Redox potential distribution inferred from self‐potential measurements associated with the corrosion of a burden metallic body

Abstract

International audienceNegative self-potential anomalies can be generated at the ground surface by ore bodies and ground water contaminated with organic compounds. These anomalies are connected to the distribution of the redox potential of the ground water. To study the relationship between redox and self-potential anomalies, a controlled sandbox experiment was performed. We used a metallic iron bar inserted in the left-hand side of a thin Plexiglas sandbox filled with a calibrated sand infiltrated by an electrolyte. The self-potential signals were measured at the surface of the tank (at different time lapses) using a pair of non-polarizing electrodes. The self-potential, the redox potential, and the pH were also measured inside the tank on a regular grid at the end of the experiment. The self-potential distribution sampled after six weeks presents a strong negative anomaly in the vicinity of the top part of the iron bar with a peak amplitude of −82 mV. The resulting distributions of the pH, redox, and self-potentials were interpreted in terms of a geobattery model combined with a description of the electrochemical mechanisms and reactions occurring at the surface of the iron bar. The corrosion of iron yields the formation of a resistive crust of fougerite at the surface of the bar. The corrosion modifies both the pH and the redox potential in the vicinity of the iron bar. The distribution of the self-potential is solved with Poisson's equation with a source term given by the divergence of a source current density at the surface of the bar. In turn, this current density is related to the distribution of the redox potential and electrical resistivity in the vicinity of the iron bar. A least-squares inversion method of the self-potential data, using a 2D finite difference simulation of the forward problem, was developed to retrieve the distribution of the redox potential

Estimation of the electrical potential distribution along metallic casing from surface self-potential profile

“…The SP profile shown on Fig. 1 exhibits a negative anomaly, reaching about -160 mV close to the casing, and returning to zero at around 15 m. The shape is very similar to those reported by Castermant et al (2008) in a laboratory experiment in which a metallic rod was let free to corrode in a tank filled with saturated sand. The idea was to use the SP profile from the piezometer to correct the observed SP anomaly on the ISR cell.…”

“…This mechanism has been successfully applied to explain the SP anomaly observed near the KTB deep well and resulting from the existence of graphitic dykes (Bigalke and Grabner, 1997). Moreover, the signal can evolve with time, when corrosion phenomena develop (e.g., Castermant et al, 2008;Rittgers et al, 2013). When measuring SP potential during pumping tests in the case of metallic casing, the anomaly resulting from the fluid flow can be obtained by subtracting the SP values measured before the test (i.e., resulting uniquely from the geobattery effect).…”

“…The idea was to use the SP profile from the piezometer to correct the observed SP anomaly on the ISR cell. In theory, from previous laboratory experiments, the potential at the surface resulting from the corrosion of the well casing can be modelled (e.g., Castermant et al, 2008), provided that information about the properties of the constituents of the casing and the chemistry of the medium, such as pH and redox potential distributions, are known. Unfortunately, in the field we did not have this information.…”

“…In the previous laboratory studies published in the literature (Castermant et al, 2008;Rittgers et al, 2013), retrieval of potential along the casing was performed from 3D (whole space) or 2D (whole surface) measurements of SP, using deterministic algorithms. In our case, we only have a 1D…”

Microbial ecology and geoelectric responses across a groundwater plume

“…Cathodic reactions driving electrical currents have been demonstrated in geobattery models (Castermant et al 2008) and in MFCs (Rabaey and Keller 2008;Oh, Min, and Logan 2004), where electron consumption at the cathode acts as driving force of the current generated.…”