A new technique for measuring broadband dielectric spectra of undisturbed soil samples

A new coaxial line cell for the determination of dielectric spectra of undisturbed soil samples was developed based on a 1.625‐inch ‐ 50 Ω coaxial system. Undisturbed soil samples were collected from a soil profile of the Taunus region (Germany) and capillary saturated followed by a step‐by‐step de‐watering in a pressure plate apparatus as well as oven‐drying at 40°C. The resultant water contents of the soil samples varied from saturation to air‐dry. Permittivity measurements were performed within a frequency range from 1 MHz to 10 GHz with a vector network analyser technique. Complex effective relative permittivity or electrical conductivity was obtained by combining quasi‐analytical and numerical inversion algorithms as well as the parameterizing of measured full set S‐parameters simultaneously under consideration of a generalized fractional dielectric relaxation model (GDR). The measurement of standard materials shows that the technique provides reliable dielectric spectra up to a restricted upper frequency of 5 GHz. For the soil samples investigated, the real part of complex effective relative permittivity ɛ′r,eff and the real part of complex effective electrical conductivity σ′eff decreased with increasing matric potential or decreasing water contents. Soil texture and porosity affect the dielectric behaviour at frequencies below 1 GHz. For frequencies above 1 GHz minor texture effects were found. The presence of organic matter decreases ɛ′r,eff and σ′eff. At 1 GHz, the empirical model of Topp et al. (1980) is in close agreement with the experimentally determined real part of the effective permittivity with RMSEs ranging from 1.21 for the basal periglacial slope deposit and 1.29 for bedrock to 3.93 for the upper periglacial slope deposit (Ah). The comparison of the experimental results with a semi‐empirical dielectric mixing model shows that data, especially for the organic‐free soils, tend to be under‐estimated below 1 GHz. The main advantage of the new method compared with conventional impedance and coaxial methods is the preservation of the natural in‐situ structure and properties such as bulk density of the investigated soil samples.