Relationships between Dielectric and Hydrogeologic Properties of Sand-Clay Mixtures

A series of laboratory experiments was performed to investigate relationships between dielectric properties and hydrogeologic properties such as lithology, porosity and water saturation of granular materials. Mixtures of sand, clay, air and water were prepared to vary hydrogeologic parameters systematically. Lithologic variation was simulated by varying the relative proportion of sand and clay in the mixtures. Compaction was simulated by packing different volumes of sand and clay (in the same relative proportion) into the sample holder. Water saturation was varied by injecting water into the pore space and by evaporative drying. The experimental data (i.e., measurements of porosity, clay content, water saturation, dielectric constant, electrical conductivity and frequency) may be used to test dielectric mixing formulas and effective medium theories. Results show that much of the scatter in dielectric property-hydrogeologic property crossplots is due to systematic variation in interrelated hydrogeologic and/or dielectric parameters. For instance, the relationship between dielectric constant and porosity for dry or saturated materials may be multi-valued if there are also changes in clay content. However, different mechanisms of porosity reduction may be associated with characteristic dielectric responses. Porosity reduction by compaction results in an increase in dielectric constant for dry materials and a decrease in dielectric constant for saturated materials. Porosity reduction by pore-filling (i.e., changing lithology) results in either an increase or a decrease in dielectric constant depending upon the volume fractions, microgeometries and surface properties of the components. These results provide insight into the types of changes one may expect to see in ground penetrating radar data from different geologic settings, and hence should be of interest to those involved in modeling, inversion and stratigraphic interpretation of ground penetrating radar data.