I-GET is an European Union funded project to develop integrated exploration methods for fractured and/or porous fluid bearing geothermal systems. The techniques are tested at typical sites in volcanic, metamorphic and sedimentary environments. We present results from the geothermal site at Gross Schönebeck located in the NE German basin, North of Berlin. The target horizons for a middle enthalpy geothermal system consist of Rotliegend sandstones and volcanic rocks at depths between 3.9 and 4.4 km. These rocks were deposited during the initial stages of the developing Southern Permian basin.
Seismic experiments (2D and 3D) were conducted to study the regional lithological and tectonic environment, and to provide attributes useful for the geotechnical characterization of the potential geothermal reservoir. Tomographic methods are applied to a refraction / wide-angle reflection seismic data set. The first arrival travel times are determined and a damped least-squares inversion is applied to derive a 2D velocity model along a 40 km transect centered at the geothermal research well. Next, the first arrival signals are aligned and spectral analysis is performed using Fourier and wavelet transform methods for comparison. The spectral data are parameterized (e.g., spectral decay) and a damped least-squares algorithm is applied to estimate the attenuation distribution. Furthermore, wide angle reflections are modelled and imaged by using the velocity model along the 40 km transect. The velocity and attenuation tomography approaches are also applied to the low-fold 3D seismic data providing information for the upper 1000 m in a 2.5 by 2.5 km area around GSB 3/90.
The resulting models of P velocity and attenuation reflect the major stratigraphic units. This is supported by the comparison with interpretations of pre-existing industry seismic data and borehole information. New insights are revealed considering the internal structure of the salt and surrounding successions. Combined interpretation of the models together with results from magnetotelluric studies using neural network techniques supports the robustness of these features.These findings provide important constraints for the modelling of the temperature field and the geomechanical conditions. Ongoing work includes analysis of amplitude variations with offset and azimuth (AVO and AVA) for the relevant target reflectors in regional 2D and 3D seismic data sets. This kind of analysis provides information on seismic anisotropy related with predominant orientations of fracture systems to be used for the geothermal energy production.
In order to evaluate to what extend the results from Gross Schoenebeck can be generalized to comparable settings in the NE German basin, we re-analyze near-vertical and wide-angle reflection seismic data from transect BASIN 9601. Particularly, we investigate the expression of the salt tectonics both in the P velocity structure and the reflection seismic images. This study is also important to test the performance of the geophysical methods developed within the I-GET project.