Integrated 3D Seismic Analysis of Pleistocene Tunnel Valleys and their infills in the German North Sea sector

Our research project is dedicated to the development of a comprehensive model for analysing the distribution, dimensions, and evolution of Pleistocene tunnel valleys and their deposits in northern Germany and adjacent areas. The primary objective is to leverage these findings to assess the likelihood of future tunnel-valley formation, with potential implications for the long-term (over the next 1 million years) safety of a radioactive waste repository.

To achieve our goal, we are relying on a 3D seismic dataset. Previously, the mapping of tunnel valleys on land is primarily based on 2D seismic and boreholes, which unfortunately do not provide the required accuracy. Therefore, we have opted to utilize a marine seismic dataset. This 3D seismic dataset 'GeoBasis3D' was acquired by the BGR in 2021.

The 3D seismic dataset is situated within the German Exclusive Economic Zone (EEZ) in the 'Entenschnabel' area. In this region, two intersecting tunnel valleys exist, with one located above the 'Belinda' salt dome. The interpretation of the tunnel-valley base based on the seismic data, and we will observe the influence of the crestal faults above the salt dome on the genesis and filling of the tunnel valley. The filling of the tunnel valleys will be described in terms of seismic facies. Different sedimentary processes can be interpreted from the seismic data. The deepest parts of the tunnel valley are directly filled, and the valley widens above. Some slumping can be detected along the steep slopes of the tunnel valley. Different phases of sedimentation can be observed within the tunnel valley, including both glacifluvial and glacilacustrine phases with parallel and homogenous reflectors. Since there are no available geological cores for the Quaternary in the area of the seismic surveys, we will have to rely on cores from Danish North Sea for the lithostratigraphic description of the sediments and for their chronological classification.

Our aim is to analyse sediment facies to draw conclusions about the backfilling process and repeated erosion phases. This will enable us to compare the findings with the development of onshore tunnel valleys in the next step. The tunnel valleys are a type of glacial erosion that can reach depths of up to 600 meters above sea level in northern Germany. They can have an impact on the long-term safety of a repository, which is required by law to be located at a minimum depth of 300 meters below ground level.