Volume 20, Issue 2 (June 1997)
Methods for Broad-Band Dielectric Permittivity Measurements (Soil-Water Mixtures, 5 Hz to 1.3 GHz)
Broad-band permittivity data enable the determination of micro- and macro-scale material characteristics and the monitoring of geo-processes. While high-frequency (> ≈ 100 MHz) permittivity measurements can be performed readily in the laboratory, low-frequency (< ≈ 10 MHz) measurements are more difficult to conduct. This paper describes two low-frequency techniques and presents broadband permittivity data for various soil-water mixtures.
Low-frequency data were gathered with an impedance analyzer in conjunction with two-terminal and four-terminal measurement systems. The two-terminal cell consisted of two copper electrodes; its range was restricted at low frequencies due to electrode polarization. The four-terminal system used separate current and voltage electrodes; its accuracy at low frequencies was limited by the ability of the equipment to resolve small phase angles.
High-frequency data obtained with a coaxial termination probe and low-frequency data obtained with these two cells are presented. Soils of very different specific surface were tested at water contents ranging from air-dry to saturated. It is shown that the conductivity of the specimen controls not only the low-frequency measurement limit (i.e., electrode polarization and phase resolution), but also the high-frequency limit due to stray inductances.