| Abstract |
In 2012 a 100 km2 large seismic survey was carried out to image faults up to 6 km depth in a granitic environment in the Westerzgebirge (Saxony, Germany). The project investigates the possibility to image deep structures in crystalline rocks for geothermal use. The investigated area is located within the so called Gera–Jachimov fault zone, a part of a prominent lineament. The upper part up to 2 km depth is known by former mining activities; so in this part seismic data can be directly correlated with geologic models. The survey was performed using Vibroseis. Line distances of 400 m and group intervals of 30 m of both source and receiver patterns result in a bin size of 15 x 15 m2 and a fold of 110. The layout of the survey is therefore outstanding for seismic exploration within a crystalline environment. The topography of the survey area is characterized by different regions: agricultural, urban and forest areas, cut by valleys and small rivers. The aim of the project, which is funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, is to investigate exploration concepts for the search for low enthalpy geothermal reservoirs in crystalline rock. Actual projects conducted by geothermal companies in Germany concentrate on hydrogeothermal reservoirs. This project should enlarge the area of possible sites for geothermal use near local urban areas by including regions with shallow crystalline basement. The seismic data image the overlying metamorphic piles, which dip in northeastern direction above the granitic intrusion. Beneath these metamorphic sediments, the granite is more seismically transparent, i.e. impedance contrasts are much lower und the seismic pattern is more chaotic. Several steep dipping reflections can be identified, which were forecasted by the former mining activities. These reflections are not continuous, but change their signal strength and signal form with depth. Time slices show lineaments with different strikes which means that different fault systems beside the main fault zone could be imaged. The reflection pattern differs within the granitic body. Areas with diffractions and elongated reflection areas can be observed. This may be an indication for different granitic bodies which form the whole crystalline complex and for faults which are not reflective themselves. The faults have been enhanced by the seismic processing. These fault systems are the first step for defining well targets of a geothermal reservoir. Further interpretation will be based on attribute analysis of the original data to characterise seismic reflection patterns. |