The derivation of an anisotropic velocity model from a combined surface and borehole seismic survey at the COSC-1 borehole, central Sweden

Abstract:

The project COSC (Collisional Orogeny in the Scandinavian Caledonides) focuses on the mid Paleozoic Caledonide Orogen in Scandinavia in order to better understand orogenic processes, from the past and in recent active mountain belts. The Scandinavian Caledonides provide a well preserved example of a Paleozoic continent-continent collision, where the surface geology in combination with geophysical data provide control of the geometry of the Caledonian structure.

In 2014 the COSC-1 borehole was successfully drilled through the Seve Nappe Complex (SNC) to ~ 2.5 km depth near the town of Åre (central Sweden). Subsequently a major seismic survey was conducted in and around the COSC-1 borehole which comprised both seismic reflection and transmission experiments. This will allow extrapolation of results from core analysis and downhole logging to the structures around the borehole. The survey consisted of three simultaneous experiments: 1) a high-resolution zero-offset Vertical Seismic Profile (VSP), 2) a multi-azimuthal walkaway VSP in combination with three long offset surface receiver lines, and 3) a limited 3D seismic survey.

In this study data from the multi-azimuthal walkaway VSP experiment and the long offset surface lines were used to derive a detailed velocity model around the borehole from the inversion of first arrival traveltimes. The P-wave velocity profile at the borehole obtained by this tomography approach correlates in general with sonic velocities measured during borehole logging and with interval velocities calculated from the zero-offset VSP data, however the absolute values are always significantly higher. This is probably due to the presence of significant seismic anisotropy in the SNC which has also been observed in ultrasonic lab measurements. The obtained anisotropic velocity model serves as the basis for the ongoing application of imaging approaches like pre-stack depth migration techniques.