Polarimetric D-InSAR for Detection of the Processes Affecting the Ground of the Periglacial Environment
Résumé
High-latitude permafrost regions store vast amounts of organic carbon. Temperature increase induces thaw of the frozen grounds, facilitating the microbial decomposition and the conversion of soil organic carbon into the greenhouse gases carbon dioxide and methane, that represents a positive feedback effect that might accelerate climate change. The thawing is restricted to some meters below the top layer of soil and a permafrost layer remains frozen below the surface. In such areas, the top layer of soil that thaws during the summer and freezes in winter -known as the active layer- warms up enough to enable plants to grow during the spring and summer. For an accurate assessment of the carbon transfers, the active layer thickness over different soils and surface types needs to be known, as well as the dynamics of soil moisture during the annual freeze/thaw cycle.
In this paper, SAR polarimetric differential interferometry is employed to follows the surface deformation during a whole freeze/thaw cycle, as it is an indicator of the active layer thickness and of the hydrological transfers. Time series of X-Band SAR data has been acquired over a site in the region of Yakutsk (Central Siberia) with a 11 day time revisit. A specific D-InSAR [1,2] approach has been developed to estimate the temporal trend of deformation affecting the periglacial environment. To improve chance to detect deformation occurred within the sensor revisit time using only two acquisitions, the sensor polarimetric capability is employed for polarimetric optimization of the coherence, and for ground movement estimation under vegetation, by exploiting the polarimetric diversity of the interferometric coherence [3,4]. The deformation patterns are then compared with the ground measurements for analyzing the thermal and hydrological processes affecting the ground during a whole freeze/thaw cycle.
Interferograms are chronologically interpreted in term of geomorphologic and thermal processes. Heat transfer induced subsidence is detected during the winter as well as water movements induced by thawing of the ground. During the break up, floods induce elevation of the banks. During summer, water movements induce typical fringe patterns, and, at the beginning of the winter, small scale fringes are seen to be correlated to the sandy ground microtopography. All these indicators are analyzed for the understanding of the processes affecting the ground of periglacial environment and for ground physical parameter estimation.