Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: a case study for the COSC-1 borehole, Sweden

Abstract

Abstract. Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply eroded orogens like the Scandinavian Caledonides allow us to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications of a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can constrain the origin of this reflectivity. To this end, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. For some intervals of the COSC-1 borehole, the core and downhole velocities deviate by up to 2 km s−1. These differences in the core and downhole velocities are most likely the result of microcracks mainly due to depressurization. However, the core and downhole velocities of the intervals with mafic rocks are generally in close agreement. Seismic anisotropy measured in laboratory samples increases from about 5 % to 26 % at depth, correlating with a transition from gneissic to schistose foliation. Thus, metamorphic foliation has a clear expression in seismic anisotropy. These results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.

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

“…Although Almqvist et al (2013) predicted that textural variations along shear zones are in part responsible for seismic reflection observed in the upper and middle crust of orogens, it appears that the mylonitization at the COSC-1 borehole does not change the seismic properties in any significant way, as it does not correlate with changes in the seismic ZOVSP data or the log-derived synthetic seismic data. This is conclusive with previous studies which showed that only some mylonitic rocks with well-developed macroscopic foliation are strongly anisotropic (Jones & Nur, 1982) and with the analysis of the P-wave velocity at the COSC-1 borehole by Wenning et al (2016) and Kästner et al (2020). They showed that the mica schists between 1,600 and 1,900 m have an anisotropy of up to 25% whereas the anisotropy decreases at depths where the mylonite sequences were described (Figure 3e).…”

“…(2017). Nor do the similar P‐wave velocity from the VSP measurements and the sonic log (Figure 4 and Kästner et al., 2020) suggest that small‐scale fractures influence the seismic imaging. Only the reflection at about 200 m depth where acoustic impedance drops (Figure 4) might relate to a texture change.…”

“…The depths of reflections do not show any correlation with the characteristics of foliation and natural fractures that have been described by Wenning et al (2017). Nor do the similar P-wave velocity from the VSP measurements and the sonic log (Figure 4 and Kästner et al, 2020) suggest that small-scale fractures influence the seismic imaging. Only the reflection at about 200 m depth where acoustic impedance drops (Figure 4) might relate to a texture change.…”

“…The gamma ray log shows a very distinct pattern for the mafic rocks which should allow their classification from downhole measurements even when core recovery is poor, especially if downhole density data are available. The sonic log shows high velocities in the same sequences, which is typical for mafic rock (Kästner et al, 2020;Pechnig et al, 1997), but the noisy pattern does not facilitate classification without additional information. MSCL density, especially in combination with gamma ray information could support a good onsite classification before XRF data is available.…”

“…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.…”