Effects of structural and compositional factors on soil electromagnetic properties

Abstract

Electromagnetic properties of clay soils can be affected by many factors, including clay mineralogy, clay percentage, flocculation of clay particles, pore fluid salt concentration, anisotropy, and water content. The influences of these factors on soil dielectric permittivity and electrical conductivity are analysed using a theoretical model developed for clay minerals and the modified Maxwell-Garnett mixing formula. The analysis shows that, at frequencies higher than 50 MHz, the real permittivity of an isotropic soil is affected primarily by water content, clay percentage and clay mineralogy and, to a lesser extent, by pore fluid salt concentration and flocculation. Three theoretical correlations are established for saturated isotropic clay–water and sand–water mixtures: (1) correlation between volumetric water content and the real permittivity at 1 GHz, (2) correlation between volumetric water content and the dielectric dispersion magnitude from 50 MHz to 1 GHz, and (3) correlation between volumetric water content and the effective electrical conductivity at 20 MHz. These correlations are validated by measured data for saturated sand, kaolinite, bentonite and mixtures of bentonite and silica flour. A new method to determine volumetric water content and specific surface area from electromagnetic measurements is proposed.

Keywords:

• water content
• anisotropy
• flocculation
• clay mineralogy
• specific surface area

Acknowledgements

The EM property measurements cited in this paper were done at the Georgia Institute of Technology with the assistance of Professor J. Carlos Santamarina. His help in the development of the EM property measurement procedures and in the overall conduct of this research is acknowledged with appreciation.