Seismic crosshole tomography as contribution to understand sedimentation processes in glacially-overdeepened valleys

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Glacially-overdeepened valleys and basins constitute major sediment traps. Their structure and their sedimentary succession allow conclusions to be made about the Pleistocene evolution of the Alps. The granted ICDP-project „Drilling Overdeepened Alpine Valleys – DOVE“ aims at these goals by drilling seven locations, distributed throughout the Alpine region, to investigate the spatio-temporal evolution of the glaciations. Applied questions refer to hydrology, geohazards, construction, and tunneling. One piece of the puzzle of overdeepened valleys is to reveal the sedimentation processes during the refill of the troughs. Within these environments, sedimentation processes may generate an anisotropic sedimentological setting. We intend to reveal seismic anisotropy of the sediments and derive the sedimentation processes, as part of the Pleistocene evolution. We hypothesize different mechanisms that lead to seismic anisotropy of the sediments: (1) fine-layering, (2) clast lineation of fluvial sediments, (3) glacio-tectonic overprint, and (4) overconsolidation by glacial loading.To convert the scarce, punctual borehole information into spatially-distributed information, geophysical methods are desirable. Since seismic near-surface methods lack resolution of seismic anisotropy so far, we will investigate the sediments using a seismic crosshole experiment. The targeted study area of this proposed project is the geophysical test site of DOVE in the overdeepened Tannwald Basin (Baden-Württemberg, Germany). It provides a unique opportunity and the ideal setting of three boreholes (ca. 140 m deep) at optimized distances (ca. 30-50 m apart) for a seismic crosshole experiment.For the experiment, we will employ different source systems that are able to generate P-waves as well as differently polarized S-waves. Polarization analysis give evidence of birefringence and direct access to anisotropy and let us identify the arrivals of different wavetypes. The first arrivals of each wavetype are input for isotropic traveltime tomography. Isotropic full waveform inversion will further increase the resolution and accuracy of seismic velocities. In a first step, P-wave and horizontally polarized S-waves will generate velocity models followed by a subsequent full P-SV waveform inversion.Using borehole data, core analysis, and sedimentological interpretation contributed by other DOVE research groups, we will integrate sedimentological and seismic anisotropy to derive sedimentation processes. The findings will be included in a methodical assessment of anisotropic VSP and surface data analysis that can be used at other DOVE locations, where only one borehole is present.