| Summary: | The BABEL project (Baltic And Bothnian Echoes from the Lithosphere) was a collaboration among British, Danish, Finnish, German and Swedish geoscientists to acquire deep-crustal reflection and wide-angle refraction data in the Baltic Shield and Gulf of Bothnia. In 1989, the collection of 2,268 km of deep marine reflection seismic data was carried out. BABEL line 7, one of several BABEL profiles, is the focus of this study and runs north of the Åland islands, in an E-W direction in the Bothnian Sea, east of the city of Gävle. The previous seismic image of the BABEL line 7 displays a considerable change in the reflectivity pattern from a weak reflective lower crust in thewest to a more highly reflective lower crust in the east, interpreted to be due to a transition from a stiff crust to a plastic crust from the west to the east. The seismic results were presented by the BABEL Working Group (1993) which focused on imaging and interpreting deep crustal structures as well as assessing the seismic velocities within the crust, the depth and nature of the Moho discontinuity and the seismic reflectivity texture in the crustal geological structures. Early Proterozoic plate tectonics in the Baltic Shield was also suggested from the reflection seismic data.The BABEL line 7 reflection data were collected with a profile length of 174 km, a group of 48 air guns towed at 7.5 m depth, and 3000 m long streamer, 60 hydrophones spaced at 50 m intervals towed at 15 m depth. Seismic data were acquired for a 25 s record length using a 4 ms sampling interval and a 75 m shot interval. Seismic data are characterized by strong source-generated noise at shallow travel times and strong but randomly distributed spurious spikes at later arrival times. In this thesis, the seismic data along BABEL line 7 were recovered and re-processed. Modern processing techniques that were not available previously, were used. A special emphasis on the shallow parts of the seismic data was given and resulted in revealingreflections as shallow as 300 ms. Some of these reflections seem to be a continuation of the deeper ones and now appear to come to the surface which can now improve thecorrelation with the surface geology. Two major apparently moderately dipping shear zones are now interpreted to reach to the surface in the re-processed data in comparison with the previous work.The deep reflections are also enhanced together with the improvement in the shallow parts which provide further insights about the nature of the Moho and its geometry along BABEL line 7. The re-processed seismic image demonstrates the potential in improving shallow and deep crustal structures along the BABEL offshore seismic data.
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