Entwicklung einer simultanen refraktions- und reflexionsseismischen 3D-Laufzeittomographie mit Anwendung auf tiefenseismische TRANSALP-Weitwinkeldaten aus den Ostalpen

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.