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Abstract. The Collisional Orogeny in the ScandinavianCaledonides (COSC) project, a contribution to the International Continental Scientific Drilling Program (ICDP), aims to provide a deeper understanding of mountain belt dynamics. Scientific investigations include a range of topics, from subduction-related tectonics to the present-day hydrological cycle. COSC investigations and drilling activities are focused in central Scandinavia, where rocks from the middle to lower crust of the orogen are exposed near the SwedishNorwegian border. Here, rock units of particular interest occur in the Seve Nappe Complex (SNC) of the so-called Middle Allochthon and include granulite facies migmatites (locally with evidence of ultra-high pressures) and amphibolite facies gneisses and mafic rocks. This complex overlies greenschist facies metasedimentary rocks of the doleriteintruded Särv Nappes and underlying, lower grade Jämt-landian Nappes (Lower Allochthon). Reflection seismic profiles have been an important component in the activities to image the subsurface structure in the area. Subhorizontal reflections in the upper 1-2 km are underlain and interlayered with strong west-to northwest-dipping reflections, suggesting significant east-vergent thrusting. Two 2.5 km deep fully cored boreholes are a major component of the project, which will improve our understanding of the subsurface structure and tectonic history of the area. Borehole COSC-1 (IGSN: http://hdl.handle.net/10273/ICDP5054EEW1001), drilled in the summer of 2014, targeted the subduction-related Seve Nappe Complex and the contact with the underlying allochthon. The COSC-2 borehole will be located further east and will investigate the lower grade, mainly Cambro-Silurian rocks of the Lower Allochthon, the Jämtlandian décollement, and penetrate into the crystalline basement rocks to identify the source of some of the northwest-dipping reflections.A series of high-resolution seismic profiles have been acquired along a composite ca. 55 km long profile to help locate the COSC drill holes. We present here the results from this COSC-related composite seismic profile (CSP), including new interpretations based on previously unpublished data acquired between 2011 and 2014. These seismic data, along with shallow drill holes in the Caledonian thrust front and previously acquired seismic, magnetotelluric, and magnetic data, are used to identify two potential drill sites for the COSC-2 borehole.

. (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, 3, pp. 1332-1346 S U M M A R YThe Scandinavian Caledonides provide a well-preserved example of a Palaeozoic continentcontinent collision, where surface geology in combination with geophysical data provides information about the geometry of parts of the Caledonian structure. The project COSC (Collisional Orogeny in the Scandinavian Caledonides) investigates the structure and physical conditions of the orogen units and the underlying basement with two approximately 2.5 km deep cored boreholes in western Jämtland, central Sweden. In 2014, the COSC-1 borehole was successfully drilled through a thick section of the Seve Nappe Complex. This tectonostratigraphic unit, mainly consisting of gneisses, belongs to the so-called Middle Allochthons and has been ductilely deformed and transported during the collisional orogeny. After the drilling, a major seismic survey was conducted in and around the COSC-1 borehole with the aim to recover findings on the structure around the borehole from core analysis and downhole logging. The survey comprised both seismic reflection and transmission experiments, and included zero-offset and multiazimuthal walkaway Vertical Seismic Profile (VSP) measurements, three long offset surface lines centred on the borehole, and a limited 3-D seismic survey. In this study, the data from the multiazimuthal walkaway VSP and the surface lines were used to derive detailed velocity models around the COSC-1 borehole by inverting the first-arrival traveltimes. The comparison of velocities from these tomography results with a velocity function calculated directly from the zero-offset VSP revealed clear differences in velocities for horizontally and vertically travelling waves. Therefore, an anisotropic VTI (transversely isotropic with vertical axis of symmetry) model was found that explains first-arrival traveltimes from both the surface and borehole seismic data. The model is described by a vertical P-wave velocity function derived from zero-offset VSP and the Thomsen parameters = 0.03 and δ = 0.3, estimated by laboratory studies and the analysis of the surface seismic and walkaway VSP data. This resulting anisotropic model provides the basis for further detailed geological and geophysical investigations in the direct vicinity of the borehole.

The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

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We present a novel 2‐D magnetotelluric (MT) inversion scheme, in which the local weights of the regularizing smoothness constraints are based on the envelope attribute of a reflection seismic image. The weights resemble those of a previously published seismic modification of the minimum gradient support method. We measure the directional gradients of the seismic envelope to modify the horizontal and vertical smoothness constraints separately. Successful application of the inversion to MT field data of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project using the envelope attribute of the COSC reflection seismic profile helped to reduce the uncertainty of the interpretation of the main décollement by demonstrating that the associated alum shales may be much thinner than suggested by a previous inversion model. Thus, the new model supports the proposed location of a future borehole COSC‐2 which is hoped to penetrate the main décollement and the underlying Precambrian basement.