Influence of Electrode Heights on 4-point and 2-point Measurements Based on Capacitively Coupled Electrodes

To survey resistive grounds and estimate electrical parameters, the capacitive resistivity imaging technique (CRI) can be used. It overcomes coupling problems that are known from DC techniques. Instead of metal stakes, thin plates are used to inject AC current into the subsurface. In field measurements capacitively coupled electrodes often do not have direct contact with the ground, because of vegetation or a rough surface. We are focusing on electrode height variations and its effect on 4-point and 2-point impedances theoretically and experimentally. To keep controlled conditions, we measured in the laboratory in a box (1.2m x 1.2m x 0.8m) filled with dry sand and gravel. We increased electrode heights from 1µm to 0.8m and repeated the measurements to verify the reproducibility. Results show, that the range of electrode heights can be separated into sections that are characterised by different properties with respect to sensitivity to height variations and electrical parameters of the subsurface. It is also beneficial to measure 2-point impedances. They can be used to assess 4-point impedance measurements. For investigations on unknown soils, it is recommended to use a wide range of electrode heights. This allows a reduction of non-uniqueness with respect to electrical parameters and electrode heights.