Subsurface Ice Structure Analysis with Longer Wavelength SAR Tomography
Banda, Francesco1; Tebaldini, Stefano1; Rocca, Fabio1; Dall, Jørgen2
1Politecnico di Milano, ITALY; 2Technical University of Denmark, DENMARK
The study of structure and dynamics of glaciers is becoming increasingly important in order to better understand their related environmental changes. Remote sensing instruments and in particular SAR provide the possibility to investigate the physics of glaciers at a large scale and continuously in time . SAR tomography is a straightforward extension of SAR from two to three dimensions . Forming an additional synthetic aperture in elevation by means of multiple acquisitions from different views, the reflectivity function of the object under investigation can be reconstructed in the entire 3D space. SAR tomography is thus a potentially powerful tool for mapping the volumetric structure of ice.
In this paper, first results from processing of airborne tomographic SAR data acquired in the framework of ESA campaign IceSAR 2012 are presented. Data were collected at P-band in South-West Greenland from Technical University of Denmark, flying POLARIS sounder  reconfigured with side-looking capability. The investigated area is the K-transect located east of Kangerlussuaq, which is one of the most studied areas in Greenland. In particular, data were gathered at SHR site, in the ablation zone, and at S10 site, in the accumulation zone. S10 data were acquired in fully polarimetric mode in order to exploit polarimetric diversity in addition to baseline diversity.
Data were focused with time domain back-projection algorithm in order to cope with the deviations of the tracks from the nominal trajectory. Subsequently, SLC products were analyzed by means of interferometric, tomographic and polarimetric techniques, in order to assess the capability of penetration of longer wavelength SAR into the ice structure.
Analysis of SHR data so far has revealed the predominance of a single and very stable scattering mechanism located at surface. Coherence values were observed to be overall high independently of the spatial and temporal baselines. An analysis of the eigenvalue spectrum of the covariance matrix of the data revealed a few relevant components. Furthermore from tomographic sections only the contribution of the surface can be clearly identified. A differential tomographic analysis  was also performed exploiting temporal diversity of the acquisitions to assess whether low coherence points are affected from volumetric or temporal decorrelation. The analysis revealed that decorrelation is caused mostly by temporal sources.
Analysis of S10 data, instead, revealed overall significantly lower coherences than SHR data. Moreover, the eigenvalue spectrum of the covariance matrix of the data appears to be more distributed. Observing tomographic sections, there seems to be significant penetration of the signal into the ice volume. A joint polarimetric/tomographic analysis was carried out in order to separate superficial and volumetric contribution. The analysis was performed using Algebraic Synthesis . This technique offers the possibility to decompose second order moment of multibaseline fully-polarimetric data into different scattering mechanisms and obtain a separate tomographic profile for each of these. A strong mechanism localized at surface and a distributed mechanism localized at subsurface were found. This seems to corroborate the idea of penetration, and thus the possibility to retrieve information about the volumetric structure of ice.
The results here discussed are preliminary. A detailed polarimetric and tomographic analysis of subsurface volume over a large scale will be presented in the full paper.
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