Abstract
When installing TDR probes to rock, void spaces (gaps) between the probe rods and rock may be en-countered. In this study, the dielectric constants of water and air were measured with various controlled gaps and ef-fects of the gaps were experimentally investigated. We fo-cused on a two-rod probe for which analytical and numer-ical solutions were available. Symmetric longitudinal gaps (identical uniform gaps along both rods), non-symmetric longitudinal gaps (uniform gaps along only one rod), and end gaps beyond the end of the rods were considered. For the symmetric cases, measured dielectric constant values of water and air were compared with analytical and/or nu-merically simulated values, and a close agreement was ob-served. For the non-symmetric cases, such agreement was seen only when the gap width was relatively small. As the gap widths increased, the dielectric constant of wa-ter was underestimated compared to the numerically esti-mated values. Such underestimation was more significant when the gaps were along the rod connected to the center conductor, suggesting that the sensitivities of the two rods were uneven. The effect of the end gaps was very lim-ited whereas the longitudinal gaps, e.g., due to loose guide holes or soil shrinkage can lead to significant measurement errors.
