Water bearing adits form a historic near-surface underground drainage and outflow systems whose functionality is of considerable importance in many mining areas all over the world. Water bearing adits are technical structures that have often been in place and in operation for more than two centuries. In most cases, they are no longer accessible directly behind the tunnel portal. Therefore, stable hydraulic, geotechnical and rock mechanical conditions are important, otherwise the hydraulic system would collapse.

In recent years, the Research Centre of Post-Mining has investigated the current situation of a large number of water bearing adits in the southern Ruhr area/Germany. To understand the interaction with the groundwater- systems hydrochemical parameters were analysed, the rate of discharge and of precipitation compared. An essential step is the spatiotemporal evaluation of available mine survey maps of the water bearing adits and the hydraulically connected mine workings. This is needed in order to determine the catchment area of a water bearing adits within the groundwater-system.

To further enhance the process understanding the results from monitoring-stations, special depth-dependent hydrochemical sampling campaigns in former shafts and infiltration/tracer tests in ditches/local streams were integrated. The entire data fusion happened in 3D geological/hydrogeological subsurface models. This integrated subsurface understanding is now the enabler for the implementation of a risk monitoring and management system for this sensitive system of water bearing adits.

Mine water rebound in German hard coal mine areas operated by RAG AG under the leadership of RAG Foundation is a showcase for post-mining associated research. It is a long-term, cost intensive and multidisciplinary project affecting highly populated urban areas like the Ruhr District. RAG is facing long-term liabilities as regulatory framework for the rebound process. Currently, mine water is pumped from levels down to 1200 meters to be discharged into local rivers. In the vicinity, monitoring water wells were drilled to identify any hydraulic potential changes in the overburden sections in support of the project’s risk assessment. Mine water management issues comprise control and forecast of mine water levels including to determine a financially and environmentally sustainable level. Mine water treatment, evaluating hydraulic barriers, but most importantly, protecting groundwater for domestic use and even more critical, regional drinking water reservoirs are mandatory prerequisites. We identified natural hydrochemical tracers, worked on in-situ processes affecting mine water chemistry and progressed with petrophysical measurements on core material including clay mineralogy screening to deliver a first step towards an integrative monitoring approach to manage such liabilities. A suite of own and literature-based hydrogeochemical and petrophysical data are presented to better describe the regional hydrology and geology.

Due to the cessation of coal mining in Germany, mine water management in the former coal districts is subject to change and of environmental and economic significance. Since there is no technical need to drain the abandoned coal mines, mine water levels rise. As a result, subsurface rock pore pressures rise changing the subsurface effective stress regime. This may induce local ground movements and activate mining-related and natural fault zones.

In order to develop a better understanding of the processes and their interaction, FloodRisk applies an interdisciplinary approach involving geological, geomechanical, geodetic, and geophysical methods. For this purpose, former German coal districts in the Saarland, Ruhr area, and Ibbenbüren serve as suitable study areas.

Based on a geological 3D subsurface model complemented by petrophysical (porosity and permeability) and structural data (fracture network characterization), geomechanical properties such as slip and dilation tendencies of faults were calculated. These data give insights into the expected fault behaviour under assumed stress conditions and provide a basis for the localisation of suitable areas for soil gas measurements (²²²Rn, ²²⁰Rn, CO₂, O₂, H₂, S, CH₄) using recently developed economic sensors. Simultaneously, seismic events and ground movements are continuously monitored using seismic network, GNSS, Nivellement, and InSAR data and subsequently compared with the evolution of mine water levels. These multidisciplinary observations will be combined in a conceptual model on flood-induced ground movements and seismicity. This enables the deduction of operations for future mine flooding and the establishment of an optimised reservoir management.

After operation of hard coal mines has been terminated in Germany, many of the mines are being flooded. In the project “FloodRisk: Earthquakes, uplift, and long-term liabilities – risk minimization during mine flooding”, the consequences of the rising mine water levels are investigated from a multidisciplinary perspective. Geodetic, geophysical, geomechanical and geological approaches are combined to develop a conceptional model for flooding induced surface displacements and seismicity. Saarland, Ruhr area and Ibbenbüren have been chosen as exemplary investigation areas.

The DMT GmbH & Co. KG (DMT) and the Geodetic Institute Karlsruhe (GIK) contribute to the project by analyzing surveying and InSAR data. DMT installs ten low-cost GNSS-Sensors near Bergwerk Ost and includes them in DMT SAFEGUARD, a proprietary GNSS monitoring system, that provides an integrated evaluation and provision service. GIK is analyzing InSAR data from Sentinel 1a/b and intends to combine GNSS, levelling and InSAR data from different orbits to derive 3D displacements. Challenges that are typical for mining areas as the spatially as well as temporally diverse displacement patterns demand for new solutions in InSAR processing. The so found displacements of the earth’s surface will be integrated with geomechanical modeling in the final phase of the project. In this presentation, the goals, challenges and approaches of FloodRisk with regard to geodesy are discussed and first results are shown.

FloodRisk is an interdisciplinary project focusing on the effects of mine water level rise in abandoned coal mine regions in Germany. Such effects are heterogeneous ground uplift, stress changes due to the change in pore pressure and the reactivation of potential faults. One of the most directly measurable effects is the induced micro seismicity.
In this study the relationship between mine water rise, fluid-induced stress changes and induced seismicity in the Haus Aden dewatering area in the eastern Ruhr area (Germany) will be investigated in more detail. For this purpose, we operate a network of short period seismic stations in the region of the former "Bergwerk Ost" colliery, which had the highest seismicity rate in the Ruhr area during active underground coal mining (closed 2010).
Level of induced seismicity started again when pumps were shut down (mid 2019). We were able to detect and localise more than 1000 microseismic events in magnitude range between -0.7 and 2.6 Mlv. Many of these events are spatially clustered and some show quite high waveform similarity. This allows relative localisation and can increase the accuracy of the location. The depth location of the earthquakes, within the limits of localisation accuracy, agrees very well with the distribution of seismicity at the time of active mining. The measured temporal trend of the mine water level shows a strong correlation with the temporal evolution of the observed micro seismicity.

The task of this European research project (PostMinQuake) is to identify mechanisms, relevant parameters and dependencies causing post-mining seismicity for several European coal regions.

When closing an underground coalmine, the mine water raises by stopping mine water pumping, which can lead to micro seismic events due to the final change of the geomechanical stress-regime.

The consortium partners (FZN-THGA, GFZ, BRGM, INERIS, CMI, SUT, SRK, IGN, Green Gas, DIAMO) will collect and analyse information from the coal basins Gardanne (France) and Upper Silesia (Czech Republic and Poland). In Germany, we will focus in the Ruhr Basin and Ibbenbüren (North-West Germany), which are places with centuries of hard coal mining history and where mining ceased in 2018.

In order to detect the processes that cause micro-seismicity, we are gathering information in the basins regarding geology; mining methods and monitoring protocols; and post-mining situation and seismicity.

This information will be analysed to identify similar approaches to develop a synthesis of good practices, compulsory and desired improvements, as well as an post-mining seismicity map for post-mining seismicity diagnosis. We also aim to develop a Ground Motion Prediction Equation, and the elaboration of criteria of hazard rating.

The final guideline could help the mining industry and decision-making bodies to manage the risks of post-mining micro seismicity, also of interest in old petroleum reservoirs when used to storage H2 or even for geothermal energy.

Acknowledgements: The Research Fund for Coal and Steel funds the PostMinQuake project under the grant agreement No 899192.