A seismic study of the mid- and lower-crust beneath the sea of Bothnia : BABEL line 1
In the Autumn of 1989, Durham University took part in the BABEL Project, a collaboration of scientists from five nations recording wide angle and normal incidence seismic data in the Baltic Shield. Recording stations were set up along the Swedish coast of the Sea of Bothnia to record marine airgun shots at wide angle. Similar stations were operated by Finnish teams on the eastern coast, and by a German team on Aland. The data recorded are of high quality and high resolution in comparison to previous wide angle surveys in the region, with a shot spacing of 75 m. A large proportion (around fifty percent) of the project involved developing software for processing this data. The in-line data from Line 1, and also those recorded at two off-line stations, have been interpreted using Cerveny's Gaussian Beam forward modelling package BEAM87, the in-line model being further constrained by 2D gravity modelling. The resulting models are compared and contrasted with normal incidence data from the same line, other models derived from BABEL data in the Sea of Bothnia and older refraction lines in the vicinity. The models show a highly complex crust whose thickness varies between 50 and 60 km along the profile. The seismic velocity is high, increasing from 5.85 km s(^-l) near the surface to 7.4 km s(^-1) at the base of the crust. Lateral velocity variations are seen in the mid-upper crust while discontinuous reflectors and diffracting bodies are seen at 30 km depth. In the central/northern part of the line the crust thickens and there is a change in seismic velocity. Using other geophysical information from the region, two hypotheses are put forward for interpreting this part of the seismic model. The first is the presence of a large igneous intrusion, and the second is the existence of a shear zone or tectonic boundary cutting Line 1. Further work will be required to confirm either or both of these hypotheses.