Microwave Dielectric Measurements For Soil Density And Moisture Content Inference In Engineering Applications

Soil is a material of interest in a number of disciplines such as geotechnical, environmental, military and agricultural engineering. Precise knowledge of dielectric properties of soil provides information about the interactions between the soil and applied electromagnetic (EM) fields. Detecting anomalies of interest in heterogeneous soil media without enough information about the background media is very challenging, if not impossible. Dielectric properties are considerably intricate, can exhibit frequency dependent behavior, and correlate with many physical and chemical properties. Measuring dielectric properties can therefore be used to estimate those physical and chemical properties, if the correlation between the dielectric properties and other physical and chemical properties is well investigated and understood. In this study, laboratory measurements and theoretical modeling of dielectric properties of sandy soils are carried out. Dielectric property measurements are executed using a minimally invasive cross-borehole microwave measurement technique. An in-situ technique to measure dielectric properties of a sandy soil at different densities and moisture contents, and their frequency dependence over a wideband of frequency (0.4 GHz to 2.2 GHz) is implemented and evaluated. Then, the correlation between variations of dielectric and physical properties of the soil (e.g. density and water content) is studied. Dielectric mixing models were used to ascertain these correlations as a forward model for future inversion. Finally, a comparison between the forward model and experimental results was undertaken to seek the level of agreement. Introduction Soils are media composed of interacting discrete solid elements with voids filled with air and/or fluids. These three phases have different physical, chemical and dielectric properties. Soil materials are considerably intricate and can exhibit significantly dispersive (i.e. frequency dependent) behavior. Therefore, soil dielectric properties must be treated as complex quantities. These complex quantities are dependent on physical and chemical conditions (e.g. temperature, crystal mineralogy in case of clay). The complex-plane representation of the complex dielectric permittivity can be presented to reveal the frequency and temperature behaviors of dielectric materials (Trabelsi and Nelson, 1997; and Trabelsi et al. 1998 and 2001). This behavior is of utmost significance in subsurface imaging. The accuracy of indirect measurement of dielectric properties by full-wave inversion of electromagnetic signals relies on the appropriateness of the model describing their frequency-dependence. At microwave frequencies, dispersion arises from factors such as relaxation mechanisms due to the presence of free water and the water bound to mineral surfaces, and interactions between ions and soil particles. To have a realistic model of propagation of electromagnetic waves in the subsurface, it is necessary to account for this phenomenon. Therefore, characterizing the frequency and physical state dependence of dielectric properties of saturated and unsaturated soils is of a prime importance. The propagation of EM waves through materials is dependent on dielectric properties, which are in turn controlled by many geologic parameters. Previous research has illustrated the potential of using