Abstract

The time domain reflectometry (TDR) technique allows simultaneous estimation of apparent permittivity (εa) and bulk electrical conductivity (σa). In highly conductive media, however, the signal is completely attenuated, which precludes permittivity measurements. This study showed that σa can be estimated in conductive media by applying long-time TDR waveform analysis to a TDR probe partially insulated with a high-dielectric coating. Four 10-cm-long three-rod TDR probes with identical geometry but different percentages of rod coating were tested: an uncoated sensor (P0) and probes with 50% (P50), 75% (P75), and 95% (P95) of the rod length coated with an insulator with a relative permittivity (εr) of 32.3. A good relationship (r2 = 0.99) was found between the εa estimated, if possible, with P0 immersed in several conductive and nonconductive media and the corresponding values estimated with P50, P75, and P95. The results also show that, for σa values ranging between 0.4 and 0.75 S m−1, simultaneous measurements of εa and σa were only possible using the partially coated probes. A good correlation was found between the σa estimated with P0 inserted in different soil columns wetted with highly saline solutions (i.e., σa > 0.2 S m−1) and those values estimated with P50, P75, and P95 (r2 = 0.96, RMSE = 1.08, and SD = 0.38). For σa < 0.2 S m−1, the accuracy of the partially coated TDR probes for estimations of σa decreased as the percentage of the rod coating increased, with errors up to 292% when σa determined by P95 was compared with that estimated by P0.

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