Abstract
Shear wave refractions and wide-angle reflections recorded on densely spaced Ocean Bottom Seismometers (OBSs) in the Vøring have been modelled by use of 2-D kinematic ray-tracing. Twenty-five three component OBSs were deployed along a 120 km long profile, and the semi-regional shear-wave velocity model derived from modelling the horizontal components is compared with a regional model from 5 OBSs along the same profile. The overall shear-wave velocity distribution of the two models are fairly similar, proving that the regional procedure with large OBS spacing provides a reliable regional shear-wave velocity model. The semi-regional model is more detailed than the regional model due to the much closer receiver spacing. The vertical resolution is improved, especially in the shallow sediments, and this enables a more detailed lithological interpretation to be carried out. Intermediate and deep sediments are also better resolved in the semi-regional model, indicating both vertical and lateral changes in the lithology. The Vp/Vs ratio inferred for the crystalline basement is 1.75 in both models, considered to be typical for the continental crust. An increase in the Vp/Vs ratio in the lower crust is observed both in the regional and in the semi-regional model. This indicates a more mafic composition in the lower crust, supporting the interpretation that the lower crust is a “mixing” of continental crust and magmatic underplating.
