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Analysis of seismic anisotropy derived from sparse three-component receivers at the Forsmark spent nuclear fuel repository site, central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
(English)Article in journal (Refereed) Submitted
Abstract [en]

The Forsmark area, located within the Fennoscandian Shield, has been chosen forstoring Sweden's spent nuclear fuel. The bedrock in the area belongs to thePaleoproterozoic Svecokarelian orogen and consists of low-temperature, fracturedcrystalline bedrock which has since been deformed both in the ductile and brittleregimes. Three major sub-vertical deformation zones that strike in the WNW to NWdirections surround a so-called tectonic lens (less deformed rock). These deformationzones have structurally anisotropy that extends into the tectonics lens. Here we presentan analysis of large offset 3-component seismic data that were acquired along with aconventional reflection seismic survey. Clear P-wave and S-wave arrivals are observedat offsets up to 4 km. We observe large time delays in the shear wave arrival timesbetween the transverse and radial components, with the transverse generally havingearlier arrival times. There is no little offset dependence in the time delays and noazimuthal variations can be documented. A 1D P-wave velocity model based on firstarrival picks shows a rapid increase in velocity in the uppermost 100 m. We suggest that most of the observed shear wave splitting occurs in these uppermost 100 m due to ahigh density of sub-horizontal fractures. To complement our analysis, we used theconventional Differential Effective Medium (DEM) theory to predict seismic velocitiesand anisotropy considering several different fracture aspect ratios (α). In order togenerate the observed time delays between the shear wave components, a large aspectratio is needed for reasonable porosities.

National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-251620OAI: oai:DiVA.org:uu-251620DiVA: diva2:806846
Available from: 2015-04-21 Created: 2015-04-21 Last updated: 2015-07-07
In thesis
1. Analysis of Seismic Data Acquired at the Forsmark Site for Storage of Spent Nuclear Fuel, Central Sweden
Open this publication in new window or tab >>Analysis of Seismic Data Acquired at the Forsmark Site for Storage of Spent Nuclear Fuel, Central Sweden
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Forsmark area, the main study area in this thesis, is located about 140 km north of Stockholm, central Sweden. It belongs to the Paleoproterozoic Svecokarelian orogen and contains several major ductile and brittle deformation zones including the Forsmark, Eckarfjärden and Singö zones. The bedrock between these zones, in general is less deformed and considered suitable for a nuclear waste repository. While several site investigations have already been carried out in the area, this thesis focuses primarily on (i) re-processing some of the existing reflection seismic lines to improve imaging of deeper structures, (ii) acquiring and processing high-resolution reflection and refraction data for better characterization of the near surface geology for the planning of a new access ramp, (iii) studying possible seismic anisotropy from active sources recorded onto sparse three-component receivers and multi-offset-azimuth vertical seismic profiling data (VSP). Reflection seismic surveys are an important component of these investigations. The re-processing helped in improving the deeper parts (1-5 km) of the seismic images and allowing three major deeper reflections to be better characterized, one of which is sub-horizontal while the other two are dipping moderately. These reflections were attributed to originate from either dolerite sills or brittle fault systems. First break traveltime tomography allowed delineating an undulating bedrock-surface topography, which is typical in the Forsmark area. Shallow reflections imaged in 3D, thanks to the acquisition design were compared with existing borehole data and explained by fractured or weak zones in the bedrock. The analysis of seismic anisotropy indicates the presence of shear-wave splitting due to transverse isotropy with a vertical symmetry axis in the uppermost hundreds of meters of crust. Open fractures and joints were interpreted to be responsible for the large delays observed between the transverse and radial components of the shear-wave arrivals, both on surface and VSP data.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 48 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1259
Keyword
Forsmark site, anisotropy, deformation zone, reflection seismic, shear-wave splitting, fractures and traveltime tomography
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-251621 (URN)978-91-554-9259-5 (ISBN)
Public defence
2015-06-12, Hamberg, Geocentrum, Villav. 16, 752 36 Uppsala, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-05-20 Created: 2015-04-21 Last updated: 2015-07-07

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Sharifi Brojerdi, Fatemeh

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