Assessing Rock Glacier Activity In Val Senales By Exploiting Multiband SAR Data Through Differential SAR Interferometry And Offset Tracking

Rock glaciers are widespread in European Alps and significant for their content of Alpine permafrost. Indeed, they are characterised by a mix ofice and rock, which is related to the presence of permafrost in mountainous areas. The landslide-like behavior of rock glacier is a complexmechanism 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 glacierflow variation at both inter-annual and seasonal time scales, showing mean velocities ranging from centimetres to meters per year. Hence, thetemperature rising due to climate change leads to changes in kinematics of rock glaciers that increase hazards for mountainous settlementsand infrastructures. Despite differential SAR interferometry (DInSAR) is a very effective tool for measuring ground stability, its application to rock glacier monitoringposes several critical issues. First, the steep topography may lead to unfavorable illuminating conditions in terms of either unfeasible detectionover layover and shadow areas, or low sensitivity to the ground displacement. Second, the presence of dense vegetation and changeable snowcover conditions causes DInSAR signal decorrelation. Third, displacement kinematics are characterised by both linear and non-linearcomponents and high displacement rates leading to measurements often corrupted by aliasing. This work investigates the rock glacier stabilityin 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 bothpersistent and distributed scatterers through SPINUA algorithm. Ad hoc processing strategies were adopted in order to overcome both signaldecorrelation due to changeable snow cover conditions, and aliasing due to very high displacement rates. The algorithm was run by selectingspring-summer acquisitions, and forced to search for solutions corresponding to phase changes behind the aliasing limit. The resulting meanline of sight (LOS) displacement map show several areas affected by ground displacements, which lay on exactly within the borders of rockglaciers derived from inventory maps. In some cases, a lack of DInSAR coherent targes occurs just within rock glacier borders, being possiblycaused by very high displacement rates not properly measured by the MTInSAR algorithm despite ad hoc processing. These areas were furtherinvestigated by exploring maps of DInSAR phase and coherence generated from consecutive Sentinel-1 acquisitions, as well as changesoccurring in orthoimages from different years. Moreover, in order to overcome the DInSAR limitations related to high deformation rates, offset tracking techniques were experimented, whichexploit SAR amplitude instead of phase. This analysis was focused on the interesting case study of Lazaun rock glacier [1]. 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 seasonaldisplacement rates up to few mm/day are reported by previous studies, which used different techniques including GNSS, inclinometers, andboth ground based and spaceborne SAR systems. Offset tracking algorithms can be used to measure displacements with a sensitivity that is afraction of the data spatial resolution. For the Lazaun case study, we adopted the intensity tracking algorithm, considering that the alternativealgorithm based on coherence tracking, is unfeasible due to the low coherence values encountered in the test area. Considering thetopography, the size of the area of interest, and the expected entity of the displacement, SAR data acquired along ascending orbits in spotlightmode are those more reliable for displacement estimation through intensity tracking. In particular, we selected six TerraSAR-X staring spotlightand six COSMO-SkyMed Second Generation (CSG), both with a pixel spacing of less than 1m, acquired in the snow free period between 2016and 2018 (TerraSAR-X) and in 2022 (CSG). These datasets were processed by optimizing the parameters according to the characteristics ofLazaun test case. The displacement maps derived along azimuth and range directions allowed to investigate both seasonal and inter-annualmovements occurring on the rock glacier. GPS field campaigns were also carried out in correspondence with some of the satellite acquisitions.A comparison of the results obtained with ground and satellite data were performed showing for the annual displacement a root mean squaredifference of 0.347 and 0.355 mm/day, with a Pearson coefficient of 0.883 and 0.895 in azimuth and range direction respectively. These resultscoming from offset tracking provide useful displacement information within the Lazaun borders, where the MTInSAR approach instead suffer oflack of coherent targets due to phase aliasing. Finally, both mean rates and displacement time series were ingested into a GIS environment together with other informative layers such asmulti-temporal mean SAR amplitude, DInSAR coherence maps, rock glacier classes (according to [2]), optical orthoimages, permafrost indexmap, and Difference Vegetation Index (NDVI). Then, the rock glacier activity was reclassified by adopting the more recent procedure proposedin [3], which is based also on the DInSAR products. This new classification was compared to that derived according to [2] showing severaldifferences. For instance, 3 out of the 6 rock glaciers classified as indefinite were reclassified as relict or translational, 6 out of the 11 rockglaciers classified as relict were reclassified as transitional, and conversely, one rock glacier classified as active was reclassified as relict.