Abstract
Deep-seated gravitational slope deformations as well as permafrost creep (i.e. rock glaciers) represent common geomorphological phenomena in high alpine areas. The systematic observation of changes over time caused by the slope movements is of high importance for a proactive management of the natural hazards related to these phenomena.
Ground-based dynamic monitoring, such as Global Positioning System (GPS) or total station, presents the disadvantage of a limited spatial coverage and is difficult to conduct in remote or impervious areas. These limitations become even more evident for monitoring dynamics at subseasonal scale.
Satellite remote sensing methods based on Synthetic Aperture Radar (SAR) data represent a set of complementary technologies at disposal to study displacement of rock glaciers and landslides at different space and time scales. In fact, SAR satellites are capable to acquire data over large distributed remote areas relatively independent from weather conditions and with high temporal frequency (up to a few days).
In this work, we propose a SAR-based approach that aims at supporting in-situ measurements for well instrumented study areas, and at monitoring rock glaciers and landslides dynamics when in-situ measurements are not available or difficult to be carried out.
The primary test site of this study is the Lazaun rock glacier in the Schnals valley. It presents displacement velocities up to 1.5 m/year, and although GPS measurement campaigns have been conducted there since 2006, it has never been monitored before by using remote sensing techniques.
SAR data with different characteristics (i.e. different spatial and temporal resolution) were processed using different techniques, i.e. differential interferometry (DInSAR), multi-temporal interferometry and amplitude tracking, to study the annual, seasonal and subseasonal rock glacier dynamics with high precision.
In detail, the high acquisition frequency of the Copernicus Sentinel-1 data (6 days) allowed studying the subseasonal displacement and understanding the displacement rate variation during the snow free period over the test area selected. Three datasets of Sentinel 1 from relative orbits 15, 117 and 168 were used, taking into account the images from the snow free period 2017 (about mid-June to November). The images were processed using Persistent Scatter (PS) and Small BAseline Subset (SBAS) techniques. Interferometric techniques allowed obtaining only the displacement along the Line of Sight (LOS) of the satellite. Exploiting the combination of the three relative orbits, displacements in three dimensions can be derived.
Six Very High Resolution (VHR) TerraSAR-X (TSX) images acquired between summer 2016 and summer 2018 were also processed by using differential interferometry (DIn-SAR). In order to overcome the limitations of DIn-SAR techniques related to high displacement rates, decorrelation and unwrapping issues, amplitude tracking technique was applied on VHR TerraSAR-X images. By exploiting the combination of the displacement along the LOS and along the satellite flight direction, displacement direction is estimated.
The accuracy of satellite-based products is assessed exploiting ground-based and proximal data, such as GPS, Ground-Based SAR (GB-SAR) and UAV-based data (acquired concurrently with the GPS campaigns). Three GB-SAR campaigns were carried out during summer 2017 and 2018. The high acquisition frequency (5 minutes) of this instrument enabled the comparison of daily displacement data with satellite SAR results.
The second test site is the Corvara mass movement, classified as a deep seated complex landslide with rotational (i.e. upper part) and flow-like (i.e. middle and lower part) behaviour. The moving volume of this slide is about 30 million m³ affecting a surface area of 2.5 km² and has been under observation since 1997. The slide can be subdivided into source, track and accumulation zones and is characterized by spatially varying displacement rates. The presence of vegetation induces a quite strong decorrelation effect in SAR. As a cconsequence in order to allow for monitoring the area using these techniques, a set of corner reflectors equipped with a support for GNSS-antenna were installed. The corner reflectors are characterized by two different size designed for X and C band respectively. The availability of periodic GPS measurements, permanent GPS stations, UAV flight campaigns, Sentinel1 and 3 TerraSAR-X data allowed to test amplitude tracking and multi-interferometric techniques in this area in a similar way with respect to Lazaun test site.
Acknowledgments
This work was conducted within the project ALPSMOTION (ALPine Slow slope Movement moniTorIng and detectiON with remote and proximal sensing), coordinated by Eurac Research-Institute for Earth Observation and funded by the Autonomous province of Bolzano, Alto Adige, “Ripartizione Diritto allo Studio, Università e Ricerca Scientifica.” The Copernicus Sentinel-1 and TSX data were processed with SARscape software (SARMAP). GB-SAR instrument has been made available by University of Pavia. GPS data were provided by University of Innsbruck. TSX Data were provided by the European Space Agency, Project Proposal id 34722, © DLR, distribution Airbus DS Geo GmbH, all rights reserved.