Factors Affecting Interfacial Polarization and Dielectric Behavior of Wet Soils

The presence of free charges and numerous discontinuities separating liquid, gas, and solid phases in partially-saturated soils give rise to a relatively understudied phenomenon of interfacial polarization that could impact bulk dielectric permittivity measurements using TDR and similar soil-embedded sensors. Evidence suggests a complex interplay between the response of complex dielectric permittivity spectrum to changes in ambient temperature and amount of ionic charges present in the wet soil. Model calculations based on Hanai's theory are supported by direct measurements using Network Analyzer showing that at low frequency (<10 MHz), the bulk dielectric permittivity increases with increasing temperature with a magnitude determined by the bulk electrical conductivity (amount of free ionic charges). Beyond a certain critical frequency, the decrease in free water permittivity dominates and the bulk dielectric permittivity decreases with increasing temperature. The dielectric permittivity inferred from TDR waveform travel time analyses is not significantly influenced by changes in the low frequency range (<10 MHz). In contrast, soil dielectric permittivity sensors operating at frequencies in the range of 10 MHz are likely to exhibit significant sensitivity to ambient conditions (temperature and electrical conductivity) affecting interfacial polarization, hence require special care in measurement interpretation.