Two-dimensional modelling and interpretation of seismic wide-angle data from the Western Gulf of Bothnia

Abstract

In late 1989, wide-angle reflection and refraction profiles were shot in the Baltic Sea, the Bothnian Sea and the Gulf of Bothnia in order to study the lithospheric structure of Archaean and Proterozoic domains. The present study concerns the crustal structure and velocity variations beneath the Gulf of Bothnia area. Data collected at six land stations, recording marine airgun shots fired along BABEL line 2 in the Gulf of Bothnia, have been interpreted. The results from traveltime inversion show an upper crustal thickness of 13–22 km with P- and S-wave velocities varying from 5.5 to 6.5 and 3.3 to 3.7 km/s, respectively. The middle crust has a thickness of 13–15 km, with P-wave velocity varying from 6.3 to 6.9 km/s and S-wave velocity from 3.6 to 3.9 km/s. The lower crustal layer with thickness ranging from 9 to 20 km shows a high P- and S-wave velocity of approximately 7.5 and 4.4 km/s, respectively. The P-and S-wave velocities for the upper mantle are about 8.0 and 4.5 km/s. Calculated Poisson's ratio ranges from 0.20 to 0.24 for the upper crust and 0.25 to 0.28 for the middle and lower crust, and 0.29-0.31 for the upper mantle, respectively. The Moho depth varies between 42 and 55 km along the profile. The Moho topography shows a smooth undulation underneath the central part of the Gulf of Bothnia. A steep offset of about 8 km occurs beneath the southernmost part of the profile. Northward-dipping reflectors have been recorded in the lower crust. These lower crustal reflections line up perfectly with the northward-dipping upper mantle reflectors displayed in the vertical reflection section from the same line.