Landslide motion observation on la Réunion Island (Indian ocean) seen by ALOS-2/PALSAR-2 based on image correlation techniques and SAR interferometry.
Résumé
The presented study consisted in processing ALOS-2 SM1 images by sub-pixel correlation and conventional differential SAR interferometry in order to assess the potential of these new data in the context of landslide deformation mapping. We selected the test site on la Réunion Island because of the presence of the well known Hell-bourg landslide, studied in the past by Remote Sensing methods, using both X, C and L-band interferometry.
The study showed that even with a reduced data set, consisting of 7 images with time spans up to 120 days, ALOS-2 PALSAR data represent a unique tool for landslide displacement monitoring in the highly vegetated context of La Réunion Island. Both the L-band and the high spatial resolution (1.8m) allow better performances in the motion measurement than other SAR data such as C-band SAR, medium resolution L-band data or high resolution X-band with weeks time-span).
We showed that the total deformation occurred between January 2016 and august 2014 reached up to 1.5m in azimuth and 70cm in slant range. The motion presents partitioning in different regimes. From the offset results, we can clearly delineate the trace of the scarps and the sectors with different velocity regimes. Time Series suggested us that significant motion occurred on a relatively short period between November 2014 and end January 2015. We suggest that this could be related to heavy rains occurring during the wet season.
The use of L-band (or larger wavelength when available) with the higher possible resolution therefore appears as the more adapted Space-borne Earth Observation tools for monitoring of very slow to slow landslides in a tropical semi-vegetated context. In this perspective the current ALOS-2 has obvious interest, provided that we access to at least bimestrial acquisitions.