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ORIGINAL RESEARCH

Measurement of Soil Bulk Electrical Conductivity Using Partially Coated TDR Probes

^a Dep. de Suelo y Agua, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, PO Box 202, 50080 Zaragoza, Spain
^b Dep. de Ingeniería Electrónica y Comunicaciones, Instituto de Ciencia de Materiales de Aragón, Consejo Superior de Investigaciones Científicas, Univ. de Zaragoza, C/María de Luna 1, 50018 Zaragoza, Spain
^* Corresponding author (david{at}eead.csic.es).

Received 6 October 2008.

The time domain reflectometry (TDR) technique allows simultaneous estimation of apparent permittivity (_a) and bulk electrical conductivity (_a). In highly conductive media, however, the signal is completely attenuated, which precludes permittivity measurements. This study showed that _a 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 (_r) of 32.3. A good relationship (r² = 0.99) was found between the _a 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 _a values ranging between 0.4 and 0.75 S m^–1, simultaneous measurements of _a and _a were only possible using the partially coated probes. A good correlation was found between the _a estimated with P₀ inserted in different soil columns wetted with highly saline solutions (i.e., _a > 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 _a < 0.2 S m^–1, the accuracy of the partially coated TDR probes for estimations of _a decreased as the percentage of the rod coating increased, with errors up to 292% when _a determined by P₉₅ was compared with that estimated by P₀.

Abbreviations: DC, direct current • TDR, time domain reflectometry