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

Kästner, Felix, Pierdominici, Simona , Elger, Judith , Zappone, Alba, Kück, Jochem and Berndt, Christian (2019) Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden. Open Access Solid Earth Discussions . DOI 10.5194/se-2019-161.

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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 to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications for 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 constrains the origin of this reflectivity. In this study, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. The core and downhole velocities deviate by up to 2 km/s. However, velocities of mafic rocks are generally in close agreement. Seismic anisotropy increases from about 5 to 26 % at depth, indicating a transition from gneissic to schistose foliation. We suggest that differences in the core and downhole velocities are most likely the result of microcracks mainly due to depressurization. Thus, seismic velocity can help to identify mafic rocks on different scales whereas the velocity signature of other lithologies is obscured in core-derived velocities. Metamorphic foliation on the other hand 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. In particular, they show that core log seismic integration via synthetic seismograms requires wireline logging data in any but mafic lithologies.

Document Type: Article
Research affiliation: GFZ
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: No
Open Access Journal?: Yes
DOI etc.: 10.5194/se-2019-161
ISSN: 1869-9537
Projects: SPP 1006
Date Deposited: 11 Nov 2019 09:42
Last Modified: 11 Nov 2019 09:42
URI: http://oceanrep.geomar.de/id/eprint/48207

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