Advancing quantitative understanding of self-potential signatures in the critical zone through long-term monitoring

Abstract

The self-potential (SP) method is a passive geophysical technique, which may offer insights about water and ionic fluxes in the vadose zone. The main obstacles presently prohibiting its routine use in quantitative vadose zone hydrology are the superposition of signals arising from various source mechanisms, difficult-to-predict electrode polarization effects that depend on electrode design and age, as well as water saturation, pore water chemistry, clay content, and temperature in the immediate vicinity of the electrodes. We present a unique long-term SP monitoring experiment focusing on the first four years of data acquired at different depths in the vadose zone within the HOBE hydrological observatory in Denmark. Using state-of-the-art SP theory combined with flow and transport simulations, we attempt to replicate the observed data and suggest reasons for observed discrepancies. The predictions are overall satisfactory during the first six months of monitoring after which both the patterns and magnitudes of the observed data change drastically. Our main observations are (1) that predicted SP magnitudes are strongly sensitive to how the effective excess charge (or alternatively, the voltage coupling coefficient) scales with water saturation implying that continued research is needed to build more accurate models of electrokinetic phenomena in unsaturated conditions, (2) that significant changes in electrode polarization occur in the shallowest electrodes at time scales of a year, most likely due to desaturation by capillarity of the fluids filling the electrodes, suggesting that electrode effects cannot be ignored and that explicit electrode modeling should be considered in future 3 monitoring studies, and (3) that multi-rate mass transfer and reactive transport modeling, with specific emphasis on fluid-mineral interactions, are needed to better predict salinity and pore water conductivity. We hope to stimulate other researchers to test new SP modeling approaches and interpretation strategies against these data by making the SP and complimentary (temperature, dielectric constant, potential/actual evapotranspiration, precipitation) data time-series available.

An effective excess charge model to describe hysteresis effects on streaming potential

“…As far as reported in literature, this is the first analytical model that accounts this phenomenon in the streaming potential. Therefore, this simple model can be a valuable starting point to the use of the SP method in hydrogeophysics studies to non-intrusively monitor unsaturated groundwater fluxes (e.g., Doussan et al, 2002;Suski et al, 2006;Jougnot et al, 2015;Voytek et al, 2019;Hu et al, 2020) and help to improve the understanding of processes occurring in the vadose zone, such as contaminant plumes (e.g. Naudet et al, 2003;Minsley et al, 2007), hydro-fracturing (e.g.…”

A monitoring system for spatiotemporal electrical self-potential measurements in cryospheric environments

“…We refrained from embedding the electrodes in bentonite clay, due to unknown freezing/thawing behavior and because salt leaching and corresponding electrical diffusion/concentration potentials can be expected in saturated flow regimes (e.g., Perrier et al, 1997), as expected for the test site. As discussed by Hu et al (2020), even in nonfreezing, unsaturated, conditions it seems that the used electrode type will degrade relatively fast. Coupled with the fact that it is still unclear how the unpolarizable electrodes will age in continuous freezing/thawing environments, it is important to carefully monitor electrode performance.…”

“…Traditionally, point measurements of relevant data, such as temperature, ice content, and snow cover thickness, provide the basis for research, whereas remote sensing provides data on surface displacement and actual mass movement events. Noninvasive geophysical imaging techniques, such as electrical resistivity tomography (ERT), ground-penetrating radar (GPR), and seismic refraction tomography have been increasingly investigated to provide structural information on the subsurface of permafrost regions (e.g., Maurer and Hauck, 2007;Hilbich et al, 2008;Hubbard et al, 2013;Merz et al, 2016;Wagner et al, 2019). However, all these methods have in common that they are sensitive to structure only, although limited indirect information on processes can be inferred by time-lapse monitoring setups (e.g., Hilbich et al, 2008Hilbich et al, , 2011Oldenborger and LeBlanc, 2018;Mollaret et al, 2019).…”

“…There is also a steady interest in characterizing landslides using active and passive electrical methods (e.g., Bogoslovsky and Ogilvy, 1977;Naudet et al, 2008;Heinze et al, 2019;Whiteley et al, 2019, and references therein). Recently, Hu et al (2020) discussed a 4-year multi-electrode time series of borehole SP measurements in a hydrological observatory, comparing measured data with modeled SP data, while taking into account water flow and transport processes.…”

“…Perrier et al (1997) present valuable information on electrode characteristics, long-term stability, and suggested installation methods, as well as a discussion of useful long-term monitoring preparations. With respect to technical aspects of long-term electrode behavior in unsaturated environments, Hu et al (2020) offer important insight into the electro-chemical details that lead to electrode effects superimposing on the process signals. They conclude that explicit electrode modeling is required in order to explain long-term monitoring SP signatures.…”