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Reflection seismic imaging of the deeper structures 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.
Geological Survey of Sweden.
2013 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 89, 21-34 p.Article in journal (Refereed) Published
Abstract [en]

The Forsmark area belongs to the Paleoproterozoic Svecokarelian orogen (c. 1.9-1.8. Ga), the principal geological entity inside the Fennoscandian Shield, and is the site where Sweden has proposed to store its spent nuclear fuel. Three major sub-vertical (at the surface), composite ductile and brittle deformation zones that strike in a WNW or NW direction are present in the area. In between these zones the bedrock is less deformed and considered suitable for a repository. We present reprocessed reflection seismic data from seven profiles in which we have focused on improving the images in the depth range 1-5. km by passing lower frequencies through the processing flow at the cost of poorer resolution in the near-surface realm. The new images indicate that sub-horizontal to moderately dipping structures are possibly more extensive at depth than previously thought. Three main deeper reflective zones have been identified, one that is sub-horizontal and two that dip moderately to the southwest. The sub-horizontal reflective zone may represent a 1.27-1.26. Ga dolerite sill at about 3. km depth. One of the moderately dipping reflective zones may originate either from another dolerite sill or from a brittle fault system. The other moderately dipping structure may be present throughout most of the area and could cut all three sub-vertical deformation zones at depth. The new images and corresponding interpretation do not require a re-evaluation of the Forsmark site for storage of spent fuel, but they do influence how to interpret the deeper structures and, as a consequence, the tectonic evolution of the area.

Place, publisher, year, edition, pages
2013. Vol. 89, 21-34 p.
Keyword [en]
Borehole, Crustal structures, Dolerite, Imaging, Shear Zones, Brittle deformation, Crustal structure, Deformation zone, Depth range, Fault system, Fennoscandian Shields, Geological-entities, Lower frequencies, Near-surface, Paleoproterozoic, Processing flow, Re-evaluation, Reflection seismic, Reflective zones, Shear zone, Spent nuclear fuel repository, Spent nuclear fuels, Tectonic evolution, Boreholes, Deformation, Gallium, Imaging techniques, Radioactive wastes, Seismology, Spent fuels, data interpretation, diabase, ductile deformation, orogeny, radioactive waste, repository, seismic data, seismic reflection, seismic tomography, structural geology, Forsmark, Sweden, Uppsala [Sweden]
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-194879DOI: 10.1016/j.jappgeo.2012.11.008ISI: 000315541100003OAI: oai:DiVA.org:uu-194879DiVA: diva2:606723
Available from: 2013-02-20 Created: 2013-02-19 Last updated: 2017-12-06
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, FatemehJuhlin, ChristopherMalehmir, Alireza

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