Ground Deformations Observed for Three Decades (1992-2022) above Old and Deep Coal Mines Reused for Deep Gas Storage Sites (Wallonia, Belgium) Using PS-InSAR Time-Series

The exploitations of the coal mines in Wallonia (Belgium) were ceased progressively till the year 1984. Groundwater pumping activities were stopped inducing a progressive groundwater rebound affecting different areas with different rates but still visible today with radar interferometry. To mitigate the threats to the environment during the postmining period, flooding and outbreak risks, surface ground deformations and the stability of civil engineering infrastructures in the vicinity of those mined areas need to be monitored. The persistent scatterer radar interferometry technique (PS-InSAR) revealed ground displacements during three decades above the two underground reservoirs (Peronnes to the West and Anderlues to the East) used for gas extraction and storage south of La Louvière city (Wallonia). 236 Interferograms were produced using images acquired by the ESA's satellites (ERS1/2, ENVISAT, and Sentinel-1A). Land surface displacements trends are heterogeneous both in time and space with different behaviours. During ERS1/2 (1992-2002) an uplift is visible in the western part of the Peronnes reservoir while the eastern part is still affected by subsidence while the Anderlues reservoir is affected by global subsidence. During ENVISAT (2003-2010), the uplifting conditions were stronger in the Peronnes reservoir and only the upper right corner is still affected by subsidence while the Anderlues reservoir records still a general subsidence phenomenon. The most recent data from Sentinel-1A revealed a general uplift through the Peronnes reservoir with decreasing LOS velocity towards the north and the south. The Anderlues reservoir is still affected by a subsidence phenomenon, but the LOS velocity values are reduced compared to the previous decades.