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

Abstract

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.

Broadband Dielectric Measurement Methods for Soft Geomaterials: Coaxial Transmission Line Cell and Open-Ended Coaxial Probe

“…The CC used in this study and the adaptor (SPINNER GmbH, Germany) [15]- [17] used to accommodate the CC are illustrated in Fig. 1.…”

“…Solving the above complex equations gives c 1 , c 2 , and c 3 , which together with the measured 11 S can be subsequently used to determine the unknown * r ε of the sample at each frequency by using (15).…”

“…However, dielectric measurements taken at reference plane I still contain unwanted information from the adapter units. In order to eliminate it, the reference plane is shifted to position II by a subsequent phase shift correction prior to the computation of * r ε [15], [18].…”

“…An iterative inversion technique called BIJ [15], [18], [19] is employed to compute * r ε from measured scattering parameters S ij after calibration and pre-processing of the dataset. But the propagation matrix algorithm (PM), as a quasi-analytical method, is used in a first step to determine * r ε by using the combined propagation factor -impedance method [20] based on the following relationship…”

“…In addition, due to the size of this type of probe, the effective measurement volume is normally limited to the order of cm 3 . A non-destructive two-port coaxial transmission line cell (CC) recently developed by Lauer et al [15] is able to measure a larger sample volume but still keep the sample undisturbed. However, this coaxial transmission line cell is not suitable for materials that are fluid or tend to shrink during measurement.…”