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Mapping and characterization of unstable slopes with Sentinel-1 multigeometry InSAR



Being a mountainous country, with long fjords and steep valley sides, Norway is particularly susceptible to large rock avalanches. In the last 100 years, over 170 people have been killed by tsunamis in fjords caused by large rock avalanches. In each case, the rock avalanche was preceded by many years of slow movement, with acceleration prior to slope failure. With several thousand kilometres of inhabited coastline and valleys, it is a challenge to identify similar hazards in an efficient manner. Once we suspect an area to be sliding, it may take several years of measurements to confirm it, and an extensive ground instrumentation to characterize the type of motion.

The Geological Survey of Norway (NGU) is responsible for hazard and risk classification of large rock slope instabilities in Norway. They also assist the Norwegian Water Resources and Energy Directorate (NVE) with long term monitoring of high risk instabilities. A very important factor in determining hazard is the determination of rates of movements. This is predominantly done using InSAR, although GNSS and in situ instrumentation (crack meters, tilt meters, borehole instrumentation, total stations etc.) are also applied at site level.

In Norway, there has been a significant interest from the public stakeholders (NGU and NVE) to use InSAR, mainly for mapping of landslides. NGU launched a development project in 2016, with Norut a prime contractor, to set up a national InSAR-based deformation mapping service, based upon satellite data from Sentinel-1. The first national deformation map, produced by using Sentinel-1 Persistent Scatterer Interferometry (PSI), was publicly released in November 2018. The system, when in operational phase, will provide updated displacement maps at a national scale, and with an open data policy.

It is however well know that when the true displacement direction differs from the satellite line-of-sight (LoS), the sensitivity decreases and interpretation of InSAR deformation measurements may become challenging. Relating InSAR displacement maps to ongoing surface displacement processes can be difficult. A knowledge of the LoS direction for the applied satellite geometry as well as factors controlling the direction of displacement (gradient and aspect of the terrain, orientation of controlling geological structures) is required to understand how much of the true three-dimensional (3D) displacement can be observed.

Combining InSAR data from ascending and descending satellite orbits can increase sensitivity for displacement by providing information about the displacement, decomposed into the East-West and Vertical vector surface. The resulting products will contain information about both the magnitude and the direction of surface displacement. Combination is possible in areas covered by at least two spatially and temporally overlapping InSAR datasets, from ascending and descending orbit geometries. By combining InSAR information determined from different lines of sight, the understanding of the type of movement taking place is improved. For example, surface parallel, mostly vertical, toppling etc.

In this project, we will develop higher-order products based on combination of different InSAR datasets in order to ease the interpretation of site-specific deformation processes. The aim of our project is to define and develop geologically meaningful InSAR products to provide meaningful information about slope processes, which could extend the use of Sentinel-1 InSAR in landslide risk management in Norway.


Prime contractor