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

Clay fine fissuring monitoring using miniature geo-electrical resistivity arrays

Abstract This article describes a miniaturised electrical imaging (resistivity tomography) technique to map the cracking pattern of a clay model. The clay used was taken from a scaled flood embankment built to study the fine fissuring due to desiccation and breaching process in flooding conditions. The potential of using a miniature array of electrodes to follow the evolution of the vertical cracks and number them during the drying process was explored. The imaging technique generated two-dimensional contoured plots of the resistivity distribution within the model before and at different stages of the desiccation process. The change in resistivity associated with the widening of the cracks were monitored as a function of time. Experiments were also carried out using a selected conductive gel to slow down the transport process into the cracks to improve the scanning capabilities of the equipment. The main vertical clay fissuring network was obtained after inversion of the experimental resistivity measurements and validated by direct observations

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

Assessing the applicability of the earth impedance method for in situ studies of tree root systems

Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root–soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root–soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root–soil interface

A review of spatial downscaling of satellite remotely sensed soil moisture

Satellite remote sensing technology has been widely used to estimate surface soil moisture. Numerous efforts have been devoted to develop global soil moisture products. However, these global soil moisture products, normally retrieved from microwave remote sensing data, are typically not suitable for regional hydrological and agricultural applications such as irrigation management and flood predictions, due to their coarse spatial resolution. Therefore, various downscaling methods have been proposed to improve the coarse resolution soil moisture products. The purpose of this paper is to review existing methods for downscaling satellite remotely sensed soil moisture. These methods are assessed and compared in terms of their advantages and limitations. This review also provides the accuracy level of these methods based on published validation studies. In the final part, problems and future trends associated with these methods are analyzed