Short communication: Potential of Sentinel 1 InSAR and offset tracking in monitoring post-cyclonic landslides activities in Reunion Island

Abstract. This study examined the results of an interferometric Synthetic Aperture Radar (InSAR) and SAR Offset Tracking (OT) study on Cirque de Salazie (CdS), Reunion Island, France, within the context of the RENOVRISK project, a multidisciplinary programme to study the cyclonic risks in the South-West Indian Ocean. CdS is one of the denser populated areas in Reunion Island. One of the aims of the project was to assess whether Sentinel 1 SAR methods could be used to measure landslide motion and/or accelerations due to post cyclonic activity on CdS. We concentrated on the post 2017 cyclonic event. We used the Copernicus Sentinel 1 data, acquired between 30/10/2017 and 06/11 2018. Sentinel 1 is a C-band SAR, and its signal can be severely affected by the presence of changing vegetation between two SAR acquisitions, particularly in CdS, where the vegetation canopy is well developed. This is why C-band radars such as the ones onboard Radarsat or Envisat, characterized by low acquisition frequency (24 and 36 days, respectively), could not be routinely used on CdS to measure landslide motion with InSAR in the past. In this study, we used InSAR and OT techniques applied to Sentinel 1 SAR. We find that C-band SAR onboard Sentinel 1 can be used to monitor landslide motion in densely vegetated areas, thanks to its high acquisition frequency (12 days). OT stacking reveals a useful complement to InSAR, especially in mapping fast moving areas. In particular, we can highlight ground motion in the Hell-Bourg, Ile à Vidot, Grand Ilet, Camp Pierrot, and Bellier landslides.