Soil Dielectric Spectra from Vector Network Analyzer Data

For more than two decades, dielectric properties (i.e., permittivity) have been used for soil water content measurements, but unexpected results have been shown for saline soils or soils high in smectite clays. Permittivity is complex with real and imaginary components, and varies with frequency. The frequency dependence is due to dipole rotation and charge migration processes under alternating current. Dielectric relaxation refers to the reorienting of molecules after an electrical field is removed. The objective of this study was to develop a procedure for determining complex permittivity spectra for soils. Six soils with a range of mineralogies were preequilibrated at four duplicated water contents. They were packed into truncated coaxial cells, and the reflection-scattering parameter was measured for frequencies between 300 kHz to 3 GHz, although the primary calculation procedure was valid to <100 MHz. The calculated complex permittivity was difficult to use for deriving unique parameters because of overlapping influence of electrical conductivity and multiple dielectric relaxation processes that extended beyond the measured frequency range. The complex resistivity was easier to interpret, clearly showing one major relaxation process, except for the driest soils. The relaxation frequency determined from complex resistivity increased significantly as water content and electrical conductivity increased. For each soil, the square root of apparent permittivity, εₐ ¹/², significantly increased as frequency decreased and as water content increased. The εₐ ¹/² showed frequency dependence ranging from 1.1-fold per order of magnitude for Cecil soil to almost 2.5-fold for Ida soil. Developing procedures to extend the measured and calculated frequency range would enhance data analysis options.