Millimeter slope seasonal deformation from multitemporal InSAR with a tropospheric delay correction: a case in Hong Kong

Rainfall induced landslides have been the No.1 geohazards in Hong Kong (HK). In coastal subtropical monsoon regions, air temperature and humidity vary substantially and frequently. Tropospheric delays (TDs) limit the accurate detection of slow slope motion (which is the common case in HK) using interferometric synthetic aperture radar (InSAR).  In this presentation, we introduce a new TD correction method for InSAR line-of-sight (LOS) measurements at individual slope scale. The TD signal was estimated from LOS time series through a blind source separation (i.e., independent component analysis). The stratified TD was isolated according to a spatially elevation-linked and temporally periodic independent-component (IC), which was determined via a correlation test and power spectrum analysis. Therefore, the TD correction was not relying on any external weather products/meteorological data and had unprecedented spatiotemporal details equivalent to the SAR images.

A case study in Tai O, HK was conducted to verify the proposed method using 63 descending CosmoSkyMed (CSK) and 143 ascending Sentinel-1 (SNT-1) images. We used meteorological data of air temperature, humidity and weather products of ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-5 and GACOS (Generic Atmospheric Correction Online Service) to validate the estimated TD. We estimated up to 3-4 cm spatiotemporally relative TD in the LOS directions of CSK and SNT-1, corresponding to a slope elevation change of ~ 400 m. The estimated TD was largely affected by specific air conditions (e.g., temperature and humidity) on the SAR image-acquisition days. In addition, we found the relative TD exhibited a slower increment rate than that of the slope elevation, suggesting the TD was not linearly related to the elevation. Ground deformation measurements from prisms and records of rainfall and tide were used to validate and interpret the InSAR deformation time series. It is interesting to find different hydrological forcings regulate the seasonal deformations in the slope (~ 10mm) and the reclamation (~ 15mm) in Tai O. Downslope movement (due to increase in pore-water pressure) occurred when rainfall accumulated from a dry season to a wet season, whereas upslope rebound (due to soil shrinkage) occurred when rainfall decreased from a wet season to a dry season. However, the periodic deformation of the reclamation substrate seemed to be more related to the sea level, instead of rainfall. The TD correction has reduced the root-mean-square error (RMSE) by 42.3%, such that InSAR LOS time series with millimeter-level accuracy (potentially 1-3 mm) were obtained in the Tai O case.