Shape and structure of the Salzach Valley, Austria, from seismic traveltime tomography and full waveform inversion

A 3-km-long wide-angle seismic survey across the Salzach Valley in the Eastern Alps, Austria, provides a detailed characterization of its structure. Reflection-and-refraction traveltime tomography with adaptive gridding was applied to obtain a minimum-structure model. More detailed images of the valley fill were derived by acoustic full waveform inversion. A multiscale frequency-domain method was applied to data in the 426 Hz range to improve the resolution from several hundred metres to less than fifty metres. The starting frequency is unusually low for the size of the survey, and it provides additional control of the long-wavelengths of the background model, which mitigates the non-linearities of the inversion. The final image is controlled by independent constraints from resistivity data and shallow wells. It shows a confined upper aquifer that overlies a 150-m-thick unsaturated zone. No strong evidence was found for consolidated sediments at the bottom of the basin. This, and the large maximum depth to bedrock of 480 m, suggests that the valley fill was repeatedly removed during strong glacial periods, exposing the bedrock to deep erosion.