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.

The successive end of hard coal mining, which took place in the various coalfields in Germany by 2018, poses the risk of possible future mining damage related to mine water rise after mine closure. In this study, an interdisciplinary approach of structural geological and geochemical methods was applied to detect active fault zones, as a first step for long-term monitoring of mine flooding. The geomechanical analysis in the area of the soil gas campaign at the former coal mine "Haus Aden" shows high dilation and moderate slip tendencies as well as the potential for critical stress of the selected fault section of the Konigsborn Fault. These features indicate the section under consideration as a preferred gas migration pathway. Soil gas measurements showed correspondingly clear anomalies of the components CO2, Rn-222 and O-2, which together with the results of the geomechanical analysis are a strong indicator for a structurally controlled migration of the gases and thus evidence of the modelled fault outcrop. A possible accompanying fault could not be identified properly, mainly because of the ambiguous CO2 concentrations. A positive correlation of CO2 and Rn-222 can be a supplementary statistical tool to better assign unclear gas anomalies. However, this approach is limited by the radioactive decay of Rn-222 and individual site parameters (groundwater, gas permeabilities). The primary mine gas CH4 could not be detected, which is probably related to its oxidative decomposition with formation of CO2. In the next step, it is planned to validate the current results with area-wide measurements if possible and then to expand selected boreholes to gas gauges. Newly developed, permanent gas sensors will be installed into these gas gauges for long-term monitoring of mine flooding. The aim is to provide new low-cost options for flooding monitoring in order to be able to estimate induced seismicity and mine damage.

When installing borehole heat exchangers, it is essential to use proper backfilling to avoid hydrogeological failure. In this study, a magnetic susceptibility system for backfilling control was investigated at the laboratory scale. The sensor penetration depth, its sensitivity and the unknown values were assessed by using magnetite-containing backfilling material samples. In addition, the capability to detect slurry levels and cavities was investigated, as well as measurement disturbances. Measured values correspond to volume susceptibilities in the SI system multiplied by a factor of 350 for translation into clear integers in the range from 0 to 11 [-]. This approach was evaluated by comparative measurements using a second susceptibility sensor. The reason for the translation is to provide an easier system for handling at construction sites. The 20-35 mm penetration depth and a sensitivity of only 10% for penetration depths > 20 mm is sufficient for slurry level detection; however, it is not sufficient for detecting all critical cavities. Differentiation between susceptibility weakening as caused by tubes, spacers and cavities is difficult; in addition, specific types of rocks can create disturbances. Optimization of penetration depth, measurement procedure as well as material susceptibility can improve quality control to avoid failure events and possibly ease future installation approval in currently forbidden hydrogeological areas.

Bei der Installation von Erdwärmesonden ist die Vermeidung hydrogeologischer Schadensfälle mit einer adäquaten Hinterfüllung essenziell. In dieser Studie wurde ein magnetisches Suszeptibilitätsmesssystem zur Hinterfüllkontrolle im Labormaßstab untersucht. An mit Magnetit dotierten Verfüllbaustoffproben wurde die Sensoreindringtiefe und Sensitivität ermittelt sowie die unbekannten Messwerte des Messsystems nachvollzogen. Zudem wurde das Potenzial hinsichtlich der Füllstands- und Fehlstellenkontrolle sowie der Einfluss möglicher Störstellen betrachtet. Die Messwerte entsprechen magnetischen Volumensuszeptibilitäten im SI-System, die vom Messsystem mit dem Faktor 350 in übersichtlichere Dezimalzahlen von 0 bis 11 [–] überführt werden. Dies wurde durch Vergleichsmessungen mit einem anderen Suszeptibilitätssensor validiert, und hat den Hintergrund der benutzerfreundlicheren Baustellenanwendung. Mit einer Eindringtiefe von 20–35 mm und einer Sensitivität von nur noch 10 % bei > 20 mm ist das Messsystem ausreichend für die Füllstandskontrolle, jedoch nicht ausreichend, um alle potenziell kritischen Fehlstellen zu erfassen. Schwächungen des Suszeptibilitätssignals durch Sondenrohre, Abstandshalter und Hohlräume sind kaum voneinander zu unterscheiden. Außerdem können bestimmte Gesteine Signalstörungen verursachen. Optimierungen der Sensoreindringtiefe, des Messprozederes sowie der Verfüllbaustoffsuszeptibilität können Beiträge zu einer verbesserten Qualitätssicherung sein. Neben der Schadensfallvermeidung können diese möglicherweise auch eine Ausweitung der Erdwärmesondentechnologie in wasserrechtlich noch unzulässigen Regionen zukünftig zulassen.