Effects of Maxwell‐Wagner polarization on soil complex dielectric permittivity under variable temperature and electrical conductivity

The presence of free charges and numerous discontinuities separating liquid, gas, and solid phases in partially saturated soils give rise to Maxwell-Wagner polarization that may significantly affect bulk dielectric permittivity measurements. Evidence suggests a complex interplay shaping the response of low-frequency (<100 MHz) dielectric permittivity to changes in ambient temperature and amount of ionic charges present in a wet soil. Model calculations based on the Maxwell-Wagner-Brugermann-Hanai (MWBH) theory are supported by direct measurements using a network analyzer, showing an increase in soil bulk dielectric permittivity with increasing temperature and with higher bulk electrical conductivity at low frequency range (<100 MHz). Beyond a certain frequency the decrease in permittivity of free water becomes dominant and results in a decrease in soil bulk dielectric permittivity with increasing temperature. The value of this crossover frequency can be predicted as a function of solution electrical conductivity (EC) as confirmed in limited tests. The dielectric permittivity inferred from TDR waveform travel time analysis is not significantly influenced by the low frequency range of the dielectric spectrum. In contrast, dielectric permittivity sensors operating at frequencies lower than 100 MHz are likely to show significant sensitivity to factors affecting the Maxwell-Wagner effect (temperature and electrical conductivity), hence requiring special care in measurement interpretation.