Deformation monitoring of the Alpine Lazaun rock glacier using offset tracking and multi-source SAR data

Abstract

Rock glaciers represent a typical landform characterizing high mountain periglacial terrains. They are composed by a mixture of frozen debris and interstitial ice and when are affected by downslope displacement related to the permafrost deformation or melting, they are called active rock glaciers. The observation of their changes over time is a critical indicator of the state of water resources and permafrost distribution and is therefore of great importance for risk scenario definition and related natural hazards management, as well as can be related to the climate change.  Synthetic Aperture Radar (SAR) data have proven to be a useful tool to estimate surface displacement phenomena in mountainous areas that can be difficult to access. In particular, Differential Interferometry can measure displacement in the line of site direction with high accuracy. However, sometimes the characteristics of the areas in terms of displacement rate and number of available images lead to some limitations, which can be overcome by using other complementary tools such as SAR offset-tracking (OT) or by integrating both techniques together. In the OT case, the estimation of the deformation in azimuth and range directions is obtained maximizing the normalized cross-correlation between pairs of images and the minimum detectable displacement depends on the spatial resolution of SAR images, in the sense that a finer spatial resolution allows estimating a lower minimum detectable displacement. Consequently, these techniques can be profitably used for medium- or long-term displacement measurements [1]. In this work, OT is used to monitor the active alpine Lazaun rock glacier, located in the Ötztal in South Tyrol (Italy). Lazaun has a snow cover free period limited typically to 3 months per year, covers an area of about 0.12 km², and its altitude is between 2480 and 2700 m a.s.l.. Previous measurements performed by using GPS have detected displacements reaching ca. 1.5 m per year [2]. Different kind of data (TerraSAR-X, Cosmo-Skymed First and Second generation, Sentinel-1 and SAOCOM) characterized by different wavelengths, exposure and resolutions have been tested to estimate the rock glacier displacements and the results have been compared with GPS measurements executed in 2016-2018 and 2022. The results show that considering the limited size of the area of interest and the displacement rate, the spatial resolution of the data is of fundamental importance and only using spotlight SAR data is possible to estimate the displacement on both seasonal and inter-annual temporal scale. This research is part of the 2021-2023 project ‘CRIOSAR: Applicazioni SAR multifrequenza alla criosfera’, funded by ASI under grant agreement n. 2018-12-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.   [1] Strozzi, Tazio, et al. "Glacier motion estimation using SAR offset-tracking procedures." IEEE Transactions on Geoscience and Remote Sensing 40.11 (2002): 2384-2391.   [2] Krainer, Karl, et al. "A 10,300-year-old permafrost core from the active rock glacier Lazaun, southern Ötztal Alps (South Tyrol, northern Italy)." Quaternary Research 83.2 (2015): 324-335.