Electromagnetic determination of clay water content: role of the microporosity

The electromagnetic determination of clay water content requires a good understanding of the main factors that affect the relationship between the clay relative permittivity K and the water content theta. The first part of this paper proposes a review of the different factors affecting it: (a) a significantly high imaginary part of the relative permittivity; (b) a frequency-dependent response; (c) a high content of bound water with a relative permittivity value lower than that of free water; (d) a geometrical effect associated with platy units that characterize the clay texture. The next sections focus on the c and d factors related to the microporosity of clayey geomaterials; both factors are studied with two theoretical tools: the Moment Method (MoM) and the Differential Effective Medium (DEM) theory. The calculations indicate that bound water effect and geometrical effect act as competing processes: when water is added in a dry clayey material, the geometrical effect contributes to increase the clay relative permittivity compared to media with isotropic particles; a significant amount of bound water generates a decrease in the clay bulk permittivity. Consequently, the results allow to identify two types of behaviour: (1) low surface area systems, typically kaolinite geomaterials, for which the geometrical effect is predominant, and (2) high surface area systems, typically smectite clays, for which a balance between the geometrical effect and the bound water effect is expected. Moreover, the MoM simulations suggest that the derivative a partial derivativekappa/partial derivativetheta is affected by the bound water content, but slightly depends on clay content. These numerical observations that may contribute to improve in situ water content monitoring should be validated by experimental investigations. (C) 2004 Elsevier B.V. All rights reserved.