A study on the mixed dielectric model of red clay

When ground-penetrating radar (GPR) detects red clay, the electric field induces polarization, causing corresponding characteristics in the dielectric constant and affecting GPR's inversion of soil moisture content. This paper investigates the physical properties of Guiyang red clay, including its composition and liquid-plastic limits. Based on the electric double-layer structure on clay particle surfaces, bound water is classified into strongly and weakly bound water. Isothermal adsorption tests determined that the maximum strongly and weakly bound water contents of Guiyang red clay are 2.9% and 17.2%, respectively. Drawing on the unfrozen water model to calculate bound water content, a four-component dielectric mixing algorithm was applied to establish a comprehensive red clay dielectric mixing model integrating moisture content, texture, and frequency factors. Dielectric property tests obtained the complex permittivity of red clay at various moisture contents. Results verified that the mixing model well reflects their relationship during the bound water phase (i.e., moisture content θ within 0 ~ 17.2%). The strong correlation between predicted and measured values demonstrates the model's feasibility and effectiveness. This provides guidance for the GPR-detectable soil moisture range, enabling more accurate inversion of soil moisture content.