Soil bulk electrical conductivity measurement using high-dielectric coated time domain reflectometry probes

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity ([epsilon].sub.a]) and hence volumetric water content ([theta]) and bulk electrical conductivity ([[sigma].sub.a]). Difficulties arise for [theta] and [[sigma].sub.a] determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that [[sigma].sub.a] can be estimated in highly conductive media using a TDR probe coated with a high-dielectric insulator (CP). To this end, the Dalton method for [[sigma].sub.a] estimations was applied to a 10-cm-long three-rod TDR probe insulated with a 0.2-mm-thick epoxy-ceramic composite coating with a relative permittivity, [[epsilon].sub.r], of 32.3. This method was calibrated on different NaCl--water solutions (0-15 dS [m.sup.-1]) and compared with the standard long-time TDR method for ([[sigma].sub.a] estimations using an UP. The method was subsequently used for determining [[sigma].sub.a] in four different soils with different values of [theta] and [[sigma].sub.a] (0-6 dS [m.sup.-1]) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the [[epsilon].sub.a] measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for [[epsilon].sub.a] estimations. For [[sigma].sub.a] values Abbreviations: CP, high-dielectric coated time domain reflectometry probe; DC, direct current; TDR, time domain reflectometry; UP, uncoated time domain reflectometry probe.