Post-mining flooding-induced ground displacements, microseismicity, and gas anomalies: a synthesis of research on the abandoned Ruhr, Saar, and Ibbenbüren hard coal districts in Germany

Abstract Raw material depletion, uneconomic production, and government policies will result in the abandonment of hundreds to thousands of underground mines worldwide within the next years. Hence, mine drainage becomes operationally and economically dispensable. Consequently, the mine flooding is initiated by the mine operator. This increases the subsurface pore fluid pressure potentially inducing seismicity and ground displacements. Here, we present a synthesis on published and unpublished research on three abandoned hard coal mining districts in Germany (Ruhr, Saar, Ibbenbüren), which are currently undergoing flooding. The interdisciplinary approach (3D fault modelling and stress analysis, seismological, ground displacement, and outgassing monitoring) suggests that flooding correlates with the temporal occurrence of post-mining microseismicity (mainly M L < 2) and ground displacements (centimeters-decimeters). Flooding-induced microseismicity in the Ruhr area is focused in pillar areas below the mine panels and is not correlated with the activation of large-scale fault systems. Anomalous outgassing into soil is a temporally variable process in the Ruhr area. Ground displacements from InSAR measurements show complex spatio-temporal patterns that correspond to developing mine water levels in different parts of the mines. The ground displacements analyzed vary during different phases of the mine life cycle: 1. Strong subsidence during production; 2. Residual subsidence after end of production; 3. Acceleration of subsidence with beginning rebound of mine water; 4. Reverse to uplift after the mine water reaches a certain impoundment depth; 5. Waning uplift after the final mine water level has been reached.