High-Rate Subsidence Monitoring in the Bobrek mine in the Upper Silesian Coal Basin Using InSAR and GNSS Technology

Intensive and repeated underground coal mining in the Upper Silesian Coal Basin (USCB), Poland – the largest coal mining region in Europe – causes large and rapid surface subsidence. The deformation rate often exceeds the maximum detectable displacement gradient of satellite radar interferometry (InSAR). Phase aliasing makes it impossible to correctly detect large subsidence, leading to underestimations of 80–90%. Therefore, effective phase aliasing correction methods are essential for using InSAR in mining areas of the USCB. The aim of this study is to present a practical application of an InSAR phase aliasing correction method – the Linear Dependency (LD) method – applied in the subsidence area of the Bobrek coal mine located in the northern part of the USCB. The method was developed at the Central Mining Institute – National Research Institute (GIG-PIB) in 2023.

We used 10.5 years of Sentinel-1 satellite images processed with the SBAS method, including both ascending and descending passes. The two LOS displacement components were decomposed into vertical and east–west (E-W) directions. Detailed analysis and LD correction were applied only to the last year of the time series (June 2024 – June 2025), for which four quarterly vertical displacement maps were generated. At the same time, quarterly RTN-GNSS measurements were carried out at 5 control points located in the areas of the largest subsidence.

A comparison of GNSS and SBAS results confirmed clear and spatially extensive phase aliasing in the study area. The largest difference between the methods was about 300 mm. Maximum subsidence measured by GNSS reached 403 mm, while SBAS detected only 106 mm, resulting in an underestimation of 65%. RMSE values at individual points reached up to 158 mm, and the average RMSE before LD correction was 122 mm.

The LD method corrects InSAR underestimation by defining a local linear relationship between the monthly subsidence rate measured by GNSS and the differences between GNSS and SBAS results at each control point. Then applying this relationship proportionally across the entire subsidence basin. After that, full reconstruction of the subsidence time series was obtained. The deformation amplitude increased significantly. After correction, maximum subsidence at the control points ranged from 174 mm to 371 mm. The largest differences after correction were 126 and 172 mm (35% underestimation), which is about twice lower than before correction. At the other points, the final differences did not exceed 22 mm.

The results clearly show that phase aliasing in the USCB is a common effect strongly related to rapid subsidence, and that standard SBAS processing cannot correctly identify high deformation rates. The LD method provides an effective way to correct phase aliasing in InSAR data on a large spatial scale, covering entire subsidence basins. This approach significantly improves the use of satellite radar interferometry for monitoring fast mining-induced subsidence in the USCB.