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Contribution à l'étude de la dynamique des glaciers rocheux dans les Alpes françaises par interférométrie radar différentielle (D-InSAR)

Abstract : Rockglaciers are one the visual expressions of mountain permafrost and have been the focus of numerous and various studies in the last two decades. Two main topics are studied by the scientific community: i) better understanding the movements of active rockglaciers and the phenomena that generate those movements; ii) assessing impact of global atmospheric change on these periglacial shapes. Here we propose to contribute to the first topic by using remote sensing method of displacement measurements already used in other countries for rockglaciers studies: Differential Interferometric Synthetic Aperture Radar (D-InSAR). D-InSAR is a method of measurement based on the phase difference between two radar images, which represent the same area but at different time intervals. The technique generates interferograms, maps of surface deformation in two-dimensions allowing for the detection and quantification (in centimeters) of variations in distance between the target and the radar between two different data acquisitions. Recent research has shown that the InSAR technique can be used to semi-quantify rockglacier deformation (under the assumption that certain conditions are respected with regard to generating and interpreting the interferograms). In the present thesis, ERS radar images (dating from 1991 to 1995) and TerraSAR-X data (dating from summer 2012) were obtained in courtesy of ESA (European Space Agency) and DLR (Deutschen Zentrums für Luft- und Raumfahrt, German Space Agency) with the aim of generating interferograms. We are interested by the detection of rockglacier movements. The ERS archives allowed us to work at French Alps scale with moderate resolution (25m in ground geometry) whereas TerraSAR-X data have better ground resolution (10m) but our analysis are more local (Haute Maurienne/Haute Tarentaise) due to the cost of the data and the time-consuming nature of the analyzes. With ERS, we selected all archives data and chose the more relevant of them. Finally 9 interferograms were generated. To analyse this amount of data two methods were employed: i) a GIS analysis of interferograms by geomorphologists (helped by ortho-images and topographic data), ii) a comparison between interferograms and existing rockglaciers shape inventory to evaluate the quality of the radar detections. At the end of the analysis a map of the French Alps with all detected rockglacier movements was produced. With TerraSAR-X data, the method of analysis was almost the same. Analyzes focused on the Haute Maurienne/Haute Tarentaise massif. A third scale of analysis is focus on Pierre Brune rockglacier which has been detected on ERS interferograms as destabilized rockglaciers (movement > 2 m/year). Further investigations have been carried out on this site (historical movements reconstitution and GPS acquisitions). Based on these studies with different scales and resolutions, we proposed a discussion about suitability of D-InSAR measurements method for long term rockglaciers monitoring as well as drawbacks and benefits of the method.
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Thomas Echelard. Contribution à l'étude de la dynamique des glaciers rocheux dans les Alpes françaises par interférométrie radar différentielle (D-InSAR). Géographie. Université de Grenoble, 2014. Français. ⟨NNT : 2014GRENU007⟩. ⟨tel-01128087⟩

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