Lithospheric structures in the Fennoscandian Shield in Sweden have been studied by a number of large-scale wide-angle seismic reflection/refraction (WARR) and normal-incidence reflection profiles since the 1960s. Among these were the over 2000 km long Fennoscandian Long Range (FENNOLORA) project in 1979 and the Baltic and Bothnian Echoes from the Lithosphere (BABEL) in 1989, which provided valuable images of the subsurface down to over 60 km depth including the Moho. Additionally, the 550 km long WARR profile, UPPLAND, was acquired in 2017, transecting across the Uppland and Ljusdal Batholiths from south to north. 

Due to the computational progress in the last 25 years, improved seismic images and velocity models could be obtained by reprocessing the recovered BABEL data set. The main finding of the reprocessed BABEL profiles is the presence of large-scale saucer-shaped intrusions of around 100 km diameter connected to an up-domed lower crust and sub-Moho reflectors. These interpreted offshore saucer-shaped intrusions together with those observed onshore suggest a nested interconnected emplacement model, implying that the deeper saucer-shaped intrusions fed the shallower ones.

Ray tracing forward modelling of the UPPLAND data revealed a unique velocity structure below the Uppland Batholith with high velocities in both the lowermost crust (~7.3 km/s) and uppermost mantle (~8.5 km/s). Such a velocity structure has not been observed anywhere else to this extent and robustness. It is interpreted to represent the presence of eclogitized material in the lowermost crust and complicates further the difference between the seismological and petrological crust-mantle boundary.

The recovery and reprocessing of the FENNOLRA data set provided new insights into the lower crust and upper mantle. Consistent with the results of the UPPLAND velocity profile where collocated, it also reveals a high-velocity lowermost crustal structure in the Fennoscandian Shield and a heterogenous upper mantle with different average velocities (8.2-8.5 km/s) across various geological terranes. Additionally, a zone with lower velocities (8.0-8.2 km/s) is detected in 70-90 km depth in the mantle.

Overall, the studies in this thesis demonstrate the potential of reprocessing legacy data, but also of acquiring new high-resolution deep seismic sounding profiles for the investigation of the crust and upper mantle structures. Aside from recovering fully digital reusable BABEL and FENNOLORA data, new and refined geophysical and geological models can be tested and provide key information for cratonic setting studies.