Seismic site characterization in and around the COSC-1 drillhole
Final Report Abstract
Within the project “Seismic site characterization in and around the COSC-1 drillhole” seismic surface and borehole data were acquired and analysed, in order to characterize the surrounding of the deep drillhole COSC-1 and to allow the extrapolation of structures and petrophysical properties from the borehole into the formation. In particular the Seve Nappe Complex (SNC) was investigated intensively. The rocks of this unit have their origin at the outer margins of the Baltica palaeocontinent and during the collisional orogeny of the Scandinavian Caledonides they were subducted deeply and subsequently hot emplaced onto the underlying allochthons. A major seismic survey was conducted that comprised both reflection and transmission experiments and consisted of three parts: (1) a high resolution zero-offset Vertical Seismic Profile (VSP) survey, (2) a multi-azimuthal walkaway VSP survey in combination with three long offset surface receiver lines and (3) a spatially limited 3D survey. The gained comprehensive data set was subsequently analysed by the project partners. From the zero-offset VSP data vertical P- and S-wave velocity profiles could be derived. The data from the walkaway VSP survey and the long offset lines delivered velocity models along different azimuths around the COSC-1 borehole through the application of a 3D tomographic approach. Both, the vertical profile from zero-offset VSP and the model derived from the tomographic approach, show a relatively strong vertical gradient for the uppermost 1000 m, which is probably due to the closing of micro-cracks and the increasing amount of mafic lithologies with depth. Strong evidence for seismic anisotropy within the SNC was found by comparing velocities derived from vertical and horizontal travel paths. As a consequence an anisotropic VTI (transversely isotropic with vertical axis of symmetry) velocity model was established, consisting of the vertical P-wave velocity from zero-offset VSP and homogeneous Thomsen parameters of δ = 0.3 and ε = 0.03. With this model both surface and borehole data first arrival traveltimes could be very well explained. This anisotropic model was then used as input for Kirchhoff pre-stack depth migration of the surface seismic data. A comparison to migration results based on an isotropic velocity model revealed a better focused and more continuous image of the present reflectors. The SNC was known from previous seismic investigations as a highly reflective unit with little visible internal structure. The analysis of the new seismic data from this project revealed it as a geometrically (3D structure with conflicting dips) and lithologically complex unit with varying physical rock properties and significant seismic anisotropy. Major differences between the physical rock properties are caused by the change of mafic lithologies (amphibolite and meta-gabbro) and gneisses, which again slightly vary depth-depending in their silica content and physical rock properties, as suggested by cluster analysis. Consequently, most prominent reflections within the SNC are related to mafic lithologies. Although spatially limited, the 3D seismic data is able to resolve also changes of different types of gneiss. Additionally, previously unknown dominant reflections below the SNC could be imaged. These are interpreted as transition zones between Lower Allochthons and the Precambrian basement. Some deeper reflections might represent dolerite intrusions or deformation zones of Caledonian or pre- Caledonian age. Their definitive origin remains enigmatic but might be clarified with the second borehole COSC-2. The COSC-1 seismic data set provided a good opportunity to investigate the feasibility of seismic interferometric (SI) approaches to an active seismic data set in crystalline rocks. The chosen virtual source method (VSM) transforms a data subset from the walkaway VSP to a new (virtual) geometry with sources and receivers within the borehole. Due to the simplifications that are assumed using VSM, the virtual sources emit only downgoing waves, which enables a direct comparison with ZOVSP data. The comparison shows that the virtual sources reproduce the direct P-wave and major reflection signals. Furthermore, the bandwidth could be increased with SI. Thus, the VSM was successfully applied to the COSC-1 data set.
Publications
- 2016, 3D reflection seismic imaging at the 2.5 km deep COSC-1 scientific borehole, central Scandinavian Caledonides, Tectonophysics, 689, 40-55
Hedin, P., Almqvist, B., Berthet, T., Juhlin, C., Buske, S., Simon, H., Giese, R., Krauß, F., Rosberg, J.-E., Alm, P.-G.
(See online at https://doi.org/10.1016/j.tecto.2015.12.013) - 2017, The derivation of an anisotropic velocity model from a combined surface and borehole seismic survey in crystalline environment at the COSC-1 borehole, central Sweden, Geophysical Journal International, 210, 1332–1346
Simon, H., Buske, S., Krauß, F., Giese, R., Hedin, P., Juhlin, C.
(See online at https://doi.org/10.1093/gji/ggx223)
