Effect of electrode shape on grounding resistances - Part 2: Experimental results and cryospheric monitoring

Although electric resistivity tomography (ERT) is now regarded as a standard tool in permafrost monitoring, high grounding resistances continue to limit the acquisition of time series over complete freeze-thaw cycles. In an attempt to alleviate the grounding resistance problem, we have tested three electrode designs featuring increasing sizes and surface area, in the laboratory and at three different field sites in Greenland. Grounding resistance measurements showed that changing the electrode shape (using plates instead of rods) reduced the grounding resistances at all sites by 28%-69% during unfrozen and frozen ground conditions. Using meshes instead of plates (the same rectangular shape and a larger effective surface area) further improved the grounding resistances by 29%-37% in winter. Replacement of rod electrodes of one entire permanent permafrost monitoring array by meshes resulted in an immediate reduction of the average grounding resistance by 73% from 1.5 to 0.4 kΩ (unfrozen conditions); in addition, the length of the acquisition period during the winter season was markedly prolonged. Grounding resistance time series from the three ERT monitoring stations in Greenland showed that the electrodes were rarely perfectly grounded and that grounding resistances exceeding 1 MΩ may occur in severe cases. We concluded that the temperature, electrode shape, and lithology at the sites have a marked impact on electrode performance. Choosing an optimized electrode design may be the deciding factor for successful data acquisition, and should therefore be considered when planning a long-term monitoring project.