FloodRisk – Postmining, mine water rebound, microseismicity, and ground movements

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Summary In 2018 the last active German hard coal mines have been abandoned and transitioned into the postmining phase. In the postmining phase, mine water pumping, necessary during active mining, becomes technically and economically unnecessary and unprofitable. As a consequence, the controlled process of mine water rebound to predefined levels increases the pore fluid pressure of subsurface rocks and changes the local stress field. As a result, faults may be (re-)activated inducing ground movements and microseismicity. This study presents an interdisciplinary approach consisting of geology, geomechanics, gas technology, geodesy, and geophysics in order to provide a process understanding how subsurface and surface are geomechanically coupled in the eastern Ruhr hard coal mining area, western Germany. Thus, mine workings and favourably oriented fractures represent the major pathways for mine water. This in accordance with CO2, 222Rn, and O2 anomalies detected along a fault. Rock matrices, however, are tight (mean porosity <1 %, mean permeability <1 mD). Furthermore, ground subsidence and microseismic events (–0.8 to 2.6 MLV) are spatially and temporally correlated with the mine water rebound and mine workings, but unrelated to tectonic faults. The applicability of these findings to other hard coal mining areas (e.g. Saarland, Ibbenbüren) will be tested.