Another satellite in our tool box is the Canadian designed and built Radarsat. Compared to most other satellites, it does not take pretty pictures. Instead, the Radarsat satellite fires a radar beam to earth at a specific wavelength, and the beam bounces off the surface of the earth and returns back the satellite. Differential Interferometric Synthetic Aperture Radar (D-InSAR) is a technique that uses repeat synthetic aperture radar (SAR) observations, separated in time, of the same area on the Earth’s surface to detect small movements of the ground surface. Since the satellite is moving in an along a fixed orbit at constant height above earth surface, the distance between the satellite and a specific point on the earth’s surface is a constant. With a repeat observation, millimeter-scale changes in the distance between the satellite and the earth’s surface can be calculated by comparing the phase shift of the wavelength of the radar signal that was fired by the satellite. A 3D model (digital elevation model) is used to remove the phase difference resulting from topography and the remaining phase difference is converted to vertical displacement which represents the displacement of the ground surface. Although this is a simplistic explanation of the process, a more detailed discussion of these processing steps may be found in Short et al. (2011).
Using SAR data collected during the summer of 2010, scientists at the Canada Centre for Remote Sensing, the Geological Survey of Canada, along with their northern partners, used differential interferometry techniques to map ground surface displacement in the Yellowknife area. Figure XY shows the ground displacement derived using stacked RADARSAT-2 D-InSAR from May 23 to September 20, 2012. The results are categorized in terms of the relative amount of displacement rather than focussing on the absolute values (units are in meters). Areas shown in red and yellow, are undergoing low and moderate downward displacement (subsidence) in the range of -1 to -3 cm and -3 to -6 cm per season, respectively. Ground that is considered stable, where no vertical change was calculated is shaded in blue. Areas where the displacement is upward, due to ground surface heave or vegetation growth of up to 6 cm, are shown in pink. At some locations, labeled ‘Incoherent’ the vertical change is presumed to be significant and is beyond the measuring capabilities of the D-InSAR process.
The ground surface displacements shown on the map are considered to be seasonal since they occurred during the May to September time period. Areas of downward displacement may result from seasonal subsidence caused by thawing of the active layer (seasonally-frozen ground) or permafrost (perennially-frozen ground) or soil compaction. Upward surface displacement may result from ground surface heave or, under certain circumstances, vegetation growth. The latter represents an apparent upward displacement that is not the product of ground surface displacement. Both downward and upward ground surface displacements may be from natural and human-induced processes.
Verification of the results of the InSAR displacement in areas identified as having moderate seasonal surface subsidence. We observed and measured indications of subsidence such as undulating roadways, areas that were presumed or measured to have been levelled when constructed, and noted and measured cracks and rotated sidewalks.
The purpose of this study is to present and interpret D-InSAR measurements of seasonal surface displacement over discontinuous permafrost terrain in Yellowknife. We conducted field verification to support the results observed by the InSAR process by documenting areas undergoing low and moderate downward displacement and noting the impacts on private property and municipal infrastructure. Mapping of surface displacement is important to assess ground and infrastructure stability, and to monitor mitigation in problem areas, and to plan future land use. Surface displacement may be from natural or human-induced processes that cause vertical movement of the ground. In Yellowknife, and other areas in northern Canada, surface displacement may be caused by freezing and thawing of the active layer (seasonally-frozen layer) or the permafrost (perennially-frozen ground). This presents challenges that increase the cost of constructing and maintaining infrastructure (e.g. roads and buildings), especially if the displacement continues in the same direction over an extended period of time. The knowledge of where the problem areas occur and the relative magnitude of displacement provides engineers and municipal planners with key information required to reduce the cost and risk of developing and maintaining northern infrastructure.
Wolfe, S., Short, N., Morse, P., Olthof, I., Schwarz, S., Stevens, C. (2013). Application of RADARSAT-2 D-InSAR season surface displacement to municipal land use in discontinuous permafrost terrain, Yellowknife, Northwest Territories, Canada. fCorresponding author, . ** Note: this work is not yet published
Short, N., Brisco, B., Couture, N., Pollard, W., Murnaghan, K. and Budkewitsch, P. 2011. A comparison of TerraSAR-X, RADARSAT-2 and ALOS-PALSAR interferometry for monitoring permafrost environments, case study from Herschel Island, Canada. Remote Sensing of Environment, 115, 3491–3506, doi:10.1016/j.rse.2011.08.012.