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DIVISION S-1—SOIL PHYSICS

Evaluation of Uncoated and Coated Time Domain Reflectometry Probes for High Electrical Conductivity Systems

Dep. of Earth and Ocean Sciences, Univ. of British Columbia, 6339 Stores Road, Vancouver, BC, Canada V6T 1Z4

* Corresponding author (cfnichol{at}hotmail.com)

High sample electrical conductivity reduces the quality of a time domain reflectrometry (TDR) waveform by the loss of signal amplitude. Two strategies are examined to obtain higher signal/noise waveforms: (i) waveform differencing by remote diode shorting and (ii) covering probe conductors with resistive coatings. Experiments using electrically conductive water solutions (0–5 dS m^-1) and three-rod Zegelin type probes show conventional dual-tangent waveform analysis and waveform differencing using manual short circuits can accurately determine travel time but the remote diode shorting method can be systematically biased by the electrical properties of the diodes used. Three-rod Zegelin-type probes with a high resistance coating on the central rod permit collection of analyzable waveforms for solutions with electrical conductivities at least as high as 70 dS m^-1. Dual-tangent analysis of the raw waveform is found to be more accurate than the remote diode shorting method within water solutions and within silica sand saturated with an electrically conductive water solution. The probe coating creates a nonlinear relationship between the apparent dielectric permittivity estimated using a coated probe and the actual sample apparent dielectric permittivity. Experimental measurements of this relationship can be fitted using an equation of the form for a coaxial cell. A three-rod coated probe with a single diode at the probe head is a practical means to collect interpretable waveforms in media with high electrical conductivity. However, measurements of travel time alone may not be sufficient to determine water content in soils with high concentrations of dissolved ions in the soil water solution.

Abbreviations: DC, direct current • MTDR, MoisturePoint TDR instrument • PIN, positive-intrinsic-negative diodes • TDR, time domain reflectometry • TEM, transverse electric and magnetic mode • TTDR, Tektronix TDR instrument

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