Application of Spaceborne and Terrestrial Radar Interferometry to Map Rock Mass Displacements in Alpine Terrain
Papke, Jessica1; Strozzi, Tazio1; Tate, Nicholas J.2
1GAMMA Remote Sensing AG, SWITZERLAND; 2University of Leicester, UNITED KINGDOM

Rock mass displacements and relief changes happen in high mountain regions at different spatial scales and velocities, and they are influenced by gravity, weather, climate and anthropogenic effects. Monitoring these activities and changes is of high interest for a better understanding of the ongoing mountain geomorphic processes and for the greater part when they occur near settlements and infrastructure. Spaceborne Synthetic Aperture Radar (SAR) and terrestrial radar interferometry are established techniques to detect ground motion [1], [2]. This case study discusses augmenting spaceborne differential SAR interferometry (DInSAR) displacements with measurements from a terrestrial radar interferometer in Alpine terrain and presents preliminary results. A continuous stack of TerraSAR-X data over the summer 2012 and observations from two fieldwork campaigns in July 2012 are used to map ongoing rock mass displacements. In focus are the "faster" moving parts (30-100 cm per year) and rock glaciers above Graechen in the Swiss Alps as introduced in [3].

In Alpine terrain which is in the higher parts only sparsely vegetated, spaceborne SAR interferometry is able to provide reliable surface displacement measurements, but there are information gaps due to radar shadow and layover caused by the rugged relief as well as in areas of decorrelation caused by too rapid movement within the revisit time of the satellite or snow cover. By acquiring terrestrial radar measurements with the GAMMA Portable Radar Interferometer (GPRI) [4] which uses another acquisition geometry and time interval, it is envisaged to address the first two of the above mentioned limitations. This is exemplarily discussed in a case study for the Distelhorn rock glacier located above Graechen in the Swiss Alps.

For this work, a continuous 11-day stack of TerraSAR-X data in stripmap mode in descending passes in the snow free season from 08/06/2012 until 07/10/2012 was exploited to compute DInSAR line-of-sight (LOS) displacement maps. A digital elevation model (DEM) at 25m resolution (DHM25 © swisstopo) was used to reduce the topographic artefacts in the phase and an atmospheric phase model based on a linear regression of atmospheric phase with respect to the DEM was applied. For small baseline unwrapped interferograms (baselines <100m), mean velocities and displacement time-series for different locations in the Graechen study area were computed using the Small Baseline Subset (SBAS) technique [5]. The X-band data was used because its spatial resolution allows not only to map the dominant rock glaciers in the Graechen study area, but also to detect relative small areas of movement, as shown for other rock glaciers and slope instabilities in the Swiss Alps by [6]. Two fieldwork campaigns were undertaken with the GPRI, each collecting 24 hours continuous 10-minute interval measurements starting on 11/07/2012 and 16/07/2012, respectively. One fieldwork site (Site 1) has direct view from North to the Distelhorn rock glacier in 1-2km distance, the other (Site 2) is further away at about 7km and oversees the whole study area from West. The GPRI is an FM-CW radar working at 17.1-17.3 GHz (Ku-Band) and provides data with 1m resolution in range and 8m resolution in azimuth at a distance of 1km. Due to its rotation around the vertical axis at acquisition time, the resulting displacement map covers a conical area and provides measurements in different line-of-sight directions. The data is interferometrically processed delivering a displacement map over 24 hours for each of the measurement campaigns.

These displacement mapping activities are part of an ongoing study to monitor the different geomorphic processes and landforms in Graechen with spaceborne and terrestrial radar interferometry. Displacement maps over different time spans (11, 22 and 33 days), mean velocities, as well as a four months displacement time-series for different positions in the study area were interferometrically computed from the TerraSAR-X data. Displacement results from 24 hours of observation with the GPRI from the two different field sites show different quality due to their different distance to the study area. The terrestrial observations from Site 1 were well suited to augment the spatial information of the spaceborne DInSAR results over the Distelhorn rock glacier. The measurements provided displacement information where spaceborne DInSAR results showed decorrelation originating from too rapid movement during the TerraSAR-X repeat cycle and in small parts where radar shadow and layover was present. Of course the different acquisition geometry of the GPRI, the rotation around the vertical axis during acquisition, the instrument position and the different look angle, resulted in displacements in other line-of-sight directions and at a different spatial resolution. Consequently they could hardly help to fill the missing gaps in the respective spaceborne DInSAR LOS displacement map. Nonetheless, the GPRI measurements could give additional displacement information since the Distelhorn rock glacier moves in different directions at the rock glacier snout. The terrestrial measurements from Site 2 were much more influenced by atmospheric artefacts due to the longer signal travel path and need further careful investigation.

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