Time evolution of mining-related residual subsidence monitored over a 24-year period using InSAR in southern Alsace, France

Abstract

From 1910 to 2002, the Mines Domaniales de Potasse d’Alsace (MDPA) exploited two potash layers, located at depths greater than 400 m in the southern Upper Rhine Graben. This mining activity has caused surface subsidence, investigated here using SAR interferometry (InSAR) and Persistent Scatterer Interferometry (PSI) approach is used to follow mining induced surface displacements over the Alsatian potassic basin. The SAR images acquired by three distinct satellites (ERS, ENVISAT, Sentinel-1) over the last 24 years cover both mining and post-mining periods. Until 1996/1997, InSAR results do not fit the high vertical displacement deduced from levelling campaigns due to unwrapping errors and are therefore not reliable to quantify the mining subsidence. From 1996/1997, InSAR results and levelling measurements become consistent, allowing us to study the subsidence following the activity of the mine. The surface subsidence decreases quickly after the end of mining operations, with current subsidence estimated below 5 mm/yr in 2018. The InSAR results obtained for the post-mining period show a correlation between residual subsidence amplitude and the mined thickness. Taking together the InSAR and levelling measurements for the whole 24 years period, the combined time series are approximated by a sum of two exponential terms, which reveals the role of the goaf compaction at depth. The study shows that SAR interferometry is well suited for monitoring small amplitude mining-induced subsidence over the residual phase, helping to better mitigate the related risk in these regions.