Wide-angle reflection migration technique with an example from the POLAR profile (northern Scandinavia)

Abstract

The 440 km long POLAR profile runs approximately SW to NE across the Baltic Shield from the Karelian Province through the Lapland Granulite Belt and the Kola Peninsula Province in northern Finland and northern Norway. The migration method developed by V.N. Pilipenko (Physics of the Earth, N1, 1983, 36–42) was applied to the wide-angle reflection data obtained along the profile. To prepare the data for the migration, P- and S-waves were analysed with more attention to the distribution of real reflectors in the crust and on the relation between the P- and S-wave velocities. Depth-migrated images of the crustal structure were obtained for both the P- and S-wave fields. This new analysis revealed structural features of the crust which had not before been imaged. The most impressive features are several inclined boundaries in the upper crust dipping northeast beneath the Lapland Granulite Belt. Another newly found element is a lower crustal boundary at a depth of 35 km in the Karelian Province which is well traced in the P-wave migration image but is not observed in the S-wave image. This boundary corresponds to a strong change in the V P/ V S ratio in the lower crust which may be interpreted in terms of mafic crustal composition. We have found that this migration technique can be applied to deep seismic sounding data resulting in new opportunities for deep crustal seismic studies. Nevertheless, more and denser observations are necessary to continue the improvement of the resolution and the reliability of the structure images.