Brandt, Inooraq

Christiansen, Anders V.

Dahlin, Torleif

Elberling, Bo

Ingeman-Nielsen, Thomas

Tomaskovicova, Sonia

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 18, 2017Optimizing electrode shapes for ERT monitoring in permafrost areasTomaskovicova, Sonia; Ingeman-Nielsen, Thomas; Christiansen, Anders V.; Brandt, Inooraq; Dahlin,Torleif; Elberling, BoPublished in:Proceedings of the XI. International Conference on PermafrostPublication date:2016Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Tomaskovicova, S., Ingeman-Nielsen, T., Christiansen, A. V., Brandt, I., Dahlin, T., & Elberling, B. (2016).Optimizing electrode shapes for ERT monitoring in permafrost areas. In Proceedings of the XI. InternationalConference on Permafrost (pp. 988-989)XI. INTERNATIONAL CONFERENCE ON PERMAFROST | 20.-24. JUNE 2016Optimizing electrode shapes for ERT monitoring in permafrost areasSonia Tomaskovicova1, Thomas Ingeman-Nielsen1, Anders V. Christiansen2, Inooraq Brandt3, Torleif Dahlin4,& Bo Elberling51Technical University of Denmark, Denmark2Aarhus University, Denmark3Orbicon, Greenland4Lund University, Sweden5University of Copenhagen, DenmarkVery large and extreme variation in grounding res-istances (GR) are some of the main challenges in themonitoring of permafrost and active layer processeswith Electrical Resistivity Tomography in sediment-ary deposits. The unfrozen, wet active layer typicallyresults in low electrode grounding resistances in sum-mer. In winter, however, when the active layer freezes,grounding resistances may increase many orders ofmagnitude, and hamper the acquisition of meaning-fully long time series.To investigate the factors conditioning the GR, wetested three stainless steel electrode types featuringincreased size and surface area:1. a rod electrode, with diameter 1 cm, length 8cm and effective surface area: 27 cm22. a square plate electrode, 10*10*0.1 cm (h*w*t),effective surface area: 204 cm23. a wire mesh electrode, 10*10*0.6 cm (h*w*t),effective surface area: 985 cm2The electrodes were tested at three different loc-alities in West Greenland: Qeqertarsuaq (69°15’N,53°30’W, 30 m a.s.l.), Ilulissat (69° 14’N, 51° 3’W,33 m a.s.l.) and Sisimiut site (66° 56’N, 53° 36’W,approx. 48 m a.s.l.). Each of these sites representsa different surface geology. The soil type at Ilulissatis a silty clay with little organic cover, at Sisimiutit is well drained fine sand likewise with very littleorganic cover, while at Qeqertarsuaq the surface de-posits consists of a coarse sand with high organiccontent. Sisimiut and Qeqertarsuaq are located inthe discontinuous permafrost zone, while Ilulissat hascontinuous permafrost.At each site we installed three 10-electrode layouts(electrode separation 30 cm), each featuring a differ-ent electrode type. The grounding resistance of eachindividual electrode was measured using the focus-oneprotocol. The focus-one protocol, is essentially a two-electrode resistance measurement, where one electrodeis connected to one terminal of the instrument, whileall the other electrodes of the layout are connectedin parallel to the other terminal, thereby effectivelyminimizing the contribution of their grounding resist-ances to the total measurement. Measurements wereconducted at all three sites summer, fall and winter,to study also the effect of ground temperature.The measured grounding resistances range fromabout 600 W to 1.2 MW (across seasons, electrodetypes and localities), and the effects of electrode type,locality (surficial geology) and ground temperatureall tested statistically significant. Plate electrodesshowed 28 to 69 % lower GR than rods during boththawed and frozen ground conditions at all sites. Meshelectrodes improved the GR by 29 to 37 % relative toplate electrodes in winter at the Ilulissat and Qeqer-tarssuaq sites. The increased effective surface area ofthe mesh electrodes seems to be an advantage whenthe electrodes are inserted or buried in fine grainedmineral or organic soils with some cohesive properties,where the soil may fill the mesh openings. At theSisimiut site, the coarser mineral soil results in noadvantage of using mesh electrodes. Under thawedconditions, the plate and mesh electrodes did not teststatistically different at any of the sites, indicatingthat the natural variation in soil and burial conditionsis larger than the effect of the larger surface area.An existing 64 electrode monitoring array installedat the Ilulissat site was used to further documentthe advantage of mesh electrodes over rod electrodes.Operating the monitoring setup, which was originallyinstalled using rod electrodes, had been a challengedue to high grounding resistances in winter, effect-ively prohibiting the collection of measurements. Re-placement of the rod electrodes with mesh electrodes988XI. INTERNATIONAL CONFERENCE ON PERMAFROST | 20.-24. JUNE 2016resulted in an immediate reduction of the averageGR by 73 % from 1.5±0.9 kΩ to 0.4±0.1 kΩ (thawedconditions). Comparable winter grounding resistanceswere reduced from 64±32.1 kΩ to 25±11.4 kΩ, andfollowing the replacement, measurements could becollected throughout the season.We conclude that temperature, electrode shape andproperties as well as lithology of the monitored sitehave significant impact on electrode grounding res-istance and array performance, particularly in thecold/dry season. Thus, optimization of the electrodedesign should be a main consideration when planninga long-term monitoring project. Under the conditionswe tested, the use of plate or mesh electrodes insteadof rod electrodes were an advantage at all sites and allseasons. Mesh electrodes constitute an improvementonly when the soil type allows to take advantage ofthe larger effective surface area (fine grained mineralor organic soils).989

Optimizing electrode shapes for ERT monitoring in permafrost areas

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Optimizing electrode shapes for ERT monitoring in permafrost areas

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