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International audience; Time-lapse electrical resistivity tomography is a widely used geophysical method to remotely monitor water saturation and the migration of contaminant plumes. This is achieved by driving a known electrical current between an electrode pair while measuring the resulting voltage between another electrode pair. The electrical resistivity structure of the subsurface can be estimated by inversion of multiple current injection and voltage pairs. The effects of heterogeneous solute concentrations (i.e., fingering of a saline tracer) below the resolution limits of the tomogram are commonly ignored (i.e, the solution is assumed to be perfectly mixed below this scale). We have adapted an experimental set-up to study the effects of sub-resolution solute heterogeneities on the effective bulk electrical resistivity. We used a 2D analogous porous medium consisting of a Hele-Shaw cell containing a single layer of 4500 cylindrical solid grains built by soft lithography. We monitored the bulk electrical resistivity at a temporal resolution of 2 s. At the same time, we monitored the spatial distribution of the water/air phases and the saline solute concentration field in the water phase using a fluorescent tracer injected together with the saline solute and a high-resolution camera (27 pixels per mm, 12 bit images). We performed saline tracer tests under full and partial water saturations. The unsaturated flow was imposed by jointly injecting air and water. The bulk resistivities measured at the scale of the medium were confronted to resistivities computed numerically from the measured spatial distributions of the fluid phases and the salinity field. We find that the air distribution, saline tracer channeling and fingering, and mixing phenomena all result in large changes in the measured and simulated bulk resistivities by creating preferential flow paths or barriers for electric current at the pore scale. Based on these findings, we postulate that sub-resolution effects contribute to one of the most important inconsistencies in conventional time-lapse ERT imaging, namely the apparent loss of tracer mass.

International audience; The self-potential (SP) method is sensitive to water fluxes in saturated and partially saturated porous media, such as those associated with rainwater infiltration and groundwater recharge. We present a field-based study at the Voulund agricultural test site, Denmark, that is, to the best of our knowledge, the first to focus on the vertical self-potential distribution prior to and during a saline tracer test. A coupled hydrogeophysical modeling framework is used tosimulate the SP response to precipitation and saline tracer infiltration. A layered hydrological model is first obtained by inverting dielectric and matric potential data. The resulting model that compares favorably with electrical resistance tomography models is subsequently used to predict the SP response. The electrokinetic contribution (caused by water fluxes in a charged porous soil) is modeled by an effective excess charge approach that considers both water saturation and pore water salinity. Our results suggest that the effective excess charge evolution prior to the tracer injection is better described by a recent flux-averaged model based on soil water retention functions than by a previously proposed volume-averaging model. This is the first time that raw (i.e., without post-processing or data-correction) vertically distributed SP measurements have been explained by a physically based model. The electrokinetic contribution cannot alone reproduce the experimental SP data during the tracer test and an electro-diffusive contribution (caused by concentration gradients) is needed. The predicted amplitude of this contribution is too small to perfectly explain the data, but the shape is in accordance with the field data. This discrepancy is attributed to imperfect descriptions of electro-diffusive phenomena in partially saturated soils, unaccounted soil heterogeneity, and discrepancies between the measured and predicted electrical conductivities in the tracer infiltration area. This study opens the way for detailed long-term field-based investigations of the SP method in vadose zone hydrology.

In order to study the high activity long life radioactive waste storage, ANDRA has built an underground laboratory in the Callovo-Oxfordian clay-rocks at 500 meters deep. Gallery excavation in deep clay-rock formations induces an Excavated Damaged Zone (EDZ) characterized by hydro-mechanical perturbations evolving with time. We have used geoelectrical methods to study hydraulic, ionic, and thermal transfer phenomena in partially saturated clay-rocks: the spectral induced polarization and the streaming potential. We have developed mechanistic models to link geo-electrical results on samples to transport properties in partially saturated conditions: effective diffusion coefficient and thermal conductivity., Afin d'étudier la faisabilité du stockage des déchets radioactifs à haute activité et à vie longue, l'ANDRA a mis en place un laboratoire de recherche souterrain dans les argilites du Callovo-Oxfordien à environ 500 mètres de profondeur. L'excavation de galeries dans ce type de formations argileuses profondes induit une zone d'endommagement (EDZ) qui se caractérise principalement par des perturbations mécaniques et hydriques qui évoluent au cours du temps. Nous nous sommes intéressé à l'étude des phénomènes de transferts hydriques, ioniques et thermiques dans les argilites partiellement saturées par des méthodes géo-électriques non intrusives : la polarisation provoquée spectrale et la polarisation spontanée. Les travaux que nous avons menés ont permis de développer des modèles mécanistiques pour relier les résultats des mesures géo-électriques sur échantillons à des propriétés de transport en milieu partiellement saturé : coefficient de diffusion effectif et conductivité thermique.

International audience; Time-lapse electrical resistivity tomography is widely used to remotelymonitor water saturation and contaminant plumes. Petrophysical relationshipsare needed to link electrical measurements to subsurface propertiesof primary interest. Most petrophysical relationships are basedon mixing laws or upscaling procedures (e.g., differential effective medium,volume averaging) that are well understood in saturated mediaat homogeneous pore water composition. The effects of heterogeneoussolute concentrations (i.e., fingering of a saline tracer) and fluid distributions(i.e., air and water) below the resolution of geophysical tomogramsare currently ignored in most hydrogeophysical studies. We adapted anexisting experimental set-up to study the effects of sub-resolution heterogeneitieson the effective bulk electrical resistivity. We used a 2Danalogous porous medium consisting of a Hele-Shaw cell containing asingle layer of 4500 cylindrical solid grains built by soft lithography.We monitored the bulk electrical resistivity between two electrodes at atemporal resolution of 2 s. At the same time, we monitored the phasedistribution and the local concentration field using a fluorescent tracerand a high-resolution camera (27 pixels per mm, 12 bit images). Our experimentsincluded drainage and imbibition, as well as high salinity tracertests under full and partial saturations. The measured bulk electricalresistivities are currently compared to those computed (1) numericallyat the pore scale based on the local salinity and saturation fields obtainedby image processing and (2) by applying petrophysical relationships definedat the scale of the porous media.We expect that this work will giverise to more robust upscaled petrophysical models and better understandingof sub-resolution effects in hydrogeophysics. The experimentalapproach will be extended to include spectral induced polarization andself-potential measurements under different water saturations.

International audience; Seismic waves propagating in a porous medium, under favorable conditions , generate measurable electromagnetic fields due to electrokinetic effects. It has been proposed, following experimental and numerical studies , that these so-called ?seismoelectromagnetic ? couplings depend on pore fluid properties. The theoretical frame describing these phenomena are based on the original Biot's theory, assuming that pores are fluid-filled. We study here the impact of a partially saturated medium on amplitudes of those seismoelectric couplings by comparing experimental data to an effective fluid model. We have built a 1 m-length-scale experiment designed for imbibition and drainage of an homogeneous silica sand ; the experimental setup includes a seismic source, acce-lerometers, electric dipoles and capacitance probes in order to monitor the seismic and seismoelectric fields during water saturation. Apparent velocities and frequency spectra are derived from seismic and electric measurements during experiments in varying saturation conditions. Amplitudes of the seismic and seismoelectric waves and their ratios (i.e. transfer functions) are discussed using a spectral analysis performed by Continuous Wavelet Transform (CWT). The experiment results reveal that the amplitude ratiosof seismic to co-seismic electric signals remain rather constant as a function of the water saturation. This result show good agreement with theoretically predicted transfer functions including effective fluid properties and some recent electrokinetic models. This experiment also show the evidence of the dynamic effect in seis-moelectric coupling.

Depuis le début des années soixante, une partie de cette activité est mise en danger par une autre activité, l'agriculture intensive. L'ancien marais salant de Moëze-Brouage abrite ces deux activités. De cette proximité est née la problématique agro-conchylicole. Il s'agira de savoir si les eaux rejetées par l'activité agricole peuvent être dommageables pour la conchyliculture. Pour le savoir, il s'agira de déterminer la qualité biologique des eaux de rejets -j agricoles en appliquant le test écotoxicologique dit « larve de bivalves ». Ce travail est donc une initiation au test « larve de bivalve» appliqué aux huîtres creuse du japon Crassostrea gigas. Dans un premier temps, nous présenterons l'étude en étudiant la problématique agroconchylicole, l'objectif et le principe du test. Nous ferons ensuite un rappel global de l'huître creuse et de sa reproduction. Dans une troisième partie, nous présenterons le test, son déroulement et le matériel qui y est nécessaire. Enfin, nous étudierons les résultats des différents tests, et nous les commenterons pour pouvoir donner les conclusions de cette pré-étude.

Self-potentials (SP) are sensitive to water fluxes and concentration gradients in both saturated and unsaturated geological media, but quantitative interpretations of SP field data may often be hindered by the superposition of different source contributions and time-varying electrode potentials. Self-potential mapping and close to two months of SP monitoring on a gravel bar were performed to investigate the origins of SP signals at a restored river section of the Thur River in northeastern Switzerland. The SP mapping and subsequent inversion of the data indicate that the SP sources are mainly located in the upper few meters in regions of soil cover rather than bare gravel. Wavelet analyses of the time-series indicate a strong, but non-linear influence of water table and water content variations, as well as rainfall intensity on the recorded SP signals. Modeling of the SP response with respect to an increase in the water table elevation and precipitation indicate that the distribution of soil properties in the vadose zone has a very strong influence. We conclude that the observed SP responses on the gravel bar are more complicated than previously proposed semi-empiric relationships between SP signals and hydraulic head or the thickness of the vadose zone. We suggest that future SP monitoring in restored river corridors should either focus on quantifying vadose zone processes by installing vertical profiles of closely spaced SP electrodes or by installing the electrodes within the river to avoid signals arising from vadose zone processes and time-varying electrochemical conditions in the vicinity of the electrodes., Hydrology and Earth System Sciences, 15 (3), ISSN:1027-5606, ISSN:1607-7938

(IF 6.75; Q1); International audience; The flow of the pore water in porous media generates an electrical current known as the streaming current. This current is due to the drag of the excess of charges contained in the electrical diffuse layer coating the surface of the grains. This current is associated with an electric field called the streaming potential field. The fluctuations of this field can be remotely measured using a set of non-polarizable electrodes located at the ground surface or in wells and a sensitive voltmeter. The self-potential method (SP) aims at passively measuring the streaming potential anomalies associated with ground water flow. We present a stochastic numerical framework for inverting self-potential data in order to localize seeps in dams and characterize their permeability and Darcy velocity. Our approach is based on the use of Markov chains Monte Carlo (McMC) method for solving the inverse problem. We performed first a validation of the method on a synthetic case study and then on large-scale field surveys on three different dams. Our approach is successful in localizing seeps and determining their permeability. A sensitivity study is performed on each of these three dams to better define the hydraulic and electrical parameters influencing the self-potential signal and the uncertainties associated with the estimation of those parameters. Our results show that the self-potential method can provide quantitative hydrogeological information for the characterization of seeps in dams and dikes.

International audience; Self-potential (SP) data are of interest to vadose zone hydrology because of their direct sensitivity to water flow and ionic transport. There is unfortunately little consensus in the literature about how to best model SP data under partially saturated conditions and different approaches (often supported by one laboratory data set alone) have been proposed. We argue herein that this lack of agreement can largely be traced to electrode effects that have not been properly taken into account. A series of drainage and imbibition experiments are considered, in which we find that previously proposed approaches to remove electrode effects are unlikely to provide adequate corrections. Instead, we explicitly model the electrode effects together with classical SP contributions using a flow and transport model. The simulated data agree overall with the observed SP signals and allow decomposing the different signal contributions to analyze them separately. By reviewing other published experimental data, we suggest that most of them include electrode effects that have not been properly taken into account. Our results suggest that previously presented SP theory works well when considering the modeling uncertainties presently associated with electrode effects. Additional work is warranted to not only develop suitable electrodes for laboratory experiments, but also to assure that associated electrode effects that appear inevitable in longer-term experiments are predictable, such that they can be incorporated in the modeling framework.

Minimally intrusive geophysical methods are required to characterize both the vadose zone of the Earth, hydrocarbon reservoirs and CO₂ sequestration. We investigate the impact of gaseous CO₂ on both electrical conductivity and electrokinetic properties of limestones under saturated and unsaturated conditions, using the spectral induced polarization and the self-potential methods. These data are contrasted with measurements performed on a Fontainebleau sandstone and data from the literature. That said, the dependence of a key parameter, the streaming coupling coefficient, with the saturation remains highly debated. Using the self-potential method, we explore the relationship between the streaming potential coupling coefficient, the capillary pressure curves and the permeability in carbonate rocks characterized by distinct textures. A new core flooding system is used to measure simultaneously both the relative permeability, the resistivity index and the streaming potential coupling coefficient in steady-state two-phase flow conditions as a function of the saturation with CO₂ or N₂. The results are compared with a recently developed theoretical model, which can accommodate either the Brooks and Corey or the van Genuchten models for the capillary pressure curves. Saturation was achieved with monovalent and divalent brines with ionic strength ranging from 1x10⁻³ Mol L⁻¹ to 1x10⁰ Mol L⁻¹. The magnitude of the coupling coefficient increases with decreasing ionic strength similarly to the trend observed for sandstones. The zeta potential has been calculated at full saturation using a modified version of the Helmholtz-Smoluchowski equation that accounts for surface electrical conductivity. Under atmospheric conditions, the magnitude of the zeta potential is decreasing with the increase of the ionic strength. We also investigate the effects of a CO₂ release and the calcite dissolution on the magnitude of the zeta potential. Finally, we use the spectral induced polarization method to determine the pore water conductivity effects on the complex electrical conductivity, the normalized chargeability and the main relaxation time during drainage in a clay free limestone. We also show evidences that these parameters could be considered as polarization parameters of the electrical double layer in the pore water conductivity range 10⁻³ - 10⁰ S m⁻¹., Les méthodes géophysiques non intrusives sont requises pour caractériser la zone vadose, les réservoirs d’hydrocarbures, ou les sites de stockages de CO₂. Nous analysons l’impact d’une injection de CO₂ gazeux sur la conductivité électrique et les propriétés électrocinétiques des roches carbonatées partiellement ou totalement saturées grâce aux méthodes de polarisations provoquée et spontanée. Les données sont analysées au regard au regard de mesures effectuées sur une roche chimiquement neutre, vis-à-vis du CO₂, c’est-à-dire , un grès de Fontainebleau. Nous comparons également nos résultats aux données de la littérature. Le coefficient de couplage électrocinétique est un paramètre clé, dont nous allons étudier la dépendance vis-à-vis de la saturation, sujet hautement débattu de nos jours. En utilisant la méthode de polarisation spontanée, nous étudions les relations entre le coefficient de couplage électrocinétique, la pression capillaire ainsi que la perméabilité relative au sein des roches carbonatées. Un système expérimental a été élaboré pour mesurer simultanément la perméabilité relative, l’indice de resistivité et le coefficient de couplage électrocinétique en écoulement diphasique de type eau-gaz, en fonction de la saturation en azote ou en dioxyde de carbone. Les résultats sont comparés à des modèles théoriques basés sur les approches de Brooks et Corey et van Genuchten, dont les exposants nous permettent d’ajuster les données avec les modèles de pression capillaire avec succès. Les échantillons sont saturés avec des saumures de différentes compositions, dont les ions sont mono- ou divalents et pour lesquelles la force ionique est comprise entre 10⁻⁴ et 10⁰ Mol L⁻¹. La valeur absolue du coefficient de couplage électrocinétique augmente lorsque la force ionique de la solution diminue, ce qui a déjà été observé dans les roches gréseuses. Le potentiel zêta a été calculé en utilisant une version modifiée de l’équation d’Helmholtz-Smoluchowski, qui prend en compte les effets liés à la conductivité de surface. Comme pour le coefficient de couplage, la valeur absolue du potentiel zêta chute lorsque la force ionique augmente. Nous nous intéressons également aux effets liés à une injection de CO₂ et à la dissolution de la calcite sur la valeur de ce potentiel zêta. Enfin, nous utilisons la méthode de polarisation provoquée pour déterminer l’influence de conductivité de l’eau porale sur la conductivité électrique complexe, la chargeabilité normalisée ainsi que le temps de relaxation en milieu carbonaté non saturé. Nous montrons que ces paramètres peuvent être considérés comme des paramètres de polarisation de la double couche électrique lorsque la conductivité de l’eau porale est comprise entre 10⁻³ et 10⁰ S m⁻¹.