Development of geoelectrical monitoring of the critical zone processes on microfluidic chips

Abstract

We miniaturize the low-frequency (<1kHz) geoelectrical acquisition using advanced micro-fabrication technologies to investigate coupled processes in the critical zone (CZ). With this innovation in the experimental acquisition, we focus on the development of the complex electrical conductivity monitoring with the spectral induced polarization (SIP) method. The interpretation of the SIP signal is based on the development of petrophysical models that relate the complex electrical conductivity to structural, hydrodynamical, and geochemical properties or distributions. State-of-the-art petrophysical models, however, suffer from a limited range of validity and presume too many microscopic mechanisms to define macroscale parameters. Thus, direct observations of the underlying processes coupled with geoelectrical monitoring are keys to deconvolute the signature of the biochemical-physical mechanisms and, then, developing more reliable models. Microfluidic experiments enable direct visualization of flows, reactions, and transport at the pore-scale thanks to transparent micromodels coupled with optical microscopy and high-resolution imaging techniques. Micromodels are a two-dimensional representation of the porous medium, ranging in complexity from single channels to replicas of natural rocks. Cutting-edge micromodels use reactive minerals to investigate the water-mineral interactions involved in the CZ. In this work, we propose a new kind of micromodels equipped with four aligned electrodes within the channel for SIP monitoring of calcite dissolution, a key multiphase process of the CZ involved in karstification. We highlight the strong correlation between SIP response and dissolution through electrical signal examination and image analysis. In particular, degassed CO2 bubbles generated by the dissolution play a major role.  Our technological advancement brings a deeper understanding of the physical interpretation of the complex electrical conductivity and will provide a further understanding of the CZ dynamic processes through SIP observation.