Improved Dielectric and Electrical Conductivity Anisotropy Measurements Using TDR in Unsaturated Mica

Abstract

Anisotropy in the vadose zone impacts the flow and transport of water and contaminants. Progress has been made in incorporating anisotropy into flow and transport models; however, obtaining accurate estimation of the extent of anisotropy in porous media remains a technical challenge. Electrical and electromagnetic (EM) measurements can be used to investigate the anisotropy of layered media. We developed a parallel‐plate time domain reflectometry (TDR) and electrical conductivity cell for accurately measuring the sample‐scale dielectric (AKa) and electrical conductivity (Aσ) anisotropy factors and used a dielectric mixture model to predict the theoretical dielectric anisotropy factor (Aϵ). Modeling of the cell EM transmission line sampling area based on electrode geometry facilitated an optimal design based on minimizing the sampling area coefficient of variation. This optimal design consists of thin (1‐mm) parallel plates with a spacing/width ratio of 1. Dielectric and electrical conductivity measurements were made in mica, layered parallel to the plates in one cell and layered perpendicular to the plates in another. The resulting water‐content‐dependent dielectric measurements yielded sample‐scale AKa factors with peak values of 2.4 occurring near 50% saturation, while electrical conductivity measurements found Aσ values approaching 10 near 75% saturation. Furthermore, the shape of the water‐content‐dependent AKa followed a Gaussian distribution, while a three‐phase dielectric mixture model predicted peak Aϵ near 40% saturation. Discrepancies in magnitude were attributed to model assumptions, packing irregularities, and heterogeneity in the mica packing structure, as noted by others. The occurrence of the peak AKa around 50% saturation corroborates anisotropy predictions for transport related to hydraulic processes.