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

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In the former Lorraine coalfield, North-East of France, mining subsidence has been occurring 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 was under consideration in the same region. This extraction project would target virgin coal layers at about 1 km depth, in which production tests were conducted between February 2017 and December 2019. Given this particular context, combining past and recent activities in the underground, it is useful to precisely characterize the current induced surface displacements in the area. For this reason, PS-InSAR (Hooper et al, 2012) has been applied to Sentinel-1 SAR images over the period 2014-2019, throughout the entire former coalfield. On one hand, 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 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 appear to be well-correlated to on-ground levelling measurements. This phenomenon is related to the ongoing flooding of the mined-out panels in the coal basin. On the other hand, due to the poor spatial coverage of the data above the CBM production test zone, no conclusion could be drawn regarding potential surface displacements related to this activity, but this result can be put into perspective with the rare observations available in active CBM production in other areas around the world. We also found out that, since mid-2018, subsidence is detected above an active anhydrite underground mine, with an absolute mean velocity about 4 mm/yr in the LoS (Line of Sight) direction over the period January 2017 – December 2019. Complementary investigations are required to determine the origin of these displacements. As a conclusion, the map of the current displacements in the study area is shown to reflect both past and present underground operations, and can even be a way to discuss possible future ones, in case new operations are projected.