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2D and 3D Reflection Seismic Studies over Scandinavian Deformation Zones
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The study of deformation zones is of great geological interest since these zones can separate rocks with different characteristics. The geometry of these structures with depth is important for interpreting the geological history of an area. Paper I to III present 2D reflection seismic data over deformation zones targeting structures in the upper 3-4 km of the crust. These seismic profiles were acquired with a crooked-line recording geometry. 2D seismic processing assumes a straight recording geometry. Most seismic processing tools were developed for sub-horizontally layered structures. However, in the crystalline rocks in Scandinavia more complex structures with contrasting dip directions and folding are common. The crooked-line recording geometries have the benefit of sampling a 3D volume. This broader sampling can be used to gain knowledge about the true geometry of subsurface structures. Correlation with geological maps and other geophysical data along with seismic data modeling can be used to differentiate reflections from faults or fracture zones from other reflectivity, e.g. mafic bodies. Fault and fracture zones may have a large impedance contrast to surrounding rocks, while ductile shear zones usually do not. The ductile shear zones can instead be interpreted based on differing reflectivity patterns between domains and correlations with geology or magnetic maps. Paper IV presents 3D reflection seismic data from a quick-clay landslide site in southern Sweden. The area is located in a deformation zone and structures in unconsolidated sediments may have been influenced by faults in the bedrock. The main target layer is located at only 20 m depth, but good surface conditions during acquisition and careful processing enabled a clear seismic image of this shallow layer to be obtained.The research presented in this thesis provides increased knowledge about subsurface structures in four geologically important areas. The unconventional processing methods used are recommended to future researchers working with data from crooked-line recording geometries in crystalline environments. The imaging of shallow structures at the quick-clay landslide site shows that the 3D reflection seismic method can be used as a complement to other geophysical measurements for shallow landslide site investigations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 57 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1102
Keyword [en]
Azimuthal binning, Crooked-line geometry, Cross-dip, Fault zone, Hard rock seismics, MTFC, Quick clay, Shear zone, UDZ
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-211215ISBN: 978-91-554-8817-8 (print)OAI: oai:DiVA.org:uu-211215DiVA: diva2:667577
Public defence
2014-01-31, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-01-09 Created: 2013-11-21 Last updated: 2014-01-24
List of papers
1. High resolution reflection seismic imaging of the Ullared Deformation Zone, southern Sweden
Open this publication in new window or tab >>High resolution reflection seismic imaging of the Ullared Deformation Zone, southern Sweden
2011 (English)In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 190, no 1-4, 25-34 p.Article in journal (Refereed) Published
Abstract [en]

The Ullared Deformation Zone (UDZ) is one of a few structures worldwide known to contain decompressed eclogite facies rocks of Precambrian age. Given the unique nature of the Ullared eclogites, a 15 km long reflection seismic profile was acquired across the UDZ in April 2007. The principal objective of the profile was to provide geometrical information on the deformation zone at depth. The profile was acquired along a crooked line which gave us the opportunity to extract strike and dip information of reflections from the data-set using non-standard seismic techniques, including a cross-dip correction. This cross-dip correction method proved to be very useful for constraining the geometry of the reflectors at depth. The most recent (from 1997) published geological interpretation of the UDZ area was based on aeromagnetic data and the UDZ was interpreted as a shear zone, or possibly a shear zone system. The seismic data and a detailed aeromagnetic analysis support the interpretation of the UDZ as a shear zone system. Based on differences in reflectivity and geometry, the UDZ can be separated into four different units. The north-easternmost unit is the only one where eclogites have been found so far. Structures of the eclogite bearing unit are interpreted to dip approximately 20-30 degrees towards northeast at depth. 

Keyword
Precambrian, Eclogite, Ullared, Reflection seismic, Cross-dip, Eastern Segment
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-161572 (URN)10.1016/j.precamres.2011.07.012 (DOI)000296209200002 ()
Available from: 2011-11-19 Created: 2011-11-15 Last updated: 2017-12-08Bibliographically approved
2. High resolution reflection seismic profiling over the Tjellefonna fault in the More-Trondelag Fault Complex, Norway
Open this publication in new window or tab >>High resolution reflection seismic profiling over the Tjellefonna fault in the More-Trondelag Fault Complex, Norway
2012 (English)In: SOLID EARTH, ISSN 1869-9510, Vol. 3, no 1, 175-188 p.Article in journal (Refereed) Published
Abstract [en]

The More-Trondelag Fault Complex (MTFC) is one of the most prominent fault zones of Norway, both onshore and offshore. In spite of its importance, very little is known of the deeper structure of the individual fault segments comprising the fault complex. Most seismic lines have been recorded offshore or focused on deeper structures. This paper presents results from two reflection seismic profiles, located on each side of the Tingvollfjord, acquired over the Tjellefonna fault in the southeastern part of the MTFC. Possible kilometer scale vertical offsets, reflecting large scale northwest-dipping normal faulting, separating the high topography to the southeast from lower topography to the northwest have been proposed for the Tjellefonna fault or the Baeverdalen lineament. In this study, however, the Tjellefonna fault is interpreted to dip approximately 50-60 degrees towards the southeast to depths of at least 1.3 km. Travel-time modeling of reflections associated with the fault was used to establish the geometry of the fault structure at depth, while detailed analysis of first P-wave arrivals in shot gathers, together with resistivity profiles, were used to define the near surface geometry of the fault zone. A continuation of the structure on the northeastern side of the Tingvollfjord is suggested by correlation of an in strike direction P-S converted reflection (generated by a fracture zone) seen on the reflection data from that side of the Tingvollfjord. The reflection seismic data correlate well with resistivity profiles and recently published near surface geophysical data. A highly reflective package forming a gentle antiform structure was also identified on both seismic profiles. This structure could be related to the folded amphibolite lenses seen on the surface or possibly by an important boundary within the gneissic basement rocks of the Western Gneiss Region. The fold hinge line of the structure is parallel with the Tjellefonna fault trace suggesting that the folding and faulting may have been related.

Keyword
Western Gneiss Region, Central Norwegian Caledonides, Northern North-Sea, Mid-Norway, Continental-Margin, Evolution, Zone, Deformation, Sweden, Model
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-185224 (URN)10.5194/se-3-175-2012 (DOI)000309884400014 ()
Available from: 2012-11-21 Created: 2012-11-21 Last updated: 2017-01-25Bibliographically approved
3. Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphasedeformation zones and magnetite‐skarn deposits
Open this publication in new window or tab >>Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphasedeformation zones and magnetite‐skarn deposits
Show others...
2011 (English)In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 116, B11307- p.Article in journal (Refereed) Published
Abstract [en]

The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora mining area. The interpretations are constrained by seismic wave velocity measurements on a series of rock samples, cross-dip analysis, prestack time migration, and swath 3-D imaging, as well as by other available geophysical and geological observations. A series of major fault zones is imaged by the seismic data, as is a large mafic intrusion. However, the most prominent feature is a package of east-dipping reflectors found east of the Dannemora area that extend down to at least 3 km depth. This package is associated with a polyphase, ductile-brittle deformation zone with the latest ductile movement showing east-side-up or reverse kinematics. Its total vertical displacement is estimated to be in the order of 2.5 km. Also clearly imaged in the seismic data is a steeply dipping reflector near the Dannemora mine that extends down to a depth of at least 2.2 km. The geological nature of this reflector is not known, but it could represent either a fluid-bearing fault zone or a deep-seated iron deposit, making it an important target for further detailed geophysical and geological investigations.

National Category
Geology
Identifiers
urn:nbn:se:uu:diva-166692 (URN)10.1029/2011JB008643 (DOI)000297675200001 ()
Available from: 2012-01-12 Created: 2012-01-12 Last updated: 2017-12-08Bibliographically approved
4. High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden
Open this publication in new window or tab >>High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden
Show others...
2014 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 79, no 2, B97-B107 p.Article in journal (Refereed) Published
Abstract [en]

Quick-clay landslides often occur in the northern hemisphere in areas that were covered by Pleistocene glaciation. They are particularly common along the shorelines of the Göta River in southwestern Sweden. Characterization of potential landslide areas and identification of features that indicate high risk are necessary to better understand the triggering mechanisms of these events. Therefore, an intensive characterization project has been initiated at the location of the Fråstad landslide in Sweden. Part of the characterization program included the acquisition of 3D reflection seismic data to image structures in the normally consolidated sediments, as well as the bedrock topography below the landslide scar. Two seismic horizons within the glacial and post-glacial sediments were observed. The shallowest seismic horizon (here referred to as S1) corresponds to a coarse-grained layer that was previously detected by eight geotechnical boreholes located within the 3D survey area. Discontinuities in S1, mapped by the 3D reflection seismic data, occur across a zone that correlates with the landslide scar boundary, suggesting that this zone may have played a role in triggering and/or in limiting the extension of the landslide. If S1 is truncated by or mixed with clays in this zone, then the outflow of water from the permeable S1 into the clays above may have increased the amount of quick-clays above this zone. The increased outflow of water may also have caused a higher pore-water pressure south of the zone, which in turn could have acted as a trigger for the landslide. This study shows the potential of using the 3D reflection seismic method as a complement to drilling and other geophysical methods when performing landslide site investigations. It also demonstrates the importance of further investigating the relationship between 3D subsurface geometries and landslide development.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-211211 (URN)10.1190/GEO2013-0225.1 (DOI)000336918700007 ()
Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved

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