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

Abstract

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 [Formula: see text] to [Formula: see text] (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 [Formula: see text] 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.

Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques

“…The highest contact impedances observed on Wetterstein were around 15 MΩ. For such poorly coupled sensors, obtaining stable ERT measurements with acceptable noise properties is extremely challenging and often beyond the capabilities of typical survey instrumentation [ Tomaškovičová et al , ].…”

“…Our geoelectrical results highlight the additional challenge of applying conventional geoelectrical methodology to monitoring of permafrost‐affected rocks, when compared to monitoring of permafrost dynamics in soils or unconsolidated sediments. Irrespective of the ambient temperatures and seasonal regime, the observed ERT contact impedances on our rock samples were at least 1–2 orders of magnitude higher than typical values observed with ERT monitoring arrays installed in soft soils or sediments [ Doetsch et al , ; Tomaškovičová et al , ]. As shown in section 3.5.1, contact impedances observed on low‐porosity, high‐strength limestone are higher still (by factors of 20–50) compared with those achievable on high‐porosity, low‐strength chalk.…”

“…Electrode coupling is a critical parameter and known source of error in geoelectrical measurements, particularly for cryospheric applications [Doetsch et al, 2015;Tomaškovičová et al, 2016]. High contact impedances can severely limit the size of the injectable current or generate unstable potential measurements, thus creating bias in some geoelectrical measurements [Zonge and Hughes, 1986;Ingeman-Nielsen et al, 2016].…”

Effect of electrode shape on grounding resistances — Part 1: The focus-one protocol

“…Such issues have continued to challenge generations of geophysicists because the limitations on total transmitted current lead to lower measured potentials and lower signal-to-noise levels (Dahlin and Loke, 1998;Dabas et al, 2000;Ishikawa, 2008;Doetsch et al, 2015). In severe cases, especially when ground freezing or drying is involved, grounding resistances may be so high that the transmitter circuitry loses the ability to properly regulate the current (or transmit at all) so that valid measurements cannot be obtained (Hilbich et al, 2009;Doetsch et al, 2015;Tomaškovičová et al, 2016).…”

“…It provides the theoretical basis for the practical experiments reported in Tomaškovičová et al (2016), in which we compare grounding resistance measurements of different electrode designs under varying environmental conditions.…”