Structures of the deep bedrock in Gothenburg. A structural documentation of the GE1 drill core

Abstract

Gothenburg Energy, SGU and Gothenburg University are currently investigating the possibilities of a deep geothermal energy plant in Gothenburg. In their first investigations there have been a 1 km drilling in Högsbo. The aim of this thesis was to provide a structural documentation of the rocks from the first 555m of the Gothenburg Energy 1 (GE1) drill core, as well as determining the rebound number and the P-wave propagation time through different lithologies of the drill core. The lithologies and radioactive isotope concentrations has been described by Hynynen (2021) and a combined log of the core will be included in both theses. The foliation throughout the core is generally dipping gently to moderately to the south, southeast and north, relative to the orientation, however the foliation is changing with depth. The fractures of the core exhibit different dip and dip direction depending on depth, but most of the fractures are dipping moderately to gently to the east and southeast. There is no indication of increased number of fractures with depth. The angular relation between mean foliation sets and mean fracture sets indicate there are older fractures which have been folded with the foliation and younger fractures which cross the foliation. Overprinting foliation and folded folds have been observed at several locations, implying at least two different deformation events have altered the bedrock. Measured Lineations on fracture planes are mostly plunging steeply, indicating vertical movements. Lineations on foliation planes are plunging gently to the N-S and NW-SE, resembling the orientations of the intersection lineations from the observed overprinting foliations. They could therefore be interpreted as lineations with similar origin. Further research is needed to draw any conclusions from the investigated mechanical properties. Similarly further investigations are needed to evaluate if Högsbo would be suitable for a deep geothermal energy plant.