Monitoring active motion of the Guobu landslide near the Laxiwa Hydropower Station in China by time-series point-like targets offset tracking

Since 2009, the Guobu landslide has been very active, posing a safety threat to the Laxiwa Hydropower Station, located only several hundred meters downstream along the upper Yellow River in China. To investigate the current state of this landslide, we analyzed two stacks of X-band TerraSAR-X (TSX) High-resolution Spotlight (HS) mode images acquired from September 2015 to April 2017 in descending orbits with different look angles. A new time-series point-like target offset tracking (TS-PTOT) method is proposed to retrieve time-series surface displacements at point-like targets (PTs) from SAR image pairs properly combined with large temporal baselines and small spatial baselines. According to an evaluation of the standard deviation of time-series displacements at stable points, the TS-PTOT method increased the measurement precision of offset tracking by about 25% over the results using a single master. Our TSX-HS TS-PTOT results manifested a spatial pattern and magnitude of displacements highly similar to DInSAR result produced with an ALOS-2 PALSAR-2 image pair. The maximum displacement rate at the upper part of the slope during the study period was around 80 cm/yr in the line-of-sight (LOS) direction, which is much lower than the displacement rate measured in 2010. Furthermore, three-dimensional (3D) displacements at those identified homologous PTs were estimated by combining two-dimensional (2D) displacements measured by TS-PTOT from the two SAR data stacks. The 3D deformation pattern of the Guobu landslide properly verified its toppling-sliding deformation mechanism.