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Using Synthetic Aperture Radar (SAR) imagery to perform interferometry (InSAR), areas of instability can be found, measured, and reported on for risk management. Using historical imagery, we discovered a considerable increase in the rate of displacement for the landslide slope in the summer prior to failure. In previous years, the area had been slowly subsiding. These InSAR results were included in the recently published paper, The 28 November 2020 Landslide, Tsunami, and Outburst Flood – A Hazard Cascade Associated With Rapid Deglaciation at Elliot Creek, British Columbia, Canada examining the landslide and debris flow events.
InSAR exploits the phase difference between two SAR images acquired at different times over the same area to estimate ground deformation with centimeter to millimeter precision. Images must be acquired using the same satellite parameters and orbit position in order to accurately perform the phase difference.
Sentinel-1A and Sentinel-1B (collectively referred to as “S1”) , were launched under the European Space Agency’s (ESA) Copernicus programme. They collect near continuous SAR imagery all over the world and this data is made publicly available. S1 typically acquires images in 250 km wide swaths with 5 m by 20 m spatial resolution, every six days in Europe and 12 days across the rest of the world. This means, every 12 days, we are able to capture an image of the area of interest with the same imaging parameters.
For this study, we used images collected from two neighboring ascending orbits (orbits 35 and 137) to perform two separate analyses. Due to the topography at the head of Elliot Creek, no descending orbit had good sensitivity over the landslide area, thus no data from the descending orbits were used. The ascending orbits were used to perform independent line-of-sight (LOS) analyses.
Only snow-free SAR images were used for the study; images with snow cover were identified and excluded based on historical optical satellite imagery. A total of 127 images were collected over the study area from June 2018 to November 2020. Of these, 73 were removed due to snow, yielding 54 images for this analysis (25 for orbit 35 and 29 for orbit 137).
An overview of the estimated displacement rate over the study area is seen above for orbits 35 and 137, respectively. Individual InSAR measurements are displayed as coloured points where the color represents the component of displacement along the satellite’s LOS. The red end of the colour scale represents displacement away from the satellite, while the blue end of the scale denotes displacement towards the satellite.
The InSAR results show precursory displacement leading up to the failure of the slope. A pronounced increase in the displacement rate is evident in summer 2020 compared to previous years. A LOS displacement of up to 150 mm was estimated from June to October 2020, yielding a LOS velocity of 300 mm per year, in summer 2020. The two ascending analyses show comparable displacement rates and displacement area extents.
Displacement areas, such as those at Elliot Lake, can be discovered ahead of failure by using the wide coverage and high precision of InSAR.
We use Sentinel data, continuously downloaded and processed in the background, for all ongoing clients. This background data is used in our Emergency Response program and is designed to quickly provide geotechnical staff with the most recent displacement information. Once initiated, we can acquire the next available Sentinel imagery over the site and provide analysis ahead of the next contracted product delivery.