The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m−1, NaCl, 25°C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in‐phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.

We investigate the thermal dependence of the complex conductivity of nine porous materials in the temperature range +20°C to −10 or −15°C. The selected samples include three soils, two granites, three clay-sands mixes, and one graphitic tight sandstone. A total of 12 experiments is conducted with one sample tested at three different salinities. Our goal is to use this database to extend the dynamic Stern layer polarization model in freezing conditions. We observe two polarization mechanisms, one associated with the effect of the change in the liquid water content and its salinity upon the polarization of the porous material. A second mechanism, at higher frequencies (>10 Hz), is likely associated with the polarization of ice. At low frequencies and above the freezing point, the in-phase and quadrature conductivities depend on temperature in a predictable way. This dependence is due to the dependence of the mobility of the charge carriers with temperature. Below the freezing point, the in-phase and quadrature conductivity follow a brutal decay with temperature. This dependence is modeled through an exponential freezing curve function. We were also able to determine how the (apparent) formation factor and surface conductivity change with temperature and water content below the freezing point. Our model is able to replicate the data at low frequencies and predicts correctly the fact that the ratio between the normalized chargeability and the surface conductivity is independent of the water content and temperature and equals a well-defined dimensionless number R.

Three‐dimensional electrical resistivity and induced polarization data were collected on an unstable Alpine rock glacier in Val Thorens (Vanoise massif, France). In addition to these field data, we also performed induced polarization measurements during freeze and thaw using a soil sample and the poorly mineralized water, both from this site. In the tomograms, the electrical conductivity and the normalized chargeability show very distinctly the presence of the rock ice mixture. The chargeability itself is however quite constant over the entire investigated area with the exception of a small area associated with the presence of carboniferous rocks rich in graphite. The background chargeability is close to the dimensionless number R = 8 × 10−2, which is consistent with the prediction of the dynamic Stern layer model for the polarization of porous media. The theory implies that this dimensionless number R is independent on saturation and temperature in agreement with field observations. A main implication of this observation is that the classical Archie's law cannot be applied to describe the electrical conductivity in this type of environments with poorly mineralized pore water. Surface conductivity dominates the measured conductivity of the materials implying in turn that the electrical conductivity is related to both the water content and cation exchange capacity of the material. We propose new equations for both the electric conductivity and the normalized chargeability in this type of environments.

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We have investigated the influence of temperature and salinity upon the spectral induced polarization of 10 samples including rocks with their mineralization (galena, chalcopyrite) plus sand mixed with semiconductors such as magnetite grains, graphite, and pyrite cubes of two different sizes. Measurements are made in a temperature-controlled bath with a high-precision impedance meter and using NaCl solutions. We cover the temperature range 5°C−50°C and the frequency range [Formula: see text] to 45 kHz. For one large pyrite cube, we also investigated six salinities from 0.1 to [Formula: see text] (at 25°C, NaCl) and three salinities for graphite. The spectra are fitted with a Cole-Cole complex parametric conductivity model for which we provide a physical meaning to the four Cole-Cole parameters. As expected, the Cole-Cole exponent and the chargeability are independent of the temperature and salinity. The instantaneous and steady state (direct current [DC]) conductivities depend on the salinity and temperature. This temperature dependence can be fitted with an Arrhenius law (combining the Stokes-Einstein and Vogel-Fulcher-Tammann equations) with an activation energy in the range of [Formula: see text]. This activation energy is the same as for the bulk pore-water conductivity demonstrating the control by the background electrolyte of these quantities, as expected. The instantaneous and DC conductivities depend on the salinity in a predictable way. The Cole-Cole relaxation time decreases with the temperature and decreases with the salinity. This behavior can be modeled with an Arrhenius law with an apparent activation energy of [Formula: see text]. A finite-element model is used further to analyze the mechanisms of polarization, and it can reproduce the temperature and salinity dependencies observed in the laboratory.

Electrical conductivity tomography is a well-established galvanometric method for imaging the subsurface electrical conductivity distribution. We characterize the conductivity distribution of a set of volcanic structures that are different in terms of activity and morphology. For that purpose, we developed a large-scale inversion code named ECT-3D aimed at handling complex topographical effects like those encountered in volcanic areas. In addition, ECT-3D offers the possibility of using as input data the two components of the electrical field recorded at independent stations. Without prior information, a Gauss-Newton method with roughness constraints is used to solve the inverse problem. The roughening operator used to impose constraints is computed on unstructured tetrahedral elements to map complex geometries. We first benchmark ECT-3D on two synthetic tests. A first test using the topography of Mt. St Helens volcano (Washington, USA) demonstrates that we can successfully reconstruct the electrical conductivity field of an edifice marked by a strong topography and strong variations in the resistivity distribution. A second case study is used to demonstrate the versatility of the code in using the two components of the electrical field recorded on independent stations along the ground surface. Then, we apply our code to real data sets recorded at (i) a thermally active area of Yellowstone caldera (Wyoming, USA), (ii) a monogenetic dome on Furnas volcano (the Azores, Portugal), and (iii) the upper portion of the caldera of Kīlauea (Hawai'i, USA). The tomographies reveal some of the major structures of these volcanoes as well as identifying alteration associated with high surface conductivities. We also review the petrophysics underlying the interpretation of the electrical conductivity of fresh and altered volcanic rocks and molten rocks to show that electrical conductivity tomography cannot be used as a stand-alone technique due to the non-uniqueness in interpreting electrical conductivity tomograms. That said, new experimental data provide evidence regarding the strong role of alteration in the vicinity of preferential fluid flow paths including magmatic conduits and hydrothermal vents.

Induced polarization well logging can be used to characterize sedimentary formations and their petrophysical properties of interest. That said, nothing is really known regarding the complex conductivity of low-porosity sedimentary rocks. To fill this gap of knowledge, we investigate the complex conductivity of 19 tight sandstones, one bioclastic turbidite, and four sand/smectite mixes. The sandstones and the bioclastic turbidite are characterized by low to very low porosities (in the range of 0.8%–12.3%) and a relatively narrow range of cation exchange capacity (CEC — [Formula: see text]). The sand-clay mixtures are prepared with pure smectite (Na-Montmorillonite, porosity approximately 90%, CEC [Formula: see text]) and a coarse sand (grain size approximately [Formula: see text]). Data quality is assessed by checking that the percentage frequency effect between two frequencies separated by a decade is proportional to the value of the phase lag measured at the geometric frequency. We also checked that the normalized chargeability determined between two frequencies is proportional to the quadrature conductivity at the geometric mean frequency. Our experimental results indicate that the surface conductivity, the normalized chargeability, and the quadrature conductivity are highly correlated to the ratio between the CEC and the bulk tortuosity of the pore space. This tortuosity is obtained as the product of the (intrinsic) formation factor with the (connected) porosity. The quadrature conductivity is proportional to the surface conductivity. All these observations are consistent with the predictions of the dynamic Stern layer model, which can be used to assess the magnitude of the polarization associated with these porous media over the full range of porosity. The next step will be to extend and assess this model to partially saturated sandstones.

Smectite-rich clay caps form permeability seals in geothermal systems. The presence of smectite is also responsible for a strong surface (interfacial) electrical conductivity and polarization due to their electrical double layer properties. We developed new complex conductivity models using both differential effective medium (DEM) and volume averaging theories accounting for both conduction and polarization of these high cation exchange capacity (CEC) materials. These models predict that the chargeability is also a non-linear function of the pore water conductivity reaching a constant value at pore water conductivity far above the so-called iso-conductivity point. The iso-conductivity point is characterized by the equality between the conductivity of the rock and the conductivity of the pore water. We apply the DEM conductivity model (which requires only two textural parameters) to smectite-rich volcanic and sedimentary rocks using data sets from the literature. When smectite is present in the volcanic rocks, the CEC of the rock is dominated by the CEC of smectite. The grain conductivity and the normalized chargeability are related to each other by a dimensionless number R = 0.10 (independent of temperature and saturation) and both are controlled by the excess of charge per unit pore volume Q V , which can be determined from the CEC and porosity. Our petrophysical model is also able to predict the permeability of the rock as well from the CEC and the porosity. It is applied to a 3-D data set at Krafla volcano (Iceland). The porosity, the CEC, the percentage of smectite, and the permeability of the clay-cap are imaged by 3-D induced polarization tomography. Electrical conductivity tomography alone does not allow separation of the contribution of the bulk pore space from the interfacial properties related to alteration and therefore should be used with caution.