Broadband apparent permittivity measurement in dispersive soils using quarter-wavelength analysis

Time-domain reflectometry (TDR) has, since the early 1980s, become a widely used and robust method for measuring the electromagnetic properties of soils. Although it allows use of relatively simple probes and measurement equipment that free users from complex analysis of small soil samples in the laboratory, it provides little information on the dispersive nature of fine-grained soils. Therefore, we developed a quarter-wavelength analysis (QWA) methodology that allows apparent permittivity spectra to be obtained for soils, while retaining the same range of cells and probes and the simplicity of data analysis associated with TDR. We tested the technique on clay soils across a wide water content range and found that it provided useful data for frequencies ranging from approximately 100 MHz to >1 GHz-the accuracy being considered no worse than two units of apparent permittivity. The results show that the methodology is capable of providing broadband dispersion data that give significantly greater detail on soil electromagnetic properties than can be expected from single-frequency TDR data. This has a number of important advantages including the potential to provide accurate water content measurements in clay soils, a means to extend TDR soil monitoring data, and the opportunity to validate research into the measurement frequency associated with simple TDR measurements and time- to frequency-domain inversions.