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Fracture zones or mafic sills?: Seismic waveform tomography at the Forsmark spent nuclear fuel repository, eastern 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. (Geophysics)
2012 (English)In: Society of Exploration Geophysicists. Expanded Abstracts with Biographies, ISSN 1052-3812, 1-5 p.Article in journal (Refereed) Published
Abstract [en]

The Swedish Nuclear Fuel and Waste Management Company (SKB) has been carrying out extensive studies at the Forsmark nuclear waste repository site in the eastern part of central Sweden since 2002. An important component of the studies has been reflection seismic surveys which are useful for locating and characterizing sites for storage of spent nuclear fuel in the crystalline bedrock. Identification of sub-horizontal to gently dipping seismic reflections is especially important since these may represent transport routes for radionuclides. Studies have shown that such reflections can be generated by water filled fracture zones that have a lower velocity than the surrounding bedrock. However, lithological changes, I.e. mafic sills, may also be responsible for reflections in some cases. At the Forsmark site, it is difficult to distinguish fracture zones from mafic sills in the standard reflection seismic processed sections. However, since mafic sills usually have a positive velocity contrast with the background velocity field while fractures zones have a negative one, the two possibilities could be differentiated if we could reconstruct the underground velocity field. Waveform tomography has the potential to perform this reconstruction, and allowing us to discriminate between fractures zones and mafic sills. In this study we test this potential of seismic waveform tomography and conclude that the reflections investigated most likely are generated at low velocity fracture zones.

Place, publisher, year, edition, pages
2012. 1-5 p.
Keyword [en]
Inverse theory, Tomography, Seismic tomography, Controlled source seismology, Fractures and faults
National Category
Earth and Related Environmental Sciences Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-187132DOI: 10.1190/segam2012-0697.1OAI: oai:DiVA.org:uu-187132DiVA: diva2:573792
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Quantifying the Seismic Response of Underground Structures via Seismic Full Waveform Inversion: Experiences from Case Studies and Synthetic Benchmarks
Open this publication in new window or tab >>Quantifying the Seismic Response of Underground Structures via Seismic Full Waveform Inversion: Experiences from Case Studies and Synthetic Benchmarks
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seismic full waveform inversion (waveform tomography) is a method to reconstruct the underground velocity field in high resolution using seismic data. The method was first introduced during the 1980’s and became computationally feasible during the late 1990’s when the method was implemented in the frequency domain. This work presents three case studies and one synthetic benchmark of full waveform inversion applications. Two of the case studies are focused on time-lapse cross-well and 2D reflection seismic data sets acquired at the Ketzin CO2 geological storage site. These studies are parts of the CO2SINK and CO2MAN projects. The results show that waveform tomography is more effective than traveltime tomography for the CO2 injection monitoring at the Ketzin site for the cross-well geometry. For the surface data sets we find it is difficult to recover the true value of the velocity anomaly due to the injection using the waveform inversion method, but it is possible to qualitatively locate the distribution of the injected CO2. The results agree well with expectations based upon conventional 2D CDP processing methods and more extensive 3D CDP processing methods in the area. A further investigation was done to study the feasibility and efficiency of seismic full waveform inversion for time-lapse monitoring of onshore CO2 geological storage sites using a reflection seismic geometry with synthetic data sets. The results show that waveform inversion may be a good complement to standard CDP processing when monitoring CO2 injection. The choice of method and strategy for waveform inversion is quite dependent on the goals of the time-lapse monitoring of the CO2 injection. The last case study is an application of the full waveform inversion method to two crooked profiles at the Forsmark site in eastern central Sweden. The main goal of this study was to help determine if the observed reflections are mainly due to fluid filled fracture zones or mafic sills. One main difficulty here is that the profiles have a crooked line geometry which corresponds to 3D seismic geometry, but a 2D based inversion method is being used. This is partly handled by a 3D to 2D coordinate projection method from traveltime inversion. The results show that these reflections are primarily due to zones of lower velocity, consistent with them being generated at water filled fracture zones.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 62 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1005
Keyword
full waveform inversion, waveform tomography, CO2 monitoring, CO2 sequestration, time lapse seismic, Inversion
National Category
Earth and Related Environmental Sciences Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-187142 (URN)978-91-554-8562-7 (ISBN)
Public defence
2013-02-01, Hambergsalen, Geocentrum, Villavagen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2013-01-10 Created: 2012-12-03 Last updated: 2013-02-11Bibliographically approved

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Zhang, FengjiaoJuhlin, Christopher

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