Soil gas surveys for post-mining flooding monitoring

A consequence of shutting down coal mines as it was done in Germany until 2018 is a rising mine water level posing the risk of possible mining damage. In the frame of the German BMBF funded project FloodRisk an interdisciplinary approach is applied to monitor such risks, in particular mine flooding induced seismicity. In this study geomechanical and geochemical methods were applied to detect active faults as a preparatory step for permanent mine flooding monitoring at the former German coal mine “Haus Aden”. A selected section of a major normal fault in the area shows potential for critical stress indicating it as a preferred gas migration pathway. Short-term soil gas measurements showed correspondingly high concentrations of CO2 and 222Rn. Both analysis results are a good indicator for a structurally controlled gas migration and thus evidence of the modelled fault outcrop. CH4 which is normally the primary mine gas could not be detected, probably related to its oxidative decomposition with formation of CO2. In the next step, selected gas sampling points will be equipped with new, low-cost, permanent gas sensors for long-term monitoring of potential post-mining induced seismicity.