Geological studies

Mapping of Ground Deformation Related to Permafrost Thawing – Nanisivik, Nunavut (Canada)

Effectively manages the implementation of infrastructure in arctic environments when faced with climatic change.

Project Start:
June 2013
Length:
20 months

Mining activity in Canada’s north has been significant over the past 40 years or so. In arctic environments, this is challenging notably with respect to accessibility and climatic conditions. The seasonal freeze and thaw of the permafrost surface active layer leads to cyclical subsidence and heave, which can cause damage to infrastructure and/or lead to environmental problems resulting from the movement of stockpiles of materials.

Context

Mining companies and government agencies that must ensure that closed mines undergo rehabilitation and conform to environmental regulations are concerned by climatic change and its impact on permafrost thawing. The Nanisivik zinc-lead mine, located on the northernmost part of Baffin Island, was closed off in 2002 after some 25 years in operation. Rehabilitation of the site, carried out soon after mine closing, was designed to minimize (1) acid runoff resulting from interactions of residues with the atmosphere and (2) risks related to physical movement of rehabilitated areas. Major modifications of the permafrost layer may cause subsidence of rehabilitated areas leading to potential environmental problems.

Challenges

Differential SAR interferometry, the technique used to measure surface displacements, requires a certain number of radar images, which must be acquired under snow-free conditions and with the same satellite-ground target geometry. The snow-free season is short (end of June to mid-September) in this type of environment. This limits the number of suitable images that allow getting valid measurements of vertical movement.

Solutions

Stereoscopic high-resolution (50 cm) optical satellite images, obtained from the GeoEye-1 satellite, were used to produce a detailed digital elevation model (DEM) and a map of surface materials for validating the results of interferometric processing and vertical movement measurements. The DEM is used to remove the topographic effects in interferometric processing. With its 11-day revisit capacity, TerraSAR-X managed to collect seven useable images, i.e., under snow-free conditions and with the same geometry, between July and September. Differential interferometry (DInSAR) processing techniques were used to derive surface vertical displacement maps.

Results

The DInSAR technique used with TerraSAR-X allowed producing displacement maps with centimetre accuracy. The results are in accordance with scientific knowledge reported by several studies in the same type of environment that state that (i) areas of significant deformation correspond to thicker permafrost active layers (thicker surficial deposition) prone to thaw instability; (ii) bedrock and thin surficial deposits are not affected by surface movements; (iii) affected surfaces are continuously subsiding during the permafrost thawing season; and (iv) the maximum extent of surface subsidence is about 3 cm during the summer season.

Key Takeaways

  • EO-based information to ensure the safety and efficiency of activities in arctic environments
  • Better overall planning and management of mining processes and infrastructure deployment
  • Reduction of costs in comparison with those of ground truth investigations
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