Seismic imaging of dyke swarms within the Sorgenfrei Tornquist Zone (Sweden) and implications for thermal energy storage
Authors: Malehmir Alireza , Bergman Bo , Andersson Benjamin , Sturk Robert , Johansson Mattis 
Published: September 2018

Abstract. There is a large interest and demand for green-type energy storage in Sweden for both short- and long-terms (hours, days, weeks and seasons). While there are a number of approaches proposed (e.g., compressed air, geothermal and thermal), only a few have commercially been demonstrated through up-scaling projects. Among these, the thermal energy storage (TES) that stores energy (excess heat or cold) in fluids is particularly appealing. The excess energy then can be stored underground in excavated caverns and used for large district heating and cooling purposes. For an up-scaling underground thermal-energy-storage project within the Tornquist suture zone of Scania in southwest of Sweden three high-resolution, each approximately one kilometer long, 5 m receiver and source spacing, seismic profiles were acquired. Geologically, the site sits within the southern margin of the Romeleåsen Fault Zone in the Sorgenfrei Tornquist Zone (STZ) where dolerite dyke swarms of Carboniferous-Permian age are observed striking in SE-NW direction for over 100 s of kilometers both on land and in offshore seismic and magnetic data (from Scania to Midland Valley in the UK). These dykes, 5–20 m thick, in the nearby quarries (within both Precambrian gneiss and quartzite) express themselves mostly sub-vertical. They can therefore act as a good water/fluid barrier, which can be an important geological factor for any thermal energy storage site. In the data acquisition, combined cabled- and wireless recorders were used to provide continuity on both sides of a major road running in the middle of the study area. Bedrock depressions are clearly depicted in the tomograms suggesting the possibility of weakness zones, highly fractured and/or weathered, in the bedrock and confirmed at several places by followed-up boreholes. Several steeply dipping (60–65°) reflections were imaged down to 400 m depth and based on a series of arguments are judged to be from dolerite dykes. This includes their orientations, strong amplitudes, regular occurrences, and correlation with downhole logging data. In addition groundwater flow measurements within the unconsolidated sediments and in bedrock suggests steeply-dipping structures are the dominant factor in directing water towards SE-NW direction, which is consistent with the strike of the dyke swarm within the STZ. To provide further insight into the origin of the reflections even the historical crustal-scale offshore BABEL lines (A-AA-AB) were revisited. Clear multi-phase faults are observed as well as a Moho step across the Tornquist zone. Overall, we favour that the reflections are of dolerite origin and their dip component (i.e., not sub-vertical) may imply a Precambrian basement (and dykes) tilting, block rotation, towards NE as a result of the Romeleåsen reverse faulting. In terms of thermal storage, these dykes then may be encountered during the excavation of the site and can complicate underground water flow should they be used as a fluid barrier in case of a leakage.