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.

The most important parameter affecting ground-penetrating radar (GPR) measurements is the complex effective relative permittivity * r,eff ε because it controls the propagation velocity and the reflection of GPR pulses. Knowing * r,eff ε of soils passed through by electromagnetic waves increases accuracy in soil thickness and interface identification. Complex effective relative permittivity * ' ' ' r,eff r,eff r,eff j ε ε ε = − of 25 soil samples with textures ranging from loamy sand to silty clay was measured using the two-electrode parallelplate method. The measurements were conducted at defined water contents for frequencies from 1 MHz to 3 GHz. The results confirm the frequency dependence of * r,eff ε and show that the dielectric behavior of soil-water mixtures is a function of water content. Applying the experimental data of this study with predictions based on the empirical model by Topp et al. (1980), we find that Topp et al.'s curve tends to underestimate the real part of * r,eff ε measured. Along with frequency and water content, soil texture and organic matter affect soil permittivity. Moreover, the real part of * r,eff ε increases at higher dry bulk densities. Output from our calibration model enables us to predict * r,eff ε for the soil samples which were tested under the actual in situ soil water content. This results in high accuracy of soil thickness prediction.KEY WORDS: ground-penetrating radar (GPR), complex effective relative permittivity, soil sample.

The frequency dependence of soil electromagnetic properties contain valuable information of the porous material due to strong contributions to the dielectric relaxation behavior by interactions between aqueous pore solution and mineral phases due to interface effects. Soil hydraulic properties such as matric potential are also influenced by different surface bonding forces due to interface processes. For this reason, a new analysis methodology was developed, which allows a simultaneous determination of the soil water characteristic curve and the dielectric relaxation behavior of undisturbed soil samples. This opens the possibility to systematically analyze coupled hydraulic/dielectric soil properties for the development of pedotransfer functions to estimate physicochemical parameters with broadband HF-EM measurement techniques.