FREQUENCY-DOMAIN METHODS FOR EXTENDING TDR MEASUREMENT -- RANGE IN SALINE SOILS

Water content and electrical conductivity of soils are routinely determined using time-domain reflectometry (TDR) based on analysis of signal travel time along buried waveguides.  In soils with appreciable electrical conductivity, travel time analysis becomes progressively inaccurate due to signal attenuation to the point of failure (typically at EC_b > 2 dS m^-1).  We demonstrate that information on bulk dielectric permittivity, lost in travel time analysis in saline soils, can be recovered in the frequency domain by using shorter waveguides to reduce signal attenuation.  We implement a robust algorithm for the time-to-frequency domain transformation (Nicolson, 1973) that does not rely on waveform differentiation.  This algorithm uses signal de-ramping to eliminate noise induced by standard differentiation and to reduce truncation errors arising from finite sample size.  The methodology was tested using coaxial cells and three-wire TDR probes under a wide range of solution electrical conductivities (0 to 24 dS m^-1).  Reliable estimates of bulk dielectric permittivity for EC’s up to 24 dS m^-1 in a silt loam soil were obtained using 2 and 3 cm TDR probes.  In addition to bulk dielectric permittivity, this method provides additional frequency-dependent and electrical conductivity information derived in the estimation of the Cole-Cole parameters.

 

Scott B. Jones and Dani Or.
Dept. Plants, Soils, and Biometeorology, Utah State University