Measurement of Soil Bulk Electrical Conductivity Using Partially Coated TDR Probes

The time domain reflectometry (TDR) technique allows simultaneous estimation of apparent permitivity (epsilon(a)) and bulk conductivity (sigma(a)). In highly conductive media, however, the signal is completely attenuated, which precludes permittivity measurements. This study showed that sigma(a) can be estimated in conductive media by applying long-time waveform analysis to a TDR probe partially insulated with a high-dielectric coating. Four 10-cm-long three-rod probes with identical geometry but different percentages of rod coating were tested: an uncoated sensor (P-0) and with 50% (P-50), 75% (P-75), and 95% (P-95) of the rod length coated with an insulator with a relative permittivity (epsilon(r)) of 32.3. A good relationship (r(2) = 0.99) was found between the epsilon(a) estimated, if possible, with P-0 immersed in several conductive and nonconductive media and the corresponding values estimated with P-50, P-75, and P-95. The results show that, for sigma(a) values ranging between 0.4 and 0.75 S m(-1), simultaneous measurements of epsilon(a) and sigma(a) were only using the partially coated probes. A good correlation was found between the sigma(a) estimated with P-0 inserted in different soil columns wetted with highly saline solutions (i.e., sigma(a)>0.2 S m(-1)) and those values estimated with P-50, P-75, P-95 (r(2) = 0.96, RMSE = 1.08, and SD = 0.38). For sigma(a) < 0.2 S m(-1), the accuracy of the partially coated TDR probes for estimations of sigma(a) decreased as the percentage of the rod coating increased, with errors up to 292% when sigma(a) determined by P-95 was compared with that estimated by P-0.