Modeling the Dielectric Properties of Granular Media to Determine Water Content
2005 Research Initiation Award Report
Investigators
David Robinson—Department of Plants, Soils, and Climate, Utah State University & Department of Geophysics, Stanford University
Timothy Doyle—Department of Physics, Utah State University
Scott Jones—Department of Plants, Soils, and Climate, Utah State University
Summary
In this project, the dielectric properties of water-bearing porous media were modeled in order to understand the fundamental mechanisms of dielectric dispersion, and to provide improved determination of water content using dielectric methods. The results of the model were then compared with experimental data and analytical approximations to verify the model. This interdisciplinary project combined the measurement capabilities in the Plants, Soils, and Biometeorology Department with the computer modeling skills in the Physics Department to provide greater scientific understanding and enhanced sensing methods for vadose zone and ground water monitoring.
A computer simulation technique was developed that uses a multipole expansion method to predict the dielectric properties of particulate materials in the radio and microwave regions. The method models the particles as spheres, and simulates the multiple scattering of the electromagnetic fields using iteration and mathematical functions called addition theorems. The model calculates the microscopic interactions between all of the particles in a particulate or granular material such as soil, sediment, or rock, and can predict the properties resulting from an arbitrary 3D microstructure of up to several thousand particles. Both ordered and random packings of up to 3000 glass particles were modeled. Comparison of the model results with experimental data and approximations verified that the model correctly predicts the effective dielectric constant of particulate mixtures with porosities of 0.30 to 0.90. The results also demonstrated the model’s ability to predict the dielectric dispersion, and showed that even simple systems such as monosize spheres can display significant dispersion behavior. The spatial distributions of the electric fields in the particle packings were also modeled. A more complete version of this report can be found at the website http://soilphysics.usu.edu/research/WaterInitiativeFunding.html.
This project has allowed us to develop a new and versatile approach to modeling dielectric measurements at the microscopic scale for determining soil water content. The model predictions match the experimental data very well, and provide additional information on the dielectric dispersion and the microscopic electric field structure. These capabilities will benefit water-monitoring technologies and will stimulate the development of new techniques and knowledge.
Papers resulting from this work:
- Robinson, D. A. and S. P. Friedman. 2005. Electrical conductivity and dielectric permittivity of sphere packing: measurements and modeling of cubic lattices, randomly packed monosize spheres and multisize mixtures. Physica A. Statistical Mechanics and its Applications, 358, 2-4: 447-465.
- Robinson, D. A., S. B. Jones, J.M. Blonquist and S.P. Friedman. 2005. A physically derived water content/permittivity calibration model for coarse-textured, layered soils. Soil Sci. Soc. Am. J. 69:1372-1378.
- Blonquist, J. M. Jr., S. B. Jones, I. Lebron and D. A. Robinson. 2006. Micro-structural and phase configuration effects determining the water content – dielectric relationship of aggregated porous media. Water Resour. Res. 42(5), W05424, doi:10.1029/2005WR004418.
- Doyle, T. E. 2006. Iterative simulation of elastic wave scattering in arbitrary dispersions of spherical particles. Accepted in J. Acoust. Soc. Am.
- Doyle, T.E., D.A. Robinson, S.B. Jones, K.H. Warnick and B.L. Carruth. Dielectric properties of arbitrary suspensions of spherical particles. In preparation for Physical Review B.
Contact Information
David Robinson
darob‹at›stanford.edu
Scott Jones
scott.jones‹at›usu.edu