Abstract Clay‐rich soils and sediments are key components of near‐surface systems, influencing water retention, ion exchange, and structural stability. Their complex dielectric response under moist conditions arises from surface–ion electrostatics and diffuse double layers that govern transport and retention processes. This study explores the broadband dielectric spectra (1 MHz–5 GHz) of four water‐saturated clays (kaolinite, illite, and two sodium‐activated bentonites) in a coaxial transmission‐line setup. The spectra were parameterized with two phenomenological relaxation models–the Generalized Dielectric Relaxation Model (GDR) and the Combined Permittivity‐Conductivity Model (CPCM)–and two mixture models: the Augmented Broadband Complex Dielectric Mixture Model (ABC‐M) and the Complex Refractive Index Model (CRIM). These approaches were evaluated for their ability to link dielectric relaxation behavior to key petrophysical parameters such as Cation Exchange Capacity (CEC), Volumetric Water Content (VWC), and porosity. The results demonstrate distinct spectral signatures correlating with clay mineralogy, particularly at low frequencies. Relaxation strength and apparent DC conductivity show systematic relationships with CEC, emphasizing the influence of clay‐specific surface properties. While expandable clays like bentonites displayed enhanced relaxation due to ion‐exchange dynamics, deviations in a soda‐activated bentonite highlighted the impact of chemical treatments on dielectric behavior. Overall, the study highlights both the potential and the limitations of broadband dielectric spectroscopy for soil and clay characterization. This study provides a systematic framework for linking clay mineral physics to applied electromagnetic methods. The results have significant implications for non‐invasive, frequency‐domain methods for characterizing soils and sediments, hydrological modeling, geotechnical evaluation, and environmental monitoring.
Schmidt et al. (2026) studied this question.