Characterizing volcano deformation with DInSAR and GNSS data: the Sotará Volcano case study.

Sotará is a stratovolcano located at a remote region of the Southern Cordillera Central, in Colombia.  It presents signs of unrest since 2011, evidenced by increased seismicity and crustal deformation formerly detected by tilt-meters, and later by inflation measured at several GNSS permanent stations operated by the Colombian Geological Survey.  In this contribution we processed a set of ascending and descending SAR scenes acquired by the Sentinel-1 Mission by using the Small Baseline Subsets (SBAS) multi temporal DInSAR approach. The region is challenging for DInSAR processing using C-Band data, because it is covered by thick forest causing temporal decorrelation, except for the peaks higher than 3500 m above sea level. As deformation in the site is subtle, i.e. in the order of 2cm/year, atmospheric contamination can potentially hide the geophysical signal, leading to misleading conclusions. To decide if atmospheric corrections should be applied, we analyzed the correlation between the unwrapped phase and topography at each interferogram before and after applying atmospheric corrections provided by the Generic Atmospheric Correction Online Service for InSAR (GACOS). We search for data clustering in the plane correlation coefficient vs. time, which allow for detecting atmospheric stratification signals and evaluating the convenience of applying corrections or not. As a result, we decided not applying atmospheric corrections, and the deformation time series without them show a good agreement with the LOS projected GNSS ones. The results were used for estimating the source parameters through inverse modelling of ascending, descending SAR data and GNSS data using the DMODELS inversion software. We detect migration of the deformation source in the Sotará volcano towards shallower positions between 2011 and 2020. This work is an example of the capability of Sentinel-1 long data series for measuring subtle deformation even in though environment conditions.