Anisotropic Kirchhoff pre-stack depth migration at the COSC-1 borehole, central Sweden

Abstract

SUMMARYA remarkably well preserved representation of a deeply eroded Palaeozoic orogen is found in the Scandinavian Caledonides, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after 400 Ma of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structures in central western Sweden comprise far transported allochthonous units, the underlying Precambrian crystalline basement, and a shallow west-dipping décollement that separates the two and is associated with a thin layer of Cambrian black shales. These structures, in particular the Seve Nappes (upper part of the Middle Allochthons), the Lower Allochthons and the highly reflective basement are the target of the two approximately 2.5 km deep fully cored scientific boreholes in central Sweden that are part of the project COSC (Collisional Orogeny in the Scandinavian Caledonides). Thus, a continuous 5 km tectonostratigraphic profile through the Caledonian nappes into Baltica’s basement will be recovered. The first borehole, COSC-1, was successfully drilled in 2014 and revealed a thick section of the seismically highly reflective Lower Seve Nappe. The Seve Nappe Complex, mainly consisting of felsic gneisses and mafic amphibolites, appears to be highly anisotropic. To allow for extrapolation of findings from core analysis and downhole logging to the structures around the borehole, several surface and borehole seismic experiments were conducted. Here, we use three long offset surface seismic profiles that are centred on the borehole COSC-1 to image the structures in the vicinity of the borehole and below it. We applied Kirchhoff pre-stack depth migration, taking into account the seismic anisotropy in the Seve Nappe Complex. We calculated Green’s functions using an anisotropic eikonal solver for a VTI (transversely isotropic with vertical axis of symmetry) velocity model, which was previously derived by the analysis of VSP (Vertical Seismic Profile) and surface seismic data. We show, that the anisotropic results are superior to the corresponding isotropic depth migration. The reflections appear significantly more continuous and better focused. The depth imaging of the long offset profiles provides a link between a high-resolution 3-D data set and the regional scale 2-D COSC Seismic Profile and complements these data sets, especially in the deeper parts below the borehole. However, many of the reflective structures can be observed in the different data sets. Most of the dominant reflections imaged originate below the bottom of the borehole and are situated within the Precambrian basement or at the transition zones between Middle and Lower Allochthons and the basement. The origin of the deeper reflections remains enigmatic, possibly representing dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.

Cross‐Scale Seismic Anisotropy Analysis in Metamorphic Rocks From the COSC‐1 Borehole in the Scandinavian Caledonides

“…investigations have shown the importance of seismic anisotropy in some of these rocks (Wenning et al, 2016), which led to improvements in the imaging and interpretation of seismic data (Simon et al, 2017(Simon et al, , 2019. In this context, we complete the study that Wenning et al (2016) have started and focus more closely on the nature and origin of the recognized anisotropy.…”

Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

“…Elastic wave anisotropy can significantly influence depth, amplitude, continuity, and focus of a seismic reflector (Simon et al., 2019; Tsvankin et al., 2010; Yan et al., 2004). The high elastic wave anisotropy of protoliths and mylonites at the Alpine Fault should be taken into account when processing and interpreting seismic data.…”

“…They processed the data following an isotropic methodology but acknowledge that seismic anisotropy should be included in future seismic processing efforts. Future studies of seismic imaging at the Alpine Fault should focus on processing the data with the inclusion of seismic anisotropy (Simon et al., 2019; Tsvankin et al., 2010). Nonetheless, to show the effect of anisotropy and microfractures on seismic imaging of the Alpine Fault, we wrap up our study by performing a simple modeling of anisotropic reflection coefficients.…”

“…Barruol et al (1992) present a 3-D ray tracing anisotropic model in a shear zone, but little interpretation on the seismic reflectivity is presented. Only recently, Simon et al (2019) perform an anisotropic Kirchhoff prestack depth migration to improve seismic imaging of the Scandinavian Caledonides by taking into account the seismic anisotropy in the Seve Nappe Complex. To date, there is no quantitative interpretation of LVZs and high reflectivity at the Alpine Fault.…”

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

“…Thus, COSC‐1 is different from previous drillings and geophysical experiments as it studies mid‐crustal rocks similar to KTB and unlike the Chinese Continental Drilling Program, but unlike KTB, the geophysical imaging at COSC‐1 is far less affected by fracture zones and the particularly steep dip of lithological boundaries in KTB, making COSC‐1 an excellent site for studying the seismic response of lithological changes at mid‐crustal levels. Several studies analyzed and interpreted the 2D and 3D data sets at COSC‐1 focusing primarily on reflections below the borehole (e.g., Hedin et al., 2012; 2016; Juhlin et al., 2016; Simon et al., 2019). Other studies integrated vertical seismic profile experiments to derive an anisotropy model (Simon et al., 2017) or used samples for laboratory anisotropy analyses (Kästner et al., 2020; Wenning et al., 2016).…”