Dielectric Properties and Influence of Conductivity in Soils at One to Fifty Megahertz
1990
Campbell, Jeffrey E.
A series of laboratory experiments were performed in order to systematically explore the dielectric response of soils to a frequency range of 1 to 50 MHz. A network analyzer and a coaxial-transmission-line type of dielectric probe were used to measure the complex (both real and imaginary) dielectric response of moist soils. The dielectric probe was placed in a modified Tempe cell in which the moisture content of the soil under investigation could be varied. Both the real and imaginary components of the dielectric permittivity of the moist soils investigated are greatest at 1 MHz and monotonically decrease with frequency. The variation in the dielectric constant among these soils is also greatest at 1 MHz and decreases with increasing frequency. The soils that have a large imaginary dielectric constant also have the largest real dielectric constant dispersion. The loss tangent for a given soil is relatively independent of moisture content over a wide range of soil moisture conditions. At low water contents, both the real and imaginary dielectric values begin to drop rapidly with decreasing water content. There is a significant temperature dependence present in the dielectric response of moist soils, which changes markedly with frequency. Ionic conductivity is the predominant mechanism causing the imaginary dielectric response and can account for many features of the observed dielectric response. A Looyenga-type mixing model incorporating ionic conductivity can account for the frequency dependence of the dielectric constant of soils.
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