A numerically based analysis of the sensitivity of conventional and alternative time domain reflectometry probes

Conventional time domain reflectometry (TDR)probes are comprised of two or three parallel metal rods. Other probes have been designed for water content profiling [Hook et al., 1992; Ferre et al., 1998b; Redman and DeRyck, 1994], surface water content measurement [White and Zegelin, 1992; Selker et al., 1993], or measurement in electrically conductive media. We use the numerical approach of Knight et al. [1997] to predict the responses of variants of these probes when surrounded by materials with different relative dielectric permittivities. These predictions are compared with published calibration curves and analytical solutions where available. Conventional rods are shown to be most sensitive to changes in the water content of the medium. The Hook et al. [1992] probe shows the highest sensitivity of the alternative designs; both surface probes can be used to measure the water content at the soil surface nonintrusively with similar sensitivities. All of the alternative probes have sensitivities that vary with the soil water content, leading to incorrect averaging of the water content if the water content varies along the probes. However, those probes that place nonmetallic components in series with the soil have more pronounced errors than those that place these materials and the soil more nearly in parallel.