hide
Free keywords:
Dissertation, Universität Hannover, 2018
Abstract:
The new generation of spaceborne Synthetic Aperture Radar (SAR) missions, such as the
TerraSARX/TanDEMX or the Sentinel1 mission, acquire data with unprecedented spatial and
temporal resolution. The short revisit times of the SAR satellites and the independence from solar illumination
and weather conditions enable regular monitoring of topography changes and displacement
variations. This is particularly useful for applications in high-mountain areas where optical spaceborne
sensors provide only limited data due to cloud coverage, and where in-situ measurements are dicult
to acquire on a regular basis.
This thesis focuses on the observation of inter- and intra-annual glacier changes and lake loadinginduced
deformations in high-mountain areas. An essential element of the work is the exploitation
of various SAR processing methods to extract seasonal displacements and topography changes that
occur with magnitudes of a 10th of a millimetre over several months up to a couple of metres within
a few days. Focus is laid on the reduction of signal disturbance sources that are particularly related
to the mountainous setting. The main sources of error in this context are atmospheric eects and
radar penetration into the ground. The investigations are carried out in three exemplary case studies
in Kyrgyzstan, Central Asia, as introduced below.
In the rst study, seasonal variations of glacier surface velocities are analysed on the example of the
Inylchek Glacier. A detailed analysis of the ow behaviour of the bending area is achieved by applying
feature tracking on TerraSARX data acquired in 2009 and 2010. Results show that the 3m resolution
StripMap data allows for a distinction between dierent surface velocities of adjacent longitudinal icestreams.
The processing of SAR data acquired every eleven days also allows the extraction of the
surface motion increase during the melting period. In addition, a 100% speed-up of the lower, rather
stagnant ablation area of the Southern Inylchek Glacier is captured in summer time, which has been
related to the glacier lake outburst ood of Lake Merzbacher.
The second analysis deals with glacier elevation changes at the Inylchek Glacier. Three 10m spatial
resolution digital elevation models (DEMs) are generated for February 2012, March 2013 and November
2013 from bistatic TanDEMX data, and are compared with each other as well as to the Cband
Shuttle Radar Topography Mission (SRTM) DEM from the year 2000. It is shown that the intraannual
comparison between the high-resolution TanDEMX DEMs is useful to even depict seasonal
changes, although uncertainties are high due to approximated radar penetration depths. A new method
to assess the glacier elevation changes in void areas based on elevation binning and slope steepness
is introduced. Mass balances calculated for the decadal changes agree well with results of previously
published long-term studies.
4
The third investigation is related to water-level induced deformations occurring at the Toktogul
Reservoir. Signicant changes of the water level lead to load changes on the crust, which results in
subsidence in the case of water level increase and uplift in case of water level lowering. The deformation
analysis is accomplished by applying the Small BAseline Subset (SBAS) method on 2004 2009 Envisat
and 2014 2016 Sentinel1 data. Whereas both datasets allow to depict an overall deformation trend
within the analysed time periods, only Sentinel1 imagery shows that the observed deformations are
closely correlated to seasonal water level changes. Results are veried by a very good agreement to
deformation rates estimated from elastic forward modelling with a numerical Earth model. The phase
of the SAR data is heavily inuenced by atmospheric eects, which are mainly related to seasonally
varying stratication of the atmosphere and the daily cycle in evaporation at the open water surface.
To determine the best method for the reduction of atmospheric eects, numerical weather model-based
approaches are compared with phase-dependent approaches. It is found that phase-based methods are
currently superior to weather model-based approaches for the application in high-mountain areas,
whereas the power-law methods works best.
