The flow of groundwater during a pumping test experiment is responsible for a measurable electrical field at the ground surface owing to the electrokinetic coupling between the Darcy velocity and the electrical current density. This electrical field can be measured passively with a network of nonpolarizable electrodes connected to a digital multichannel multimeter with a high internal impedance (>10 Mohm). These so‐called self‐potential signals can be used to track the pattern of groundwater flow in the subsurface. A field test was performed using a set of 53 Pb/PbCl2 electrodes plus an additional electrode used as a unique reference in the field and a set of five piezometers to monitor the position of the piezometric surface. Using appropriate Green's functions, the electrical response is analyzed in terms of piezometric head distribution. This new methodology, which we call “electrography,” allows visualization of preferential fluid flow pathways and the distribution of heads during pumping test experiments. Using a conditioning technique, this method could allow inversion of the hydraulic conductivity distribution around a pumping well.

Accurate mapping of the electrical conductivity and redox potential of groundwater is important in delineating the shape of a contaminant plume. A map of redox potential in an aquifer is indicative of biodegradation of organic matter and of concentrations of redox-active components; a map of electrical conductivity provides information on the mineralisation of the groundwater. Both maps can be used to optimise the position of pumping wells for remediation. The self-potential method (SP) and electrical resistivity tomography (ERT) have been applied to the contaminant plume associated with the Entressen landfill in south-east France. The self-potential depends on groundwater flow (electrokinetic contribution) and redox conditions (electro-redox contribution). Using the variation of the piezometric head in the aquifer, the electrokinetic contribution is removed from the SP signals. A good linear correlation (R²=0.85) is obtained between the residual SP data and the redox potential values measured in monitoring wells. This relationship is used to draw a redox potential map of the overall contaminated site. The electrical conductivity of the subsoil is obtained from 3D-ERT analysis. There is good linear correlation (R²=0.91) between the electrical conductivity of the aquifer determined from the 3D-ERT image and the conductivity of the groundwater measured in boreholes. This indicates that the formation factor is nearly homogeneous in the shallow aquifer at the scale of the ERT. From this correlation, a map of the pore water conductivity of the aquifer is obtained.

[1] La Fossa cone is an active stratovolcano located on Vulcano Island in the Aeolian Archipelago (southern Italy). Its activity is characterized by explosive phreatic and phreatomagmatic eruptions producing wet and dry pyroclastic surges, pumice fall deposits, and highly viscous lava flows. Nine 2-D electrical resistivity tomograms (ERTs; electrode spacing 20 m, with a depth of investigation >200 m) were obtained to image the edifice. In addition, we also measured the self-potential, the CO 2 flux from the soil, and the temperature along these profiles at the same locations. These data provide complementary information to interpret the ERT profiles. The ERT profiles allow us to identify the main structural boundaries (and their associated fluid circulations) defining the shallow architecture of the Fossa cone. The hydrothermal system is identified by very low values of the electrical resistivity (<20 W m). Its lateral extension is clearly limited by the crater boundaries, which are relatively resistive (>400 W m). Inside the crater it is possible to follow the plumbing system of the main fumarolic areas. On the flank of the edifice a thick layer of tuff is also marked by very low resistivity values (in the range 1-20 W m) because of its composition in clays and zeolites. The ashes and pyroclastic materials ejected during the nineteenth-century eruptions and partially covering the flank of the volcano correspond to relatively resistive materials (several hundreds to several thousands W m). We carried out laboratory measurements of the electrical resistivity and the streaming potential coupling coefficient of the main materials forming the volcanic edifice. A 2-D simulation of the groundwater flow is performed over the edifice using a commercial finite element code. Input parameters are the topography, the ERT cross section, and the value of the measured streaming current coupling coefficient. From this simulation we computed the self-potential field, and we found good agreement with the measured self-potential data by adjusting the boundary conditions for the flux of water. Inverse modeling shows that self-potential data can be used to determine the pattern of groundwater flow and potentially to assess water budget at the scale of the volcanic edifice. Citation: Revil, A., et al. (2008), Inner structure of La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian Sea, Italy) revealed by high-resolution electric resistivity tomography coupled with self-potential, temperature, and CO 2 diffuse degassing measurements,

The importance of estimating spatially variable aquifer parameters such as transmissivity is widely recognized for studies in resource evaluation and contaminant transport. A useful approach for mapping such parameters is inverse modeling of data from series of pumping tests, that is, via hydraulic tomography. This inversion of field hydraulic tomographic data requires development of numerical forward models that can accurately represent test conditions while maintaining computational efficiency. One issue this presents is specification of boundary and initial conditions, whose location, type, and value may be poorly constrained. To circumvent this issue when modeling unconfined steady-state pumping tests, we present a strategy that analyzes field data using a potential difference method and that uses dipole pumping tests as the aquifer stimulation. By using our potential difference approach, which is similar to modeling drawdown in confined settings, we remove the need for specifying poorly known boundary condition values and natural source/sink terms within the problem domain. Dipole pumping tests are complementary to this strategy in that they can be more realistically modeled than single-well tests due to their conservative nature, quick achievement of steady state, and the insensitivity of near-field response to far-field boundary conditions. After developing the mathematical theory, our approach is first validated through a synthetic example. We then apply our method to the inversion of data from a field campaign at the Boise Hydrogeophysical Research Site. Results from inversion of nine pumping tests show expected geologic features, and uncertainty bounds indicate that hydraulic conductivity is well constrained within the central site area.

[1] Finding the geometry of aquifers in an active volcano is important for evaluating the hazards associated with phreatomagmatic phenomena and incidentally to address the problem of water supply. A combination of electrical resistivity tomography (ERT), self-potential, CO 2 , and temperature measurements provides insights about the location and pattern of ground water flow at Stromboli volcano. The measurements were conducted along a NE-SW profile across the island from Scari to Ginostra, crossing the summit (Pizzo) area. ERT data (electrode spacing 20 m, depth of penetration of $200 m) shows the shallow architecture through the distribution of the resistivities. The hydrothermal system is characterized by low values of the resistivity (<50 W m) while the surrounding rocks are resistive (>2000 W m) except on the North-East flank of the volcano where a cold aquifer is detected at a depth of $80 m (resistivity in the range 70-300 W m). CO 2 and temperature measurements corroborate the delineation of the hydrothermal body in the summit part of the volcano while a negative self-potential anomaly underlines the position of the cold aquifer. Citation:

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We use high-resolution electrical resistivity imaging to delineate the geometry of complex landslides in the Lucanian Apennine chain of southern Italy, to identify the discontinuity between the landslide material and bedrock, and to locate possible surfaces of reactivation. The Lucanian Apennine chain is characterized by high hydrogeological hazard and shows a complete panorama of mass movements. In this area, all typologies of landslides markedly predisposed and tightly controlled by the geostructural characteristics, are found: rotational and translational slides, rototranslational slides, earth and mudflows, as well as deep-seated gravitational slope phenomena with a predominance of rototranslational slides evolving as earthflow slides. Three test sites, characterized by complex geology and a high hydrogeologic hazard, are studied. The Giarrossa and Varco Izzo earthflow slides are located to the west and east of the town of Potenza, whereas the Latronico slide is located close to the town of Latronico. Electrical images are produced from dipole-dipole geoelectric data acquired along arrays spanning selected profiles positioned perpendicular and parallel to the landslide bodies. High-resolution electrical resistivity images are attained through the use of geologic and borehole constraints in the interpretation phase. Integration and comparison of our results with other geophysical data delineate the geometries and hydrologic characteristics of the landslide structures.

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