We conducted seismic crosshole experiments with a sparker source in order to obtain a high-resolution subsurface velocity model in the glacially overdeepened Tannwald Basin (ICDP site 5068_1). The data show complex wave fields that contain a lot of information but also present challenges. Nevertheless, isotropic first-arrival travel-time tomography provides the first high-resolution subsurface models that correlate well with the sonic logs and the core recovered from one of the three boreholes.