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

Abstract

Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in centralSweden, an old exhumed orogenic thrust zone that is characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about their origin in relation to composition and structural influences on measurements at different seismic scales. Here, we present a new integrative study of cross-scale seismic anisotropy analyses combining mineralogical composition, microstructural analyses and seismic laboratory experiments from the COSC-1 borehole, which sampled a 2.5 km-deep section of metamorphic rocks deformed in an orogenic root now preserved in the Lower Seve Nappe. While there is strong crystallographic preferred orientation in most samples in general, variations in anisotropy depend mostly on bulk mineral composition and dominant core lithology as shown by a strong correlation between these. This relationship enables to identify three distinct seismic anisotropy facies providing a continuous anisotropy profile along the borehole. Moreover, comparison of laboratory seismic measurements and electron-backscatter diffraction data reveals a strong scale-dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross-validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy over seismic scales. 1 I n t r o d u c t i o n Seismic anisotropy is a powerful tool to investigate the continental crust at depth. In the upper crust, the anisotropy of seismic wave propagation is generally associated with layered sedimentary basins or extensively fractured regions, where in the former the anisotropy is caused by bedding (e.g., Bois et al.,

Anisotropic velocity models for (3-D) seismic imaging of the Lower Seve Nappe in Jämtland, Sweden

“…However, Alkhalifah & Larner (1994) indicated that the use of Thomsen's parameters is permissible even for media with higher anisotropy. While the transverse isotropic symmetry is a valid assumption for the mica-rich units, much of the upper units (amphibolite and felsic gneiss) at the borehole may also be described by a lower symmetry system, such as orthorhombic symmetry (Kästner et al 2021). Despite these simplifications, we could observe a significant increase in the amplitudes and continuity of reflections.…”

“…To test its effect on the reflection image, we therefore assumed a vertically transverse isotropic medium with values of ε and δ much lower than 1. According to Kästner et al (2021), the latter is justified for most of the upper parts of the COSC-1 borehole, whereas the deeper borehole parts show considerably higher values of anisotropy (Fig. 3).…”

“…In model 2, we used the mean seismic anisotropy based on seismic laboratory measurements of 16 core samples, which were taken from various lithologies and depths from the COSC-1 drill cores (Wenning et al 2016;Kästner et al 2020a). In model 3, we chose a mean anisotropy parameter based on the seismic anisotropy profile proposed in Kästner et al (2021), which was derived from an anisotropic-facies approach of the COSC-1 core lithology. Similarly, to obtain model 4, we determined two averages for two prominent depth ranges (−500 to 1250 m; 1250 to 2000 m), according to the general trend of the facies-based anisotropy profile (Table 1, Fig.…”

Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

“…The role of mica in seismic anisotropy has been relatively well studied because mica is recognized as a major contributor to observed seismic anisotropy in middle crustal settings owing to its high anisotropy and the common development of preferred shape and crystallographic orientation (e.g., Barruol & Mainprice, 1993; Christensen, 1965; Dempsey et al., 2011; Kästner et al., 2021; Lloyd et al., 2009; Shapiro et al., 2004; Ward et al., 2012). However, we are not aware of studies that have explored the relationship between mica content and seismic velocity contrast, so here we employ synthetic microstructures to explore this relationship and compare our results to published velocity data from natural rocks with varying mica content.…”

“…The microfracture closure pressure (Pc), above which velocities increase linearly, is dependent on rock type and shape of pores and microfractures (e.g., Walsh, 1965). For taking intrinsic seismic properties of the rocks and comparison with EBSD analysis of the SCSZ samples and the other 21 rocks (Ji et al., 2015; Kästner et al., 2021; Lloyd et al., 2009; Naus‐Thijssen, Goupee, Johnson, et al., 2011; Watling, 2017), we obtained the pressure derivative (d V /dP) and the velocity intercept V 0 at zero pressure by making and extrapolating a linear regression fit to the high‐pressure part of each velocity–pressure curve above Pc (e.g., Almqvist & Mainprice, 2017; Burlini & Fountain, 1993; Ji et al., 2007; Kästner et al., 2021; Kern et al., 2001; Khazanehdari et al., 2000). This relationship in the linear regime is described by $$V(\mathrm{P})={V}_{0}+(\mathrm{d}V/\mathrm{dP})\mathrm{P}$$.…”