Transverse sample area of two- and three-rod time domain reflectometry probes: Electrical conductivity

Numerical models have been applied successfully to the analysis of the sensitivity and transverse spatial sample areas of time domain reflectometry (TDR) probes to lateral variations in dielectric permittivity (ε). However, no similar treatment has been presented for the spatial sensitivity of TDR to lateral variations in electrical conductivity (σ). The objective of this investigation was to examine the response of conventional two- and three-rod probes to sharp changes in σ within their sample areas. The spatial weighting, predicted numerically for probes of varying geometries with sharp ε boundaries and two different σ contrasts in the plane perpendicular to the direction of wave propagation, shows good agreement with the TDR-measured σ. For low-loss conditions the sensitivity distribution of TDR is shown to be independent of the value of σ. This demonstrates that the spatial sensitivities to dielectric permittivity and electrical conductivity are the same for these conditions and that TDR-measured water contents can be used to correct TDR-measured σ for water content effects.