Time domain reflectometry measurement of bulk permittivity of porous mixtures containing bound water

Dani Or and Scott B. Jones

Dept. Plants, Soils, and Biometeorology
Utah State University
Logan, Utah, 84322-4820, USA

Time domain reflectometry has become an important measurement technique for determination of porous media water content and electrical conductivity in the earth sciences due to its simplicity and accuracy. Water content is inferred from the bulk dielectric permittivity of the medium usually based on travel time analysis along simple waveguides, whereas bulk electrical conductivity is inferred from TDR signal attenuation along a waveguide. The objectives are to (1) review TDR applications and primary factors affecting dielectric permittivity measurements and inference of water content of porous mixtures; (2) discuss approaches for estimation of critical parameters required for permittivity modeling such as water binding surface area, and constituent composition, shape and distribution; and (3) model porous mixture permittivity using dielectric mixing theories. Primary factors influencing permittivity measurements are free and bound water contents, chemical composition, temperature, constituent shape and phase configuration, and measurement frequency. The bound water content of porous materials often exhibits a reduced dielectric signature (within TDR frequency range) compared to that of free water due to strong association with hydrophilic surfaces, especially in the presence of appreciable amounts of clay minerals and similar materials. Bound water associated with solid surfaces exhibits a 'thermodielectric' effect that has been modeled using the temperature-dependent volume exchange between the bound and free water phases. Particle shape, layering and phase configuration, and bound water effects on permittivity predictions are accounted for in a three-phase dielectric mixing model.