A 3D refraction and reflection seismic travel time tomography was developed on the basis of the widespread Local Earthquake Tomography (LET) method SIMULPS. To include crustal scale refraction seismic observations in the inversion the accuracy of this method was adjusted for long ray paths and high resolution. Floating or discontinuous reflectors are modelled on separate grids by bi-cubic splines, and an appropriate reflection ray tracer was developed using the Approximate Ray Tracing/Pseudo Bending (ART/PB) method. Accuracy was adjusted by several means: arcuate ART trajectories are roughly adjusted to the velocity field before bending, multiple ART travel time minima are perturbed and step length along the ray path for distant observations is reduced by iterative resegmentation. The resulting ray tracer also finds head waves as long as interface geometry is not too complex. The extensions have been tested with some simple synthetic models focusing on problems connected with discontinuities and low velocity zones.
The inversion algorithm was then applied to a wide-angle data set from the Eastern Alps recorded by seismological three-component stations during the TRANSALP campaign. From Vibroseis records a high resolved model for the upper crust was derived, which was extended to depth with low resolution using distant observations from dynamite shots. The resulting model correlates well with known geologic structures in the upper crust. The middle and lower crust shows distinct velocity functions for the European and the Adriatic part of the profile. The European Moho is resolved from the Northern Calcareous Alps in 40 km depth to the Alpine root in ca. 55 km depth, where it seems to lose its reflective character. Only few reflections from the Adriatic Moho were recorded yielding a depth of ca. 40 km. Lower crustal structure is interpreted as a result of southward subduction of Penninic oceanic crust before collision.
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