A novel framework for combining polarimetric Sentinel-1 InSAR time series in subsidence monitoring - A case study of Sydney

The rapid growth of the city of Sydney, Australia over the last decades, has led to significant development of residential and transportation infrastructure. Land subsidence associated with the urban development can lead to serious issues which should be thoroughly understood and carefully managed. To address this challenge, an enhanced polarisation time-series InSAR (Pol-TS-InSAR) processing framework was developed, using the dual polarisation (DP) Sentinel-1 data to integrate information from different polarimetric channels with different weighting during the TS-InSAR deformation analysis. Ninety DP Sentinel-1 images acquired between 2019 and 2022 are analysed using Pol-TS-InSAR to map the land subsidence in Sydney, with the assistance of the GPS measurements. Improvement of measurement points density from Pol-TS-InSAR is observed compared to the single polarimetric TS-InSAR counterpart for all land use types (ranging between 68% and 208%). The comparison between the Pol-TS-InSAR measurements and GPS measurements shows an absolute mean difference and RMS difference of 0.75 mm/yr and 0.95 mm/yr, respectively, in vertical direction. The results of the ground subsidence analysis revealed that the main subsidence factors in Sydney are related to groundwater extraction, mining activities, underground tunnel construction and landfill. The latter two factors were less well-known prior to this study. In additional to these factors, land subsidence related to high-rise building construction has also been observed, even though the impact seems to be less significant than other factors.