The state of seismic exploration of the GeoLaB Tromm site – lessons learned so far

As part of the geoscientific exploration, the GeoLaB Tromm site has been investigated by two seismic surveying campaigns, complementing the so far available structural and geophysical information from surface mapping, gravity and magnetic surveying, electrical resistivity tomography (ERT), and other methods. The contribution of the seismic exploration is expected to provide data for a structural characterization of the underground down to the depth range of a potential underground laboratory, and below, i.e., from the earth’s surface down to approximately 1 km below ground level.
Seismic imaging projects in mountainous areas, such as the Odenwald around the Tromm site, face various challenges, such as limited accessibility for placing seismic source and receiver locations according to the needed spatial resolution, or receiving a complex reflected/backscattered wave field due to a complex underground structure, violating common assumptions made in classical seismic exploration approaches. Therefore, an initial pilot seismic survey, consisting of 2 perpendicular profiles (“Pilot Seismic Tromm 2024”) was acquired in September 2024. A major task of this survey was to assess the feasibility of structural imaging of the Tromm granite and surrounding structures and to provide initial geophysical parameters important for a more detailed planning of a full-scale 3D exploration (e.g., the distribution of seismic velocities).
... mehr
One year later, after the exploratory boreholes have been completed, this pilot seismic survey was complemented by the “VSP and 2D Seismic Tromm 2025”. This survey consisted, on the one hand, in a Zero-Offset-VSP (Vertical Seismic Profiling) acquisition along the GeoLaB-1 borehole, providing high-resolution seismic data for a precise velocity-depth profile along that borehole, and for imaging small-scale reflective features along the borehole. On the other hand, the seismic vibro source was operated along the two profiles acquired in 2024, while recording the signals in the GeoLaB-1 borehole, and along the vibro-profiles. The downhole-recording of the vibro source operations along the lines will provide imaging data focusing around the track of the borehole and may contribute to identifying small-scale fault zones around the borehole.
The data acquired by the Pilot Seismic Tromm 2024 was processed and imaged in the time and depth domains by a contractor, and depth-migrated sections are available along the two vibro lines. The main feature, identified on both sections, is, in the area of the Tromm granite, rather weak and diffuse reflectivity in the first 400 m below ground level, followed by a clear unconformity at 400 m depth, below which complex structural patterns are observed. For further discussion of the sections, please refer to Tolba et al. (this vol.).
The currently available seismic results have shown that the densely spaced acquisition of vibro-seismic lines in this mountainous region is able to identify major unconformities in the underground and to indicate potential fault zones. These observations, and the knowledge gained on the distribution of seismic velocities, support the conceptual design of further seismic 3D imaging of the Tromm site, once the decision has been made to build an underground laboratory.