Dielectric Constant of Sand Using TDR and FDR Measurements and Prediction Models

Abstract

The effects of soil dry density and water content are being examined through experimental time-domain-reflectometry (TDR) and frequency-domain reflectometry (FDR) methods in correlation with electromagnetic simulations. The infiltration rate (hydraulic conductivity) of water in sand is exceptionally high, resulting in heterogeneous moisture distribution through the soil. The effective dielectric constant of the soil/water/air mixture is dependent on the soil's dry density and moisture content. Both TDR and FDR methods are performed on a coaxial transmission line filled with a soil/water/air mixture. The flow of the water through the soil creates a dynamic situation in which the soil/water/air electrical impedance changes over time. The resulting soil has heterogeneous water content, creating varying electrical impedance values along the length of the coaxial line. The soil compaction, i.e., dense or loose, has significant impact on the heterogeneity of the moisture content through the soil and the dry density of the soil. In each case, the effective dielectric constant is determined from the data collected from TDR and FDR experiments, and the values are compared with the predictions using established empirical models by Topp, Hilhorst, and Hendrickx. With the exception of the data represented as a function of the degree of saturation, the Hendrickx model appears to best represent the measured dielectric constants since it falls within two standard deviations of the measured data. A computer simulation technology (CST) Microwave Studio is used to supplement experimental observations of various soil moisture contents in a coaxial cell. Simulations confirm that the change in the dielectric constant through the soil is a result of the heterogeneous moisture distribution. It was found that the soil moisture content has a major impact on the resulting dielectric constant from measurements or modeling. In the coaxial-cell device, soil moisture migration during the testing period results in a heterogeneous moisture regime and a temporal dielectric constant. This is particularly exaggerated for high-hydraulic-conductivity soils such as sand.