Self-potential signals generated by the corrosion of buried metallic objects with application to contaminant plumes

Abstract

Large-amplitude ([Formula: see text]) 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 [Formula: see text] 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.

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

“…This distribution is very similar to the laboratory results obtained by Castermant et al (2008) and Rittgers et al (2013), making us confident in the efficiency of our algorithm. Note here that we introduce a non-deterministic approach for using SP surface profiles (largely badly constrained problem), contrary to Rittgers et al (2013) who used deterministic approach for 3D SP measurements in the whole space (relatively well constrained problem), allowing us to establish a statistics on the possible potential distribution.…”

“…The retrieved distribution of potential in the whole space (Fig. 12) compared in a very satisfactorily way to the observations made by Rittgers et al (2013) during the corrosion of a small vertical metallic hollow cylinder in a sandbox connecting a saturated and a unsaturated zone in a metric sandbox, being negative in the upper zone, and positive in the lower one. This coherent result made us very confident with our inversion procedure.…”

“…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).…”

“…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

“…This would allow for a more robust modelling approach rather the simple empirical methods that may not always adhere to the geometry of the electric field that are adopted here. Such experiments could also allow changes in water table which could mimic the seasonal changes noted by (Giampaolo et al 2014) which could also have a significant role in iron cycling (Rittgers et al 2013) where our long term datasets at this site were lacking. Unclassified OTUs also represent a significant gap in our full understanding of the microbial ecology of the site these could represent additional degradative and electrogenic species.…”