Self-Potential Signals Generated by the Corrosion of Buried Metallic Objects with Application to Contaminant Plumes

Large-amplitude (\u3e100 mV) negative electric (self)-potential anomalies are often observed in the vicinity of buried metallic objects and ore bodies or over groundwater plumes associated with organic contaminants. To explain the physical and chemical mechanisms that generate such electrical signals, a controlled laboratory experiment was carried out involving two metallic cylinders buried with vertical and horizontal orientations and centered through and in the capillary fringe within a sandbox. The 2D and 3D self-potential (SP) data were collected at several time steps along with collocated pH and redox potential measurements. Large dipolar SP and redox potential anomalies developed in association with the progressive corrosion of the vertical pipe, although no anomalies were observed in the vicinity of the horizontal pipe. This discrepancy was due to the orientation of the pipes with the vertical pipe subjected to a significantly larger EH gradient. Accounting for the electrical conductivity distribution, the SP data were inverted to recover the source current density vector field using a deterministic least-squares 4D (time-lapse) finite-element modeling approach. These results were then used to retrieve the 3D distribution of the redox potential along the vertical metallic cylinder. The results of the inversion were found to be in excellent agreement with the measured distribution of the redox potential. This experiment indicated that passively recorded electrical signals can be used to nonintrusively monitor corrosion processes. In addition, vertical electrical potential profiles measured through a mature hydrocarbon contaminated site were consistent with the sandbox observations, lending support to the geobattery model over organic contaminant plumes

Complex conductivity of soils

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 m21, NaCl, 258C) 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

Thermoelectric self-potential and resistivity data localize the burning front of underground coal fires

Self-potential signals and resistivity data can be jointly inverted or analyzed to track the position of the burning front of an underground coal-seam fire. We first investigate the magnitude of the thermoelectric coupling associated with the presence of a thermal anomaly (thermoelectric current associated with a thermal gradient). A sandbox experiment is developed and modeled to show that in presence of a heat source, a negative self-potential anomaly is expected at the ground surface. The expected sensitivity coefficient is typically on the order of -0.5 mV degrees C-1 in a silica sand saturated by demineralized water. Geophysical field measurements gathered at Marshall (near Boulder, CO) show clearly the position of the burning front in the electrical resistivity tomogram and in the self-potential data gathered at the ground surface with a negative self-potential anomaly of about -50 mV. To localize more accurately the position of the burning front, we developed a strategy based on two steps: (1) We first jointly invert resistivity and self-potential data using a cross-gradient approach, and (2) a joint interpretation of the resistivity and self-potential data is made using a normalized burning front index (NBI). The value of the NBI ranges from 0 to 1 with 1 indicating a high probability to find the burning front (strictly speaking, the NBI is, however, not a probably density). We validate first this strategy using synthetic data and then we apply it to the field data. A clear source is localized at the expected position of the burning front of the coal-seam fire. The NBI determined from the joint inversion is only slightly better than the value determined from independent inversion of the two geophysical data sets

Complex electrical conductivity model, influence of surface conductivity and new strategies for the inversion of resistivity and induced polarization data

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Time-lapse three-dimensional inversion of complex conductivity data using an active time constrained (ATC) approach

Induced polarization (more precisely the magnitude and phase of impedance of the subsurface) is measured using a network of electrodes located at the ground surface or in boreholes. This method yields important information related to the distribution of permeability and contaminants in the shallow subsurface. We propose a new time-lapse 3-D modelling and inversion algorithm to image the evolution of complex conductivity over time.We discretize the subsurface using hexahedron cells. Each cell is assigned a complex resistivity or conductivity value. Using the finite-element approach, we model the in-phase and out-of-phase (quadrature) electrical potentials on the 3-D grid, which are then transformed into apparent complex resistivity. Inhomogeneous Dirichlet boundary conditions are used at the boundary of the domain. The calculation of the Jacobian matrix is based on the principles of reciprocity. The goal of time-lapse inversion is to determine the change in the complex resistivity of each cell of the spatial grid as a function of time. Each model along the time axis is called a ‘reference space model’. This approach can be simplified into an inverse problem looking for the optimum of several reference space models using the approximation that the material properties vary linearly in time between two subsequent reference models. Regularizations in both space domain and time domain reduce inversion artefacts and improve the stability of the inversion problem. In addition, the use of the time-lapse equations allows the simultaneous inversion of data obtained at different times in just one inversion step (4-D inversion). The advantages of this new inversion algorithm are demonstrated on synthetic time-lapse data resulting from the simulation of a salt tracer test in a heterogeneous random material described by an anisotropic semi-variogram

Imaging with cross-hole seismoelectric tomography

International audienceWe propose a cross-hole imaging approach based on seismoelectric conversions (SC) associated with the transmission of seismic waves from seismic sources located in a borehole to receivers (electrodes) located in a second borehole. The seismoelectric (seismic-to-electric) problem is solved using Biot theory coupled with a generalized Ohm's law with an electrokinetic streaming current contribution. The components of the displacement of the solid phase, the fluid pressure, and the electrical potential are solved using a finite element approach with Perfect Match Layer (PML) boundary conditions for the seismic waves and boundary conditions mimicking an infinite material for th electrostatic problem. We develop an inversion algorithm using the electrical disturbances recorded in the second borehole to localize the position of the heterogeneities responsible for the SC. Because of the ill-posed nature of the inverse problem (inherent to all potential-field problems), regularization is used to constrain the solution at each time in the SC-time window comprised between the time of the seismic shot and the time of the first arrival of the seismic waves in the second borehole. All the inverted volumetric current source densities are aggregated together to produce an image of the position of the heterogeneities between the two boreholes. Two simple synthetic case studies are presented to test this concept. The first case study corresponds to a vertical discontinuity between two homogeneous sub-domains. The second case study corresponds to a poroelastic inclusion (partially saturated by oil) embedded into an homogenous poroelastic formation. In both cases, the position of the heterogeneity is recovered using only the electrical disturbances associated with the SC. That said, a joint inversion of the seismic and seismoelectric data could improve these results

Induced polarization tomography applied to the detection and the monitoring of leaks in embankments

International audienceDuring an induced polarization survey, both electrical conductivity and chargeability can be imaged. Recent petrophysical models have been developed to provide a consistent picture of these two parameters in terms of water and clay contents of soils. We test the ability of this method at a test site in which a controlled artificial leakage can be generated in an embankment surrounding an experimental basin. 3D tomography of the conductivity and normalized chargeability are performed during such a controlled leakage. Conductivity and induced polarization measurements were also performed on a core sample from the site. The sample was also characterized in terms of porosity and cation exchange capacity. Combining the 3D survey and these laboratory measurements, a 3D tomogram of the relative variation in water content (before leakage and during leakage) was estimated. It clearly exhibits the ground water flow path through the embankment from the outlet of the tube used to generate the leak to the bottom of the embankment. In addition, a self-potential survey was performed over the zone of leakage. This survey evidences also the projection of the ground water flow path over the ground surface. Both methods are found to provide a consistent picture. A 2.5D time lapse tomography of the electrical conductivity and normalized chargeability was also performed and evidences the position of the preferential flow paths below the profile. These results confirm the ability and efficiency of induced polarization to provide reliable information pertaining to the detection of leakages in dams and embankments

Characterization of Focused Seepage Through an Earthfill Dam Using Geoelectrical Methods

International audienceResistivity and self‐potential tomography can be used to investigate anomalous seepage inside heterogeneous earthen dams. The self‐potential (SP) signals provide a unique signature to groundwater flow because the source current density responsible for the SP signals is proportional to the Darcy velocity. The distribution of the SP signals is also influenced by the distribution of the resistivity; therefore, resistivity and SP need to be used in concert to elucidate groundwater flow pathways. In this study, a survey is conducted at a small earthen dam in Colorado where anomalous seepage is observed on the downstream face at the dam toe. The data reveal SP and direct current resistivity anomalies that are used to delineate three anomalous seepage zones within the dam and to estimate the source of the localized seepage discharge. The SP data are inverted in two dimensions using the resistivity distribution to determine the distribution of the Darcy velocity responsible for the observed seepage. The inverted Darcy velocity agrees with an estimation of the Darcy velocity from the hydraulic conductivity obtained from a slug test and the observed head gradient