Measuring water content in saline sands using impulse time domain transmission techniques

This paper discusses two time domain transmission (TDT) electromagnetic methods for measuring soil water content in a sand and examines the impact of pore water salinity on the resulting measurements. The first technique calculates the time taken by an impulse traveling one way through the medium relative to its time taken in air. The second method converts this impulse to the frequency domain via a Fast Fourier Transform (FFT) and calculates the travel time from the difference in phase measured in air and that measured in the medium at each frequency, resulting in a measurement of the frequency-dependent travel time. The relationship between travel time and water content was determined for pore water electrical conductivities (EC) ranging from 0.5 to 40 dS m-1. At 0.5 dS m-1 the relationship was similar to that found by previous researchers using time domain reflectometry (TDR) measurements. At pore water EC (> or =) 5 dS m-1 the travel time was faster than that found for 0.5 dS m-1 at the same water content, contradicting traditional thinking based on transmission line theory and differing from results of TDR methods. In addition, for pore water EC (> or =) 5 dS m-1, the relationship determined between travel time and water content was shown to be independent of pore water EC, to the precision of the TDT measurement technique. As a result, the impulse TDT method and this calibration relationship may improve our ability to measure soil water content under natural field conditions and may encourage further investigation of the impact of salinity on the spectral dielectric response of porous media.