The Tannwald Basin (between Biberach and Bad Waldsee, Baden-Würrtemberg) was formed as overdeepened basin by at least three ice advances of the Rhine Glacier during Middle and Late Pleistocene (Ellwanger et al., 2011). Three boreholes, two flush drillings and one cored, provide a unique opportunity to obtain high-resolution images of the subsurface by borehole seismic measurements. A correlation of the new results with geological investigations will characterize the sedimentary infill and sedimentation processes in higher detail than before.

Seismic methods and inversion   

In the project, LIAG acquired crosshole seismic data in three boreholes in the Tannwald Basin (ICDP site 5068_1). This kind of data contains higher frequencies than surface data as its high frequency content is not attenuated in the low-velocity weathering layer. A high-resolution full-waveform inversion allows us to derive subsurface models that can directly compared to the lithology derived from the core of borehole C.

Data that we acquired with polarizing shear wave sources indicates that the Quaternary glacial sediments exhibit seismic anisotropy, which we could not resolve by previous surface measurements. In an anisotropic medium, wave velocity depends on the direction of propagation and/or of the direction of oscillation. Waves that propagate horizontally through the medium promise to supply more information on potential seismic anisotropy when compared to the mostly vertically traveled waves recorded at the surface. Additionally, different geometries of vertical seismic profiling (e.g., walk-away VSP, walk-around VSP) will help to fully characterize and quantify seismic anisotropy as well as reliably derive it from surface measurements in the future. From seismic anisotropy and sedimentological information of the boreholes, we will infer sedimentation processes that help to understand the evolution of overdeepened valleys.