DIELECTRIC-PROPERTIES OF WET SOILS IN THE FREQUENCY-RANGE 1-3000 MHZ

The effective relative dielectric constant epsilon(e, r) and the effective conductivity sigma(e) have each been determined as a function of frequency in the range 1-3000 MHz at volumetric water contents of up to approximately 0.74 for clays, 0.83 for a peat and 0.56 for a silt. At frequencies above about 25 MHz (depending on soil type), epsilon(e, r) increases with water content for all samples. However, at lower frequencies, epsilon(e, r) only increases with water content as long as the wet density also increases, which is the case for water contents up to a critical value lying between 0.35 and 0.48. At higher water contents, epsilon(e, r) and the wet density decrease with increasing water content. Consequently, curves of epsilon(e, r) versus frequency for two wet samples with different water contents, at least one of them higher than the critical value, are seen to cross at about 25 MHz. Below the critical value the curve of the sample with the lower water content is below the other curve at all freqencies applied. At a given frequency, sigma(e) has a maximum as a function of water content. This is tentatively explained by assuming that sigma(e) is the sum of pore water conductivity (increasing with water content until all salts in the soil are dissolved into the water and then decreasing) and surface water conductivity (increasing with wet density and therefore increasing with water content up to the critical value and then decreasing). At frequencies higher than 1000 MHz, epsilon(e, r) depends only weakly on salinity (which is represented by the measured conductivity). It shows an increasing dependence if the frequency is decreased towards 1 MHz. The highest values of epsilon(e, r) and sigma(e), measured in this work, occur for a sample of wet, nearly saturated silt originating from a location below sea-level near to the Dead Sea, Israel: epsilon(e, r) decreases continuously from a value of about 10(4) at 3 MHz to about 10(2) at 200 MHz, while simga(e) rises from about 4 S/m to 5 S/m at these respective frequencies. The dependence of the wavelength on the loss-tangent is strong and the wavelength is considerably smaller than it would be in a dielectric. This is the only sample for which sigma(e) increases with water content, even if the latter is above its critical value. Therefore it is assumed that the pore water conductivity is greater than the surface water conductivity if the volumetric water content is lower than 0.564, the maximum value applied. The measurements give evidence for the presence of a relaxation at about 3 MHz for all samples examined.