Analysis of P- and S-wave velocities in the COSC-1 borehole, central Sweden

Abstract

The Collisional Orogeny in the Scandinavian Caledonides (COSC) drilling project, supported by the International Continental Drilling Program (ICDP) and Swedish Research Council, was designed to study mountain building processes and the geologic structure and composition of the Caledonian orogeny in central Sweden. The 2.5 km deep scientific COSC-1 borehole (ICDP 5054-1-A) was successfully drilled during the spring and summer of 2014, targeting the Lower Seve Nappe Complex (SNC). With a variety of conducted seismic studies, nearly 100% core recovery, downhole well-logging and on-core measurements through the Lower SNC a comprehensive data set was obtained. This study will focus on the analysis of seismic P- and S-wave velocities of the full-waveform sonic (FWS) data set. The resulting velocities, vp and vs, have been used in seismic models and for comparison to other relevant studies conducted in the COSC-1 borehole. vp and vs showed correlation with velocities obtained from Zero-Offset VSP (ZVSP) and core measurements, indicating a varying vp around a constant velocity and a gradually increasing vs with depth. Correlation has been found between a gradually decreasing vp/vs ratio and an increasing SiO2 content with depth, implying a gradual transition in the middle crustal composition from a mafic to a felsic environment with an average SiO2 content of 69.89±9.23 wt% and vp/vs ratio of 1.70±0.14. Seismic modelling of the derived vp, vs and density resulted in 1D synthetic seismograms and a 2D elastic and isotropic finite-difference model, which are correlated to ZVSP and surface seismic. Seismic modelling obtained similar results as the ZVSP, both indicating a reflective Lower SNC. Reflective interfaces in the basal shear zone (>1710 m) are interpreted as due to amphibole-rich gneiss and mica schist interfaces. Similar reflectivity sequences are found in the 3D migrated section, but yielding a poorer correlation with the seismic models. Comparisons of the FWS data set show good correlations with other seismic methods which can be used in the further development of COSC, but also differences which may have to be accounted for in the future.