Laboratory‐Constrained Seismic Properties of the COSC‐2 Borehole, Scandinavian Caledonides: Implications for Crustal Anisotropy and Reflectivity

Abstract Seismic surveys are essential for studying the Earth's subsurface. However, the interpretation of seismic surveys requires a link between field data and rock physical properties by linking laboratory‐derived data from drill core samples with borehole information and seismic images. Here, we study the seismic properties of the second “Collisional Orogeny in the Scandinavian Caledonides” borehole (COSC‐2), which intersects parautochthonous turbidites, highly deformed black shales (Alum Shale), autochthonous sediments, and the underlying Baltic basement. We selected six core samples of the major lithologies (graywacke, lithic arenite, Alum Shale, arkosic arenite, porphyry, and dolerite) to measure ultrasonic P‐ and S‐wave velocities ( and ) under dry conditions at confining pressures of 10–240 MPa. For anisotropic sedimentary samples, we measured and in three mutually perpendicular sub‐samples oriented relative to foliation and lineation. Combining our measurements with theoretical concepts of elasticity, we estimated key influences on seismic properties. Our results show: (a) Pressure solution seams in arenites have little effect on seismic anisotropy. (b) Strongly anisotropic graywacke and Alum Shale samples have orthorhombic or lower symmetry, challenging the common transverse isotropy assumption for clay‐rich rocks. Accounting for laboratory‐derived anisotropy may improve the reflectivity in seismic surveys. (c) Organic matter in Alum Shale reduces seismic velocities, enhances seismic anisotropy, and causes Alum Shale to be a prominent reflector. (d) Despite the low porosity, microcracks in the dolerite cause velocities to be significantly higher under saturated conditions than under dry conditions, which must be considered when interpreting the reflectivity of dolerites intruded into porphyries.