Soil Bulk Electrical Conductivity Measurement using High-Dielectric Coated Time Domain Reflectometry Probes

摘要

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity (epsilon a) and hence volumetric water content (theta) and bulk electrical conductivity (sigma a). Difficulties arise for theta and sigma a determination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that sigma 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 theta 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 r, of 32.3. This method was calibrated on different NaCl-water solutions (0-15 dS m-1) and compared with the standard long-time TDR method for a estimations using an UP. The method was subsequently used for determining sigma a in four different soils with different values of theta and sigma a (0-6 dS m-1) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the epsilon a measured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for epsilon a estimations. For epsilon a values <4 dS m-1, the UP allows determinations of epsilon a and the most accurate estimations of sigma a using the standard TDR method (R2 = 0.99). For higher sigma a values, however, estimations of sigma a were only possible using the CP, where sigma a was satisfactorily determined (R2 = 0.99) using the Dalton method. To this end, a previous calibration between the real and the apparent bulk electrical conductivity estimated with the CP was required.