Streaming Potential Coupling Coefficient Research Articles

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 CO2 diffuse degassing measurements

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 CO2 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 Ω m). Its lateral extension is clearly limited by the crater boundaries, which are relatively resistive (>400 Ω 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 Ω 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 Ω 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.

Forward Modeling and validation of a new formulation to compute self-potential signals associated with ground water flow

Abstract. The classical formulation of the coupled hydroelectrical flow in porous media is based on a linear formulation of two coupled constitutive equations for the electrical current density and the seepage velocity of the water phase and obeying Onsager's reciprocity. This formulation shows that the streaming current density is controlled by the gradient of the fluid pressure of the water phase and a streaming current coupling coefficient that depends on the so-called zeta potential. Recently a new formulation has been introduced in which the streaming current density is directly connected to the seepage velocity of the water phase and to the excess of electrical charge per unit pore volume in the porous material. The advantages of this formulation are numerous. First this new formulation is more intuitive not only in terms of establishing a constitutive equation for the generalized Ohm's law but also in specifying boundary conditions for the influence of the flow field upon the streaming potential. With the new formulation, the streaming potential coupling coefficient shows a decrease of its magnitude with permeability in agreement with published results. The new formulation has been extended in the inertial laminar flow regime and to unsaturated conditions with applications to the vadose zone. This formulation is suitable to model self-potential signals in the field. We investigate infiltration of water from an agricultural ditch, vertical infiltration of water into a sinkhole, and preferential horizontal flow of ground water in a paleochannel. For the three cases reported in the present study, a good match is obtained between finite element simulations performed and field observations. Thus, this formulation could be useful for the inverse mapping of the geometry of groundwater flow from self-potential field measurements.

A laboratory apparatus for streaming potential and resistivity measurements on soil samples

We describe an apparatus designed to perform streaming potential and resistivity measurements on unconsolidated soil samples. The apparatus enables the use of both unidirectional and oscillatory flow methods to measure the streaming potential coupling coefficient (C); the direct current resistivity method is used to measure the bulk resistivity (rho) of the soil sample. Measuring both of these parameters on the same sample under the same conditions enables us to properly characterize the streaming current cross-coupling coefficient (L). The apparatus is designed to test reconstituted saturated soil samples up to a maximum grain size of 9.5 mm, and hydraulic gradients from less than 0.1 up to a maximum of 4 m of H(2)Om in flow-through experiments. Excellent agreement between C values measured using the unidirectional and oscillatory flow methods validates the oscillatory flow method for unconsolidated samples.

Streaming potentials of granular media: Influence of the Dukhin and Reynolds numbers

Laboratory experiments are performed to understand the controlling parameters of the electrical field associated with the seepage of water through a porous material. We use seven glass bead packs with varying mean grain size in an effort to obtain a standard material for the investigation of these electrical potentials. The mean grain size of these samples is in the range 56–3000 μm. We use pure NaCl electrolytes with conductivity in the range 10−4 to 10−1 S m−1 at 25°C. The flow conditions cover viscous and inertial laminar flow conditions but not turbulent flow. In the relationship between the streaming potential coupling coefficient and the grain size, three distinct domains are defined by the values of two dimensionless numbers, the Dukhin and the Reynolds numbers. The Dukhin number represents the ratio between the surface conductivity of the grains (due to conduction in the electrical double layer coating the surface of the grains) and the pore water electrical conductivity. At high Dukhin numbers (≫1) and low Reynolds numbers (≪1), the magnitude of the streaming potential coupling coefficient decreases with the increase of the Dukhin number and depends on the mean grain diameter (and therefore permeability) of the medium. At low Dukhin and Reynolds numbers (≪1), the streaming potential coupling coefficient becomes independent of the microstructure and is given by the well‐known Helmholtz‐Smoluchowski equation widely used in the literature. At high Reynolds numbers, the magnitude of the streaming potential coupling coefficient decreases with the increase of the Reynolds number in agreement with a new model developed in this paper. A numerical application is made illustrating the relation between the self‐potential signal and the intensity of seepage through a leakage in an embankment.

Electrokinetic coupling in unsaturated porous media

We consider a charged porous material that is saturated by two fluid phases that are immiscible and continuous on the scale of a representative elementary volume. The wetting phase for the grains is water and the nonwetting phase is assumed to be an electrically insulating viscous fluid. We use a volume-averaging approach to derive the linear constitutive equations for the electrical current density as well as the seepage velocities of the wetting and nonwetting phases on the scale of a representative elementary volume. These macroscopic constitutive equations are obtained by volume-averaging Ampère's law together with the Nernst–Planck equation and the Stokes equations. The material properties entering the macroscopic constitutive equations are explicitly described as functions of the saturation of the water phase, the electrical formation factor, and parameters that describe the capillary pressure function, the relative permeability functions, and the variation of electrical conductivity with saturation. New equations are derived for the streaming potential and electro-osmosis coupling coefficients. A primary drainage and imbibition experiment is simulated numerically to demonstrate that the relative streaming potential coupling coefficient depends not only on the water saturation, but also on the material properties of the sample, as well as the saturation history. We also compare the predicted streaming potential coupling coefficients with experimental data from four dolomite core samples. Measurements on these samples include electrical conductivity, capillary pressure, the streaming potential coupling coefficient at various levels of saturation, and the permeability at saturation of the rock samples. We found very good agreement between these experimental data and the model predictions.

Electrical properties of zeolitized volcaniclastic materials

The interpretation of electromagnetic anomalies associated with volcanic activity requires a good understanding of two rock properties, the electrical conductivity and the streaming potential coupling coefficient. We measured these properties on 22 consolidated tuff samples containing clays and zeolites typically found in hydrothermal systems or in other areas of high water‐rock interactions associated with active volcanic areas. These rocks exhibit unusually high surface conductivity and electrical cementation exponent (2.5–3.3). These features are explained by the highly complex texture of these rocks due to hydration/dissolution processes of the volcanic glass and the formation of clay minerals and zeolites as alteration products. At a pore fluid conductivity of 1.0 ± 0.2 S m−1, the streaming potential coupling coefficient ranges from −3.55 to −10.7 mV MPa−1. The zeta potential (a key electrochemical property of the pore water/mineral interface) determined from these measurements at T0 = 20°C and pH ∼6–7 and corrected for surface conductivity is ∼−28 ± 8 mV at 0.1 M NaCl. Two clay‐free samples exhibit a zeta potential ∼−16 ± 5 mV, a value associated with zeolites (clinoptilolite). The temperature dependence of the ζ potential is rather strong with ζ(T) = ζ(T0) [1 + νζ (T − T0)], where νζ ≈ 4.2 × 10−2 C−1 in the temperature range 20–60°C. These data are applied to the understanding of large‐scale self‐potential anomalies located at the ground surface of Miyake‐Jima volcano.

Streaming electrical potential anomaly along faults in geothermal areas

Electrical potential anomalies are often measured associated with geothermal areas and volcanoes. In these systems, fluid flow is usually mostly restricted to faults and fracture networks. An equation describing electrical potential anomalies of electrokinetic nature associated with fluid upflow induced by a thermal source along faults is derived. The electrical potential anomaly is related to the depth of the thermal reservoir, the temperature difference between the surface and the reservoir of the geothermal area, the thermal expansion of water, and the streaming electrical potential coupling coefficient in the fault zone. A quantitative calculation of the electrokinetic anomaly is provided by comparing this model to a self‐potential survey of the Cerro‐Prieto geothermal field [Fitterman and Corwin, 1982]. The predictions of the model agree well with the field measurements.

Inhibition of Cartilage Degradation and Changes in Physical Properties Induced by IL-1β and Retinoic Acid Using Matrix Metalloproteinase Inhibitors

Bovine cartilage explants were treated with 100 ng/ml recombinant human interleukin-1β (IL-1β) or 1 μmall-transretinoic acid (RA) and changes in biochemical, biomechanical, and physicochemical properties were assessed. Additionally, samples cultured with IL-1β or RA were treated with 4 μmrecombinant human tissue inhibitor of metalloproteinases-1 (TIMP-1) or a synthetic metalloproteinase inhibitor (L-758,354) to inhibit this degradation. Treatment with IL-1β or RA each resulted in >90% GAG loss after 8 days in culture. Addition of TIMP or L-758,354 to the culture media inhibited IL-1β-induced loss of tissue GAG by 40 and 65%, respectively, and inhibited RA-induced GAG loss by 35 and 65%, respectively. Analysis of degradation products in the culture media using a G1 antibody indicated that IL-1β- and RA-treated plugs released 68-kDa fragments of aggrecan, corresponding to a segment of the aggrecan core protein from the G1 domain to the C-terminus NITEGE, consistent with “aggrecanase” activity. Release of the G1 fragment was inhibited by treatment with L-758,354. Both IL-1β and RA induced significant loss of hyaluronan from cartilage explants after 8 days of exposure and HA loss was also inhibited by addition of L-756,354 to the culture media. IL-1β, but not RA, induced a significant increase in swelling ratio (wet weight in 0.01mNaCl normalized to wet weight in DMEM) after 8 days in culture, consistent with degradation of the collagen network, and the increase in tissue swelling was inhibited by treatment with TIMP-1 or L-758,354. Exposure to IL-1β or RA resulted in significant changes in cartilage physical properties including streaming potential, equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient after 8 days in culture, and these changes were inhibited by 40–90% by exposure to TIMP and by 50–90% by exposure to L-758,354. Measurement of dynamic streaming potential showed that changes due to treatment with IL-1β alone were highly dependent in compression frequency, with dramatic changes seen at high frequency prior to changes in mechanical properties, and little initial change seen at low frequency. Streaming potential and equilibrium modulus of explants treated with RA decreased to 10% of their initial values after 8 days in culture, but decreased to only 40 and 90%, respectively, when treated with RA plus TIMP-1.

Streaming potential from multiphase flow

In trying to understand the affect of electrokinetics on the spontaneous potential (SP) log, the focus has generally been on the solid‐brine streaming potential. Within the accuracy of the measurements, the streaming‐potential coupling coefficient is shown to be independent of the permeability of the rock. The solid‐brine streaming potential is of much smaller magnitude than the electrostatic potentials from gas‐liquid and liquid‐liquid flow. Air bubbles were found to increase the streaming potential coupling coefficient by more than two orders of magnitude over the value for single‐phase brine flow. Thus, two‐phase gas‐liquid flow is more likely to have a significant impact on the SP log than is single phase liquid flow. Two‐phase oil‐brine flow may also produce a larger electrokinetic potential than single‐phase flow. The magnitude of the electrokinetic potential caused by oil‐brine flow will depend on the composition of the oil and the brine. Trace materials can have a major impact on the electrokinetic potential of hydrocarbons. In a system with multiphase flow, the solid‐liquid interaction is probably the smallest component of the electrokinetic potential.