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Analysis of Seismic Data Acquired at the Forsmark Site for Storage of Spent Nuclear Fuel, Central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
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 [en]
Forsmark site, anisotropy, deformation zone, reflection seismic, shear-wave splitting, fractures and traveltime tomography
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-251621ISBN: 978-91-554-9259-5 (print)OAI: oai:DiVA.org:uu-251621DiVA: diva2:806848
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
List of papers
1. Reflection seismic imaging of the deeper structures at the Forsmark spent nuclear fuel repository site, central Sweden
Open this publication in new window or tab >>Reflection seismic imaging of the deeper structures at the Forsmark spent nuclear fuel repository site, central 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.

Keyword
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:nbn:se:uu:diva-194879 (URN)10.1016/j.jappgeo.2012.11.008 (DOI)000315541100003 ()
Available from: 2013-02-20 Created: 2013-02-19 Last updated: 2017-12-06
2. High resolution seismic imaging at the planned tunnel entrance to the Forsmark repository for spent nuclear fuel, central Sweden
Open this publication in new window or tab >>High resolution seismic imaging at the planned tunnel entrance to the Forsmark repository for spent nuclear fuel, central Sweden
Show others...
2014 (English)In: Near Surface Geophysics, ISSN 1569-4445, E-ISSN 1873-0604, Vol. 12, 709-719 p.Article in journal (Refereed) Published
Abstract [en]

The Swedish Nuclear Waste Management Company (SKB) plans to build a repository for storage of high-level radioactive spent nuclear fuel at the Forsmark site in central Sweden at a depth of about 470 m. The planned repository will cover an area of about 3.6 km(2) at this depth. Prior to beginning excavation and tunneling, some detailed geophysical surveys are being performed at the planned site. One of these was a refraction seismic survey to determine depth to bedrock in the vicinity of the planned access ramp. Two lines, each about 300 m long and spaced about 35 m apart, were acquired in August 2011. Since the bedrock topography is known to be highly variable, a close receiver (2 m) and source (6 m) spacing was required to map it. This close spacing allowed the data also to be treated as reflection seismic data and some adjustments to the acquisition procedure were made in the field with this in mind to aid in the later processing. The main adjustment was that seismic data were recorded on all geophone stations simultaneously. That is, as shots were fired along one line, data were recorded along both that line and the other one. Likewise, when shots were fired along the other line, data were recorded along that line and the first line. This adjustment allowed semi-3D coverage between the lines. Results from first break traveltime tomography along the lines indicate a depth to bedrock that is greater than that found from geotechnical observations along the lines. This discrepancy is attributed to the uppermost bedrock being highly fractured and having a velocity significantly below that expected from the intact bedrock deeper down. Reflection seismic processing of the data shows a reflection at about 20 ms (about 60 m). The reflection is interpreted to have a gentle northwesterly dip component to it. Comparison with core data in the area suggests that the reflection is from a thin (a few metres thick) fracture zone, although none of the boreholes actually penetrate the reflector where it is mapped by the seismic data. This fracture zone may be part of a larger fracture zone mapped by core drilling further to the east. The newly mapped reflector may be crossed by the ramp when excavation begins. Further seismic surveying towards the west is required to verify if this will be the case.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-239029 (URN)10.3997/1873-0604.2014028 (DOI)000347643700003 ()
Available from: 2014-12-18 Created: 2014-12-18 Last updated: 2017-12-05
3. Analysis of seismic anisotropy derived from sparse three-component receivers at the Forsmark spent nuclear fuel repository site, central Sweden
Open this publication in new window or tab >>Analysis of seismic anisotropy derived from sparse three-component receivers at the Forsmark spent nuclear fuel repository site, central Sweden
(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:nbn:se:uu:diva-251620 (URN)
Available from: 2015-04-21 Created: 2015-04-21 Last updated: 2015-07-07
4. Analysis of P- and S-waves in vertical seismic profile data acquired at the Swedish site for storage of spent nuclear fuel, Forsmark
Open this publication in new window or tab >>Analysis of P- and S-waves in vertical seismic profile data acquired at the Swedish site for storage of spent nuclear fuel, Forsmark
(English)Manuscript (preprint) (Other academic)
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-251619 (URN)
Available from: 2015-04-21 Created: 2015-04-21 Last updated: 2015-07-07

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