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Waves, scale, sand, and water : dielectric constant of unconsolidated sediments Chan, Christina Ye

Abstract

Field dielectric measurements are used to estimate the water content of the subsurface. In order to estimate water content accurately, the Earth's heterogeneity should be taken into account. Layering is a simple form of heterogeneity which is a close approximation in many sedimentary environments. The interplay between the average layer thickness of a sedimentary system and the wavelength of the EM wave used for the dielectric measurement is important in determining average dielectric constant. When the layers are thick compared to the wavelength, the system falls under the ray theory regime; when the layers are thin, the system falls under the effective medium theory regime. Using numerical and experimental techniques, I confirm these two regimes. I also investigate the transition zone between the regimes and find that it falls at a wavelength to layer thickness ratio of around 4. The breadth of the zone is affected by the dielectric constants of the components, the proportions of the components, and the distribution of the layers, but not the conductivity of the soils. Because many sedimentary environments have layering, the presence of these layers must be accounted for when using the average dielectric constant measured in the field to estimate water content. I compare relationships between dielectric constant and water content which take into account the presence of layering with relationships which assume homogeneity. Modeling dielectric constant as a function of lithology and water content, I find differences among the dielectric constants predicted from the different relationships. I show the potential error in water content estimation if a layered system is assumed to be homogeneous. I also present a flow chart for more accurately estimating water content and saturation from field measurements. This method not only gives the global water content of the whole system but also gives the water contents of the individual sedimentary layers if they are present. In this thesis, I present research which can provide more accurate estimates of water content from dielectric measurements. These investigations advance the knowledge of EM wave propagation and increase the accuracy of estimating water content from field dielectric measurements of the subsurface.

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