Complex Effective Relative Permittivity of Soil Samples from the Taunus Region (Germany)

The most important parameter affecting ground-penetrating radar (GPR) measurements is the complex effective relative permittivity ε_{r, eff}^* because it controls the propagation velocity and the reflection of GPR pulses. Knowing ε_{r, eff}^* of soils passed through by electromagnetic waves increases accuracy in soil thickness and interface identification. Complex effective relative permittivity ε_{r, eff}^* =ε_{r, eff}^' − jε"_{r, eff}^* 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 ε_{r, eff}^* 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 ε_{r, eff}^* measured. Along with frequency and water content, soil texture and organic matter affect soil permittivity. Moreover, the real part of ε_{r, eff}^* increases at higher dry bulk densities. Output from our calibration model enables us to predict ε_{r, eff}^* for the soil samples which were tested under the actual in situ soil water content. This results in high accuracy of soil thickness prediction.