How can mining data be used for regional stress derivation? – Recommendations based on examples from the Ruhr area

Due to the wide availability of stress measurements in mines it is tempting to infer the regional stress state from stress observations in mines. However, our study demonstrates limitations of this approach and how to overcome them. In this study we used hydraulic fracturing measurement data from shallow boreholes at different mining depth levels in the eastern Ruhr area and compared them with stress information from deep boreholes to infer the regional stress state. We defined selection criteria, which resulted in more robust values for S_hmin  magnitudes because we eliminated the data that had been influenced by the mine galleries, mining sequence and nearby faults. When deriving S_Hmax , special consideration was given to the effect of pore pressure. Despite the fact that mines are filled with air, pore pressure cannot be automatically assumed to be zero. The pore pressure in the tested intervals is highly dependent on the excavation damage zone and the permeability of the rock. We show that careful selection of data and consideration of pore pressure (for S_Hmax  values) is essential to distinguish between local and regional stresses in mining areas. We therefore recommend the use of independent pore pressure observations and, where available, deep vertical borehole data. The resulting stress state in our study is indicative of normal fault tectonics, contradicting previous studies that infer a strike-slip tectonic regime. This results in less critical stress states of faults in the study area.