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

Abstract

. (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.

Seismic P Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

“…Additionally, anisotropy can be expected for P wave velocities as foliated schist is highly anisotropic (e.g., Allen et al, ; Christensen & Okaya, ; Godfrey et al, ; Li et al, ; Okaya et al, ) and could explain traveltime differences between differing main ray travel paths for borehole and surface receivers. In a similar experiment in Sweden, Simon et al () were able to determine Thomsen parameters and thus an anisotropic velocity model (vertical transverse isotropy, VTI) making use of the different dominant ray paths in surface and borehole analysis. However, the Alpine Fault in the Whataroa Valley dips at ∼50° (Lay et al, ; Toy et al, ) and the fabric dip is between 45° and 55° (Massiot et al, ; Townend et al, ) so that the simplest anisotropy approach would be a tilted transverse isotropy case.…”

Core‐Log‐Seismic Integration in Metamorphic Rocks and Its Implication for the Regional Geology: A Case Study for the ICDP Drilling Project COSC‐1, Sweden

“…Krauß (2018) showed that the implementation of velocities from the borehole seismic measurements into the depth conversion of the 3D seismic data adjusts the position of reflections to the ones in the ZOVSP data. P‐wave velocity anisotropy, in particular the mica schist and amphibolites (Kästner et al., 2020; Simon et al., 2017; Wenning et al., 2016), could affect the quality of the seismic data, too. Furthermore, Wrona et al.…”

“…reflections to the ones in the ZOVSP data. P-wave velocity anisotropy, in particular the mica schist and amphibolites (Kästner et al, 2020;Simon et al, 2017;Wenning et al, 2016), could affect the quality of the seismic data, too. Furthermore, Wrona et al (2020) showed that the dip of reflectors and the noise level hamper the recognizability of multiple, sub-parallel, inclined reflections, converging reflections and cross-cutting reflections in seismic imaging.…”

Seismic P Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

“…Additionally, anisotropy can be expected for P wave velocities as foliated schist is highly anisotropic (e.g., Allen et al, 2017;Christensen & Okaya, 2007;Godfrey et al, 2000;Li et al, 2018;Okaya et al, 1995) and could explain traveltime differences between differing main ray travel paths for borehole and surface receivers. In a similar experiment in Sweden, Simon et al (2017) were able to determine Thomsen parameters and thus an anisotropic velocity model (vertical transverse isotropy, VTI) making use of the different dominant ray paths in surface and borehole analysis. However, the Alpine Fault in the Whataroa Valley dips at ∼50 • (Lay et al, 2016;Toy et al, 2017) and the fabric dip is between 45 • and 55 Townend et al, 2017) so that the simplest anisotropy approach would be a tilted transverse isotropy case.…”