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ORIGINAL RESEARCH

Measurement of Soil Bulk Electrical Conductivity Using Partially Coated TDR Probes

D. Moret-Fernándeza,*, F. Lerab, J. L. Arrúea and M. V. Lópeza

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

Correspondence: * Corresponding author (david{at}eead.csic.es).

Received for publication 6 October 2008. The time domain reflectometry (TDR) technique allows simultaneous estimation of apparent permittivity ({varepsilon}a) and bulk electrical 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 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 ({varepsilon}r) of 32.3. A good relationship (r2 = 0.99) was found between the {varepsilon}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 {sigma}a values ranging between 0.4 and 0.75 S m–1, simultaneous measurements of {varepsilon}a and {sigma}a were only possible using the partially coated probes. A good correlation was found between the {sigma}a estimated with P0 inserted in different soil columns wetted with highly saline solutions (i.e., {sigma}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 {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 P95 was compared with that estimated by P0.

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






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