Abstract
Rock glaciers are widespread in European Alps and significant for their content of Alpine permafrost. Indeed, they are characterised by a mix of ice and rock, which is related to the presence of permafrost in mountainous areas. The landslide-like behavior of rock glacier is a complex mechanism influenced by the interaction of several factors such as topographical predisposition, internal structure, debris granulometry, temperature, hydrology, and stress conditions. The external temperature is considered one of the most important factors controlling rock glacier flow variation at both inter-annual and seasonal time scales, showing mean velocities ranging from centimetres to meters per year. Hence, the temperature rising due to climate change leads to changes in kinematics of rock glaciers that increase hazards for mountainous settlements and infrastructures.
Despite differential SAR interferometry (DInSAR) is a very effective tool for measuring ground stability, its application to rock glacier monitoring poses several critical issues. First, the steep topography may lead to unfavorable illuminating conditions in terms of either unfeasible detection over layover and shadow areas, or low sensitivity to the ground displacement. Second, the presence of dense vegetation and changeable snow cover conditions cause DInSAR signal decorrelation. Third, displacement kinematics are characterised by both linear and non-linear components and high displacement rates leading to measurements often corrupted by aliasing. This work investigates the rock glacier stability in Val Senales (Italian Alps) by exploiting both the interferometric phase and amplitude of SAR image stack at C-band and X-band.
A multi-temporal DInSAR processing of 345 Sentinel-1 SAR images acquired between 2015 and 2022 was performed by exploiting both persistent and distributed scatterers through SPINUA algorithm. Ad hoc processing strategies were adopted in order to overcome both signal decorrelation due to changeable snow cover conditions, and aliasing due to very high displacement rates. The algorithm was run by selecting spring-summer acquisitions, and forced to search for solutions corresponding to phase changes behind the aliasing limit. The resulting mean line of sight (LOS) displacement map show several areas affected by ground displacements, which lay on exactly within the borders of rock glaciers derived from inventory maps. In some cases, a lack of DInSAR coherent targes occurs just within rock glacier borders, being possibly caused by very high displacement rates not properly measured by the MTInSAR algorithm despite ad hoc processing. These areas were further investigated by exploring maps of DInSAR phase and coherence generated from consecutive Sentinel-1 acquisitions, as well as changes occurring in orthoimages from different years.
Moreover, in order to overcome the DInSAR limitations related to high deformation rates, offset tracking techniques were experimented, which exploit SAR amplitude instead of phase. This analysis was focused on the interesting case study of Lazaun rock glacier. It is a tongue-shaped, 660 m long and 200 m wide, active rock glacier located in Senales Valley (Italy) at about 2600 m asl. Interannual and seasonal displacement rates up to few mm/day have been reported by previous studies, which used different techniques including GNSS, inclinometers, and both ground based and spaceborne SAR systems.
Offset tracking algorithms can be used to measure displacements with a sensitivity that is a fraction of the data spatial resolution. In particular, the Intensity Tracking algorithm was used in this case, considering that it is not possible to use its alternative, the Coherence Tracking algorithm, due to the low coherence values encountered in the test area. Considering the topography, the size of the area of interest and the expected entity of the displacement, Ascending SAR spotlight data have been selected and acquired.
At this aim six TerraSAR-X staring spotlight and six COSMO-SkyMed Second Generation (CSG), both with a pixel spacing of less than 1m have been processed optimizing the parameters to the characteristics of the area under study to get displacement maps in azimuth and range directions. These images have been acquired in the snow free period between 2016 and 2018 (TerraSAR-X) and in 2022 (CSG) and have allowed to estimate seasonal and inter-annual displacement maps. GPS field campaigns were carried out in correspondence with some of the satellite acquisitions. In this case, a comparison of the results obtained with ground and satellite data has been possible reaching in the case of annual displacement a Root Mean Square Difference of 0.347 and 0.355 mm/day with a Pearson Coefficient of 0.883 and 0.895 in azimuth and range direction respectively.
Results coming from offset tracking provide useful information on the displacements occurring within the Lazaun borders, which is complementary with respect to measurements performed through MTInSAR, which instead suffer of lack of coherent targets due to phase aliasing.
Finally, the mean velocities and displacement time series were ingested into a GIS environment together with other informative layers such as multi-temporal mean SAR amplitude, DInSAR coherence maps, rock glacier classes (according to [2]), optical orthoimages, permafrost index map, and Difference Vegetation Index (NDVI). Then, the rock glacier activity was reclassified by adopting the more recent procedure proposed in [3], which is based also on the DInSAR products. This new classification has been compared to that derived according to [2] showing several differences. For instance, 3 out of the 6 rock glaciers classified as indefinite were reclassified as relict or translational, 6 out of the 11 rock glaciers classified as relict were reclassified as transitional, and conversely, one rock glacier classified as active was reclassified as relict
References
* Krainer, K., Bressan, D., Dietre, B., Haas, J. N., Hajdas, I., Lang, K., Mair, V., Nickus, U., Reidl, D., Thies, H., & Tonidandel, D. (2015). A 10,300-year-old permafrost core from the active rock glacier Lazaun, southern Ötztal Alps (South Tyrol, northern Italy). Quaternary Research, 83(2), 324–335. https://doi.org/10.1016/j.yqres.2014.12.005
* E. Bollmann, L. Rieg, L., M. Spross, R. Sailer, k. Bucher, M. Maukisch, M. Monreal, A. Zischg, V. Mair, K. Lang, and J. Stötter, “Blockgletscherkataster in Südtirol-Erstellung und Analyse,” Permafrost in Südtirol, Innsbrucker Geographische Studien. J. Stötter & R. Sailer Eds., pp. 147–171, 2012.
* IPA Action Group - Rock glacier inventories and kinematics. Towards standard guidelines for inventorying rock glaciers: practical concepts (version 2.0), pp. 1–10, 2022
Acknowledgments
This work was carried out in the framework of the project “CRIOSAR: Applicazioni SAR multifrequenza alla criosfera”, funded by ASI under grant agreement n. ASI N. 2021-12-U.0. TerraSAR-X data were provided by the European Space Agency, Project Proposal id 34722, © DLR, distribution Airbus DS Geo GmbH, all rights reserved.
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