Regional and semi-regional modelling of wide-angle shear waves in OBS data from the Vøring Basin, N. Norway’a comparison

Abstract

Shear wave refractions and wide-angle reflections recorded on densely spaced Ocean Bottom Seismometers (OBSs) in the Vøring Basin 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. © 2003, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.