Evaluation of Uncoated and Coated Time Domain Reflectometry Probes for High Electrical Conductivity Systems Craig Nichol,* Roger Beckie, and Leslie Smith ABSTRACT column experiment and a field-scale experiment (Nichol et al., 2000). In our application, a high soil water electri- High sample electrical conductivity reduces the quality of a time cal conductivity (5–20 dS m -1 ) caused by oxidizing sul- domain reflectrometry (TDR) waveform by the loss of signal ampli- tude. Two strategies are examined to obtain higher signal/noise wave- fide minerals in the mine waste leads to poor signal forms: (i) waveform differencing by remote diode shorting and (ii) quality using conventional TDR techniques. Sufficient covering probe conductors with resistive coatings. Experiments using signal amplitude could not be reliably obtained using a electrically conductive water solutions (0–5 dS m -1 ) and three-rod Zege- short probe of conventional design. Two techniques for lin type probes show conventional dual-tangent waveform analysis obtaining improved signals were selected: (i) waveform and waveform differencing using manual short circuits can accurately differencing using remote diode shorting (Hook and determine travel time but the remote diode shorting method can be Livingstone, 1992), and (ii) the use of a resistive probe systematically biased by the electrical properties of the diodes used. coating (Kelly et al., 1995; Ferre ´ et al., 1996; Mojid et Three-rod Zegelin-type probes with a high resistance coating on the al., 1998). Our aim is to derive a method to reliably central rod permit collection of analyzable waveforms for solutions with collect waveforms that can be interpreted by automated 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 waveform fitting to derive an estimate of travel time remote diode shorting method within water solutions and within silica and hence apparent dielectric permittivity. sand saturated with an electrically conductive water solution. The probe In this paper, we test both techniques for the collec- coating creates a nonlinear relationship between the apparent dielec- tion of interpretable waveforms in high electrical con- tric permittivity estimated using a coated probe and the actual sample ductivity media. We first provide a brief summary of apparent dielectric permittivity. Experimental measurements of this the TDR method and discuss how it is affected by bulk relationship can be fitted using an equation of the form for a coaxial soil electrical conductivity. We then describe the rele- cell. A three-rod coated probe with a single diode at the probe head vant theory of waveform differencing by remote diode is a practical means to collect interpretable waveforms in media with shorting and the use of probes covered with a high- high electrical conductivity. However, measurements of travel time resistance coating. A series of experiments are then pre- alone may not be sufficient to determine water content in soils with high concentrations of dissolved ions in the soil water solution. sented. The first examines the performance of a short (160 mm) uncoated probe using water solutions of vari- able electrical conductivity. The second experiments an- alyse the performance of the waveform differencing S oil water content can be measured in the field method using the remote diode shorting method and using gravimetric methods, neutron scattering, or manual short circuits under the same conditions as the techniques based upon the thermal or electrical proper- first experiments. We then examine the performance ties of soil-air-water mixtures. Electrical methods are of a coated probe under the same conditions for both advantageous because they are easily automated to con- conventional TDR waveform collection and analysis, duct unattended measurements with high sampling rates and using the remote-diode shorting method. We next at multiple locations. Time domain reflectrometry is a present the performance of a coated probe in silica commonly used electrical technique. With knowledge sand saturated with water solutions of varying electrical of the electrical properties of soil-air-water mixtures, conductivity. A last experiment demonstrates a practical and appropriate experimental calibrations, the bulk soil means of compensating for the effect of a probe coating dielectric permittivity can be related to the volumetric on the estimation of sample dielectric permittivity. water content and the bulk soil electrical conductivity can be related to the soil water electrical conductance. THEORY While TDR methods have gained wide acceptance and usage in soils, relatively less attention has been given Time domain reflectometry can be used to determine the to TDR’s application to high electrical conductivity ma- relative apparent bulk dielectric permittivity (ε app ), the bulk terials such as mining waste. In these materials, conven- electrical conductivity ( DC ), and in certain circumstances, the frequency-dependent real and imaginary parts of the complex tional TDR probe designs and signal analysis techniques sample dielectric permittivity (Hoekstra and Delaney, 1974; fail to provide sufficiently accurate estimates of the sam- Dalton et al., 1984; Topp et al., 1988; Heimovaara et al., 1996; ple dielectric properties. This paper describes an ap- Friel and Or, 1999). Time domain reflectometry measures the proach to design an automated TDR system for the propagation of a fast rise time, step voltage pulse through a measurement of water content in unsaturated mine coaxial cable to a waveguide (probe) in contact with the sam- waste rock. The TDR system is used in both a laboratory ple. Part of the incident pulse energy reflects back to the TDR Dep. of Earth and Ocean Sciences, Univ. of British Columbia, 6339 Abbreviations: DC, direct current; MTDR, MoisturePoint TDR in- Stores Road, Vancouver, BC, Canada V6T 1Z4. Received 10 May strument; PIN, positive-intrinsic-negative diodes; TDR, time domain 2001. *Corresponding author (cfnichol@hotmail.com). reflectometry; TEM, transverse electric and magnetic mode; TTDR, Tektronix TDR instrument. Published in Soil Sci. Soc. Am. J. 66:1454–1465 (2002). 1454 Published September, 2002