Effect of degree of saturation on the electrical conductivity of soils: role of surface conduction

Abstract

The estimation of the degree of saturation (S) based on electrical conductivity, σmix (or electrical resistivity) surveys typically relies on the S-exponent of Archie’s equation. However, the S-exponent varies with soil type, indicating significant variability and the potential for miscalculation of S. This study focuses on the variability of the S-exponent in Archie’s equation across soil types and investigates the relationship between σmix and S in terms of pore water conduction (Kw) and surface conduction (Ks) through theoretical modeling and experimentation. Silica sand and two types of clays were tested at various initial porosities and pore water concentrations, and σmix was measured over a range of S values (20% to 100%). For sand, where surface conduction is negligible, Archie’s equation with a constant S-exponent (approximately 2) accurately predicted σmix. However, clays exhibited varying S-exponents, ranging from 1.5 to 2.1, depending on pore water conductivity. This suggests that a new model is needed to accurately predict σmix in unsaturated clayey soils. A newly proposed σmix estimation formula, incorporating separate S-exponents for Kw and Ks, provided accurate predictions across various test conditions. These findings enhance the understanding of electrical conductivity in unsaturated soils and offer a simple yet effective model for its prediction.