InSAR Product Analysis (IPA): a QGIS tool for slope instability assessment based on SAR interferometry

Differential SAR interferometry (DInSAR) and Multi-Temporal DInSAR (MTInSAR) are largely exploited for measuring slope stabilities. However, both suffer from the typical critical environmental setting of areas affected by slope instability. First, the steep topography may lead to unfavourable illuminating conditions in terms of either unfeasible detection over layover and shadow areas or low sensitivity to the ground displacement. Second, the presence of dense vegetation and changeable cover conditions causes DInSAR signal decorrelation and a low density of MTinSAR coherent targets (CTs). Third, displacement kinematics are characterised by non-linear components and high displacement rates, leading to measurements corrupted by aliasing. All these critical issues negatively impact the applicability and interpretation of this well-established technology.We developed a QGIS plugin based on the PyQGIS library, which, starting from standard DInSAR/MTInSAR products and a few ancillary layers, derives additional products useful for supporting the interpretation of the DInSAR results and the assessment of the slope stability over the area under investigation.First, the tool estimates the visibility of the area of interest (AOI) with respect to the satellite line of sight (LOS). It combines the satellite acquisition geometry and the ground geomorphic information to derive an index of visibility, which allows end users to check the applicability of DInSAR analysis over the AOI just based on geometrical factors and before performing DInSAR processing.If MTInSAR displacement products are available, the IPA tool derives further outputs. First, it computes the percentage of the AOI surface covered by CTs. This allows end users to estimate how significant the information derivable from MTInSAR within the AOI is.Moreover, the reliability of DInSAR products also depends on the orientation of the slope within the AOI. For instance, for slopes facing north or south, the downslope movement is basically perpendicular to the LOS direction, thus leading to unfeasible DInSAR-based estimation of displacements. Hence, the IPA tool estimates the percentage of downslope movement captured from the DInSAR geometry along the LOS and, for each CT, computes the downslope mean displacement rate corresponding to the LOS component measured by MTInSAR.These IPA products are combined with other layers such as NDVI, DInSAR coherence, and landslide inventory for performing a feasibility analysis before DInSAR/MTInSAR processing, for checking the reliability of DInSAR/MTInSAR products to assess the slope instability, and for supporting the interpretation of the DInSAR displacement in analysing slope instabilities.Finally, the IPA tool performs a displacement time series analysis based on automated procedures recently developed for identifying CTs with nonlinear signals and based on fuzzy entropy and Fisher statistics. This allows a focus on a smaller set of CTs affected by nonlinear displacements (including warning signals) and potentially deserving further geophysical or geotechnical analysis.The work introduces the methodologies and provides some examples based on DInSAR displacement products derived by processing Sentinel-1 data. Acknowledgment This work was supported by the Italian Ministry of Education, University, and Research, D.D. 104/02.02.2022,  PRIN 2022 under project MIRAGE - Mass movement Investigation and prediction through geomorphology, Remote sensing, and Artificial intelligence (2022X539XM).