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, σ_mix (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 σ_mix and S in terms of pore water conduction (K_w) and surface conduction (K_s) through theoretical modeling and experimentation. Silica sand and two types of clays were tested at various initial porosities and pore water concentrations, and σ_mix 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 σ_mix. 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 σ_mix in unsaturated clayey soils. A newly proposed σ_mix estimation formula, incorporating separate S-exponents for K_w and K_s, 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.