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  • 1
    Electronic Resource
    Electronic Resource
    Structure of a Volcanic Continental Margin Derived from Ocean-bottom Seismographic Data: The Northern Vøring Margin, off Norway (1998)
    Springer
    Pure and applied geophysics 152 (1998), S. 1-21 
    Details
    ISSN: 1420-9136
    Keywords: Key words: Vøring margin, Ocean-Bottom Seismograph, crustal structure, volcanic continental margin.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —This paper presents a crustal model derived from an Ocean Bottom Seismograph (OBS) study along the northern Vøring margin off Norway. The profile was acquired to map the crustal structure in the northernmost part of the Vøring Basin, and to link crustal models of the Lofoten and central Vøring Basin obtained by previous OBS studies. The Vøring margin, as well as the Lofoten margin to the north, was created by continental breakup between Norway and Greenland in late Paleocene-early Eocene. The rifting and continental breakup process were accompanied by intense extrusive and intrusive magmatic activities. The OBS data provide the whole crustal structure along the northern Vøring margin, in the area where the deep crustal structure cannot be resolved by conventional multichannel reflection data due to sill intrusions in the sedimentary sequence. The shallow part of the crustal model is characterized by up to 10 km thick sediments, a sequence of flood basalts and sill intrusions. The P-wave velocities in the flood basalts and sill intrusions are estimated to 5.0 km/s and 4.7–5.8 km/s, respectively. The model indicates an abrupt thickening of the upper crystalline crust from approx.3 km in the NE, to about 10 km towards the SE, with velocities of 6.0–6.2 km/s. The lower crustal velocities are not well resolved due to lack of clear refraction arrivals from the lower crust. However, the observed amplitude versus offsets are best explained by a model with a change in lower crustal velocities from 6.8 to 7.2 km/s beneath the Bivrost lineament. The modelling infers the presence of a lower crustal reflector beneath the lineament, which represents the landward continuation of the Bivrost lineament. Reflection arrivals from the Moho reveal a Moho depth of 23 km in the middle of the profile and 18– 20 km in the northeastern part of the profile. A 370 km long crustal section from the central part of the Vøring Basin to the Lofoten margin, obtained by the results of this study and previous OBS studies, shows a simple thinned continental crust on the Lofoten margin, and a high velocity lower crust underlying an upper crust of varying thickness in the Vøring Basin. The transition between these structures is situated beneath the Bivrost lineament in the lower crust, and beneath the basement high about 40 km south of the lineament in the upper crust.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000240050138
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  • 2
    Electronic Resource
    Electronic Resource
    Comparison between a Regional and Semi-regional Crustal OBS Model in the Vøring Basin, Mid-Norway Margin (1997)
    Springer
    Pure and applied geophysics 149 (1997), S. 641-665 
    Details
    ISSN: 1420-9136
    Keywords: Key words: Vøring Basin, crustal structure, 3-component OBSs.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —Semi-regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Vøring Basin, mid-Norway margin, have been modeled by use of 2-D ray-tracing. The semi-regional model, derived from the study of twenty-five OBSs deployed along a 120-km long profile, is compared with a regional model consisting of five OBSs from the same profile. The semi-regional model is somewhat more detailed than the regional model, due to the considerably closer receiver spacing. The overall geometry and velocity distribution of the two models are remarkably similar, however, proving that the regional procedure with large OBS spacing provides a reliable regional model.¶Intrusions of sills, related to early Tertiary continental rifting and break up, are important at intermediate and deep sedimentary levels (2–10 km below sea floor) in most parts of the area. The semi-regional modeling suggests that one of the deepest sills extends much further east and is substantially thicker (locally more than 500 m) than indicated in the regional model. Another important difference is a high-velocity body within the upper crystalline crust at 11–12 km depth in the NW part of the area, indicating that the closer OBS spacing in the semi-regional modeling allows detection of local intra-crustal intrusions. Local differences are also inferred in the lower crust; at about 20 km depth a structure is inferred within the lower crust from wide-angle reflections. This might suggest that the high-velocity lower crustal layer, interpreted as magmatic underplating, consists of a mixture of underplated/intruded magmatic material and blocks of continental lower crust.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000240050045
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  • 3
    Electronic Resource
    Electronic Resource
    Modelling shear waves in OBS data from the Vøring basin (Northern Norway) by 2-D ray-tracing (1996)
    Springer
    Pure and applied geophysics 147 (1996), S. 611-629 
    Details
    ISSN: 1420-9136
    Keywords: Vøring basin ; 3C OBS data ; shear waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Three component recordings from an array of five ocean bottom seismographs in the northwestern part of the Vøring basin have been used to obtain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves are identified by means of travel-time differences compared to the compressional (P) waves, and by analyzing their particle motions. The model has been obtained by kinematic (travel-time) ray-tracing modelling of the OBS horizontal components. The shear-wave modelling indicates that mode conversions occur at several high velocity interfaces (sills) in the 4–10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set. An averageV p /V s ratio of 2.1 is inferred for the layers above the uppermost sill, indicative of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in theV p /V s ratio (1.7) below the first sill may in part be atributed to well consolidated sediments, and to a change in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5–10 km depthV p /V s ratios of 1.85 indicate a lower sand/shale ratio consistent with the expected lithologies. The averageV p /V s ratio inferred for the crust is 1.75, which is consistent with values obtained north of Vøring, in the Lofoten area. An eastward thinning of the crystalline basement is supported by the shear-wave modelling.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01089694
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  • 4
    Electronic Resource
    Electronic Resource
    Crustal Structure Beneath Lofoten, N. Norway, From Vertical Incidence and Wide-Angle Seismic Data (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 114 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A high-quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off Lofoten, northern Norway. A seismic wide-angle experiment was performed in 1988 along the same profile, using seven three-component Ocean Bottom Seismographs (OBS) with 20-25 km spacing and shotpoint intervals of 240 m.The study of the data has shown that the combination of the multichannel reflection and the wide-angle (OBS) technique provides information about the crustal structure beneath the Lofoten shelf that could not have been achieved using only one of the techniques. the multichannel reflection data provide a detailed image of the shallow (Cretaceous) structures, which represents an important basis for inversion of the OBS data. the lower crust and the Moho are also well mapped in some parts of the area with the multichannel reflection technique.The OBS data reveal that significant amounts of pre-Cretaceous sediments exist along almost the entire profile, with a maximum thickness of about 5 km in the Vestfjorden Basin. From the OBS data the thickness of the lower crust is inferred to decrease from about 11.5 km under the Røst High to about 2 km below the Lofoten Ridge. the OBS data indicate further that the Moho position under the Vestfjorden Basin is considerably deeper than can be inferred from the reflection data.About 10km below Moho a strong dipping event is observed in the OBS data. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or presence of layers of partially hydrated peridotite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb01471.x
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  • 5
    Electronic Resource
    Electronic Resource
    REFLECTION AND POLARIZATION OF TUBE WAVES AS SEEN IN VSP DATA (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 40 (1992), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: P-wave and S-wave data acquired with vertical seismic profiling (VSP) often include tube waves propagating in the borehole, although considerable efforts are generally made to ensure that these waves are not recorded. However, several theoretical studies have indicated that tube waves could provide important information about the rock formation and thus should not be considered as pure noise.In order to study some of these aspects experimentally, tube waves were acquired by VSP in a well in the Paris Basin both before and after casing. A sparker was used as source inside the borehole, which ensured that the data recorded contained high-amplitude tube waves.It is shown that the casing is an obstacle which prevents the study of formation parameters, and thus further tube-wave acquisitions should be carried out in open holes only.The before-casing tube-wave reflection log is compared to a synthetic log computed from the sonic log. The high resolution of the tube waves is of particular interest, revealing layers that are too thin to be detected in body-wave surveys.It has recently been suggested that the projection of the tube-wave polarization in the horizontal plane can be used to determine directions of stress-induced anisotropy in the rock formation. Strong polarization anomalies are observed in the data sets but are attributed to tool problems rather than any rock-formation feature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1992.tb00544.x
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  • 6
    Electronic Resource
    Electronic Resource
    Ocean Bottom Seismographs used in a crustal study of an area covered with flood-basalt off Lofoten, N. Norway (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Terra nova 5 (1993), S. 0 
    Details
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Seismic refraction measurements were made in August 1988 to study the crustal structure off Lofoten, Northern Norway. Twenty-four 3-component Ocean Bottom Seismographs (OBS) were used, of which seven were deployed in the area covered by landward-flowing basalt deposited during the early Eocene break-up between Norway and Greenland. The main purpose of the OBS survey was to investigate whether this method can be used to map structures below the basalt, which is not easy to penetrate with conventional seismic reflection techniques. The records obtained showed that the OBS data contain considerable information about structures below the flood-basalt; preopening sediments up to 4.0 km thick is indicated below the 1.0–2.5 km-thick landward-flowing basalt. The success of the OBS survey indicates that such measurements can become an important tool in investigations on passive volcanic margins and, potentially, in other areas where highly reflective boundaries make the reflection technique difficult to apply.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3121.1993.tb00228.x
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