, for example. We propose here to investigate deeper horizons. We have performed an experiment to estimate the impact of sixteen years of subsurface drainage on the hydraulic properties of an Albeluvisol. We have focused our analysis on the heterogeneous E&Bt horizon, composed by two elementary pedological volumes (EPV) organised in a complex 3D structure. The analysis of serial sections demonstrated that the proportion and spatial arrangement of the EPVs evolved with time. Based on 3D reconstructions of the structure of the two EPVs and on the knowledge of their hydraulic properties, we have quantified the evolution of the hydraulic properties at the horizon scale, by using a 3D water transfer model. An application of this work will consist in including the temporal evolution of soil hydraulic into a model of soil evolution