Surface displacements induced by underground activities from 2014 to 2019 in the former Lorraine coalfield, northeastern France

In the former Lorraine coalfield, northeastern France, mining subsidence has occurred over decades. After the end of coal mining in 2004, uplift started to be detected throughout the basin. In recent years, a coalbed methane (CBM) extraction project has been considered in the same region. This extraction project targets virgin coal layers at about 1 km depth, with production tests carried out between February 2017 and December 2019. Given this particular context, combination of historical and recent subsurface activities, it is useful to characterise the current induced surface displacements in the area. For this reason, PS-InSAR (Hooper A, Bekaert D, Spaans K, Arıkan M. 2012. Recent advances in SAR interferometry time series analysis for measuring crustal deformation. Tectonophysics 514-517: 1–13.) has been applied to Sentinel-1 SAR images over the period 2014-2019, covering the entire former coalfield. Our results confirm the occurrence of uplift movements above the abandoned mine panels, with a maximum mean velocity of +9 mm/yr in the LoS (Line of Sight) direction, over the whole period. This uplift decreases through time; its rate is more important over the first half of the study period. When comparisons are available, our results correlate well to on-ground levelling measurements. This phenomenon is related to the ongoing flooding of the mined-out panels in the coal basin. Due to the poor spatial coverage of the data above the CBM production test zone, no conclusion could be drawn regarding potential surface displacements associated with this activity, but this result may be viewed in light of the rare observations available in active CBM production in other regions around the world. We also found out that, since mid-2018, subsidence has been detected above an active anhydrite underground mine, with an absolute mean velocity approximately 4 mm/yr in the Line-of-Sight (LoS) velocity over the period January 2017 to December 2019. Further investigations are required to determine the origin of these displacements. In conclusion, the map of the current displacements in the study area reflects both past (mining) and present (quarrying) underground operations, and may also serve to inform discussions on possible future ones (CBM extraction).