HR: 0830h
AN: NS41B-10    [Abstracts]
TI: Measurement and Modeling of the TDR Signal Propagation Through Layered Dielectric Media and of the Effective Permittivity of Sandy Soils
AU: * Friedman, S P
EM: vwsfried@agri.gov.il
AF: Institute of Soil, Water and Environmental Sciences (ARO), The Volcani Center, Bet Dagan, 50250 Israel
AU: Robinson, D A
AF: Dept. Plants, Soils and Biometeorology, Utah State University, Logan, UT 84322-4820 United States
AU: Jones, S B
AF: Dept. Plants, Soils and Biometeorology, Utah State University, Logan, UT 84322-4820 United States
AU: Blonquist, M J
AF: Dept. Plants, Soils and Biometeorology, Utah State University, Logan, UT 84322-4820 United States
AU: Schaap, M G
EM: mschaap@ussl.ars.usda.gov
AF: George E. Brown Jr., Salinity Lab, 450 W. Big Springs Rd., Riverside, CA 92507 United States
AB: Layered dielectric materials are often encountered in the natural environment due to differences in water content caused either by a wetting or drying front. This is especially true for coarse-grained materials such as sandy soils, sediments and some rocks that have very distinctive layers of water content. This poster examines the issue of how the permittivity along a TDR probe is averaged as a function of layer thickness and probe orientation. Measurements of apparent permittivity, Ka, using TDR are presented for two, three and multi layer materials. TDR waveforms are modeled for multiple layers of varying thickness and show a change in the averaging of the apparent permittivity from refractive index to arithmetic when more thin layers are present. Analysis of the modeled results shows that the averaging regime is frequency-dependent. However, broadband techniques applied to materials with a few layers will generally produce refractive averaging. A transition to arithmetic averaging is found for systems having many (>4 layers). Narrow-band methods may be very sensitive to layering and may perform in a highly non-refractive way when layering with a strong permittivity contrast is present. Many empirical formulas relating TDR-measured permittivity, Ka, to volumetric water content have been proposed owing to the lack of a robust and accurate physically-based model describing this relationship across a range of soils. Soil-specific calibrations are often infeasible due to the time-consuming gravimetric sampling required for adequate calibration where limited resources often prevail. In this poster we propose a physically-based sample scale model for the permittivity - water content relationship in coarse-grained media using modeled or calibrated two-point anchoring. Materials tested include mono-size glass spheres and quartz sand grains in addition to two sandy soils. The physically-based model accounts for particle shape and bulk density using a two-phase, particle-scale mixing model and refers to a wetting or draining profile with a sharp wetting or drying front. Our measurements indicate the absence of dielectric hysteresis for the materials studied. An alternate calibration approach only requires the measured soil effective permittivities for dry and saturated conditions (i.e., two-phase mixtures) and knowledge of the bulk density. For the effective permittivity of dry sandy soils we recommend to use a common value of 3 for the solid phase permittivity. The results provide insight into the appropriate "refractive index" modeling of layered (wetting/drying) soil profiles with the grain-scale modeled two-phase permittivity providing bounds for the sample-scale three-phase porous medium.
DE: 1866 Soil moisture
DE: 3914 Electrical properties
DE: 5109 Magnetic and electrical properties
DE: 6909 Electromagnetic metrology
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly