Precision measurements of dielectric material properties over a wide frequency range have advanced considerably with the use of network analyzers and time-domain spectrometers (Cole and Cole, 1941, 1942; Fellner-Feldegg, 1969; Cole, 1975a, b; Cole et al., 1980; Feldman et al., 1992; Berberian, 1993; Feldman et al., 1996; Berberian and King, 2002; Huisman et al., 2004). The frequency range of these methods (1–10 000 MHz) encompasses those of ground-penetrating radar (GPR) and time-domain reflectometry (TDR) systems and the relaxation frequencies of polar liquids and dispersive clays. Some of these materials, especially the swelling clays (e.g., bentonite, montmorillonite), appear to have discrete relaxation times rather than the continuum thought to be characteristic of complex materials (Ishida et al., 2000). In this note we test an interpretational procedure that incorporates a genetic optimization algorithm to model dispersive dielectric properties using a Debye model with discrete relaxation times. Using network analyzer measurements of bentonite (scattering coefficients), we produce a Debye model that is physically reasonable and self-consistent, and it satisfies the Kramers-Kronig relationships.

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