Frequency dependent electric properties of homogeneous multi-phase lossy media in the ground-penetrating radar frequency range

• Presence of clay responsible for the non linear behavior of the electric permittivity
• Possibility to detect clay independently of water
• Ability to individuate clay is strongly reduced for frequencies above 1 GHz
• Correction of Topp model for clayey soils
• Volumetric mixing model calibration to soil specific parameters

The electric properties of multiphase aggregate mixtures are evaluated for a given mineralogic composition at frequencies between 300 kHz and 3 GHz. Two measurement techniques are employed: a coaxial transmission line and a monostatic stepped-frequency ground-penetrating radar (GPR). The effect of increasing water content is analyzed in several sand clay mixtures. For the end-member case of maximum clay (25% in weight) and increasing water content, investigations are compared between the two measurement techniques. The electrical properties of materials are influenced by the amount of water, but clay affects the frequency dependency of soils showing distinctive features regardless of the mineralogy. The microwave attenuation, expressed by the quality factor Q, is partly dependent on frequency and on the water content. The performance of one empirical and one volumetric mixing model is evaluated to assess the capability of indirectly retrieving the volumetric water content for a known mixture. The results are encouraging for applications in the field of pavement engineering with the aim of clay detection. The models used show similar behaviors, but measured data are better modeled using third order polynomial equations.