Monitoring of ground settlement around air vent of subway tunnel in metropolitan city using SBAS-InSAR and wavelet transform

There is a lack of quantitative constraints on the mechanism of groundwater fluctuations on the settlement of urban rail transit appurtenant structures. In this study, four air vents along the Bundang Line in Seoul, South Korea, were selected to analyze 168 Sentinel-1A C-band images acquired between July 2018 and January 2025. Using the SBAS-InSAR (Small Baseline Subset Interferometric Synthetic Aperture Radar) technique, a deformation time series covering the entire line was derived. Based on the SBAS-InSAR results, the maximum settlement rate of Bundang Line is -4.4 mm/year, and the maximum cumulative settlement is about 20 mm. Subsequently, the results of 3 wavelet transform tools to quantify the seasonal deformation of Air Vent and the underground water level show that the change of the underground water level is the primary external driver of the seasonal deformation of Air Vents, and part of the air vent deformation lags the underground water level by about 20 days. In this study, underground water level fluctuation is identified as the main driver of settlement in the Air Vent of Bundang Line, and response lags of 20-45 d are given for each section. The combination of SBAS-InSAR time series based on Sentinel-1A imagery and wavelet analysis can capture this hydrologic-deformation linkage in time with millimeter-level accuracy and monthly time delay, providing reliable data support for early warning of ground settlement along the subway tunnel and decision making for disaster mitigation.