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  • 1
    Electronic Resource
    Electronic Resource
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Publishing Ltd
    Geophysical prospecting 52 (2004), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: We describe two practicable approaches for an efficient computation of seismic traveltimes and amplitudes. The first approach is based on a combined finite-difference solution of the eikonal equation and the transport equation (the ‘FD approach’). These equations are formulated as hyperbolic conservation laws; the eikonal equation is solved numerically by a third-order ENO–Godunov scheme for the traveltimes whereas the transport equation is solved by a first-order upwind scheme for the amplitudes. The schemes are implemented in 2D using polar coordinates. The results are first-arrival traveltimes and the corresponding amplitudes. The second approach uses ray tracing (the ‘ray approach’) and employs a wavefront construction (WFC) method to calculate the traveltimes. Geometrical spreading factors are then computed from these traveltimes via the ray propagator without the need for dynamic ray tracing or numerical differentiation. With this procedure it is also possible to obtain multivalued traveltimes and the corresponding geometrical spreading factors.Both methods are compared using the Marmousi model. The results show that the FD eikonal traveltimes are highly accurate and perfectly match the WFC traveltimes. The resulting FD amplitudes are smooth and consistent with the geometrical spreading factors obtained from the ray approach. Hence, both approaches can be used for fast and reliable computation of seismic first-arrival traveltimes and amplitudes in complex models. In addition, the capabilities of the ray approach for computing traveltimes and spreading factors of later arrivals are demonstrated with the help of the Shell benchmark model.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 156 (1999), S. 157-171 
    ISSN: 1420-9136
    Keywords: Key Words: Crustal structure, deep seismic reflection, explosion seismology, 3-D Kirchhoff migration.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract — This paper presents an overview of the results obtained from a 3-D prestack depth migration of the ISO89-3D data set. The algorithm is implemented as a Kirchhoff-type migration, in which the migrated image is generated by weighted summation along diffraction surfaces through the shot record section. The diffraction surfaces are computed by a 3-D finite difference solution of the eikonal equation. A 3-D macro-velocity model derived mainly from wide-angle tomographic inversion served as input for the travel-time calculations. The results of the migration are presented as slices through a volume covering an area of 21 km × 21 km in the horizontal and 15 km in the vertical direction, centered around the KTB drill hole. In these slices the continuation of the Franconian Lineament or SE1 reflector, respectively, can be identified over most of the survey area as a northeast dipping reflector plane. Its signature appears partly curved and discontinuous and with different strength of reflection down to a maximum depth of 9 km. About 5 km to the south-southeast of the KTB drill hole the uppermost top reflection of the Erbendorf body (EB) can be recognized at approximately the same depth. The slices clearly show its complicated internal structure consisting of several apparently separated reflective parts. Moreover, the geometry and the shape of a few other subsurface structures are described.
    Type of Medium: Electronic Resource
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  • 3
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    In:  Pure and Applied Geophysics 156: 157-171
    Publication Date: 1999
    Keywords: seismic processing/methodology, ISO 89, KTB, Oberpfalz, Bohemian Massif, reflection seismics
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  • 4
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
    Format: application/pdf
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  • 5
    Publication Date: 2006-07-01
    Print ISSN: 0039-3169
    Electronic ISSN: 1573-1626
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Published by Springer
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  • 6
    Publication Date: 1999-09-01
    Print ISSN: 0033-4553
    Electronic ISSN: 1420-9136
    Topics: Geosciences , Physics
    Published by Springer
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  • 7
    Publication Date: 2019
    Description: 〈span〉〈div〉Summary〈/div〉A remarkably well preserved representation of a deeply eroded Palaeozoic orogen is found in the Scandinavian Caledonides, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after four hundred million years of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structures in central western Sweden comprise far transported allochthonous units, the underlying Precambrian crystalline basement, and a shallow west-dipping décollement that separates the two and is associated with a thin layer of Cambrian black shales. These structures, in particular the Seve Nappes (upper part of the Middle Allochthons), the Lower Allochthons and the highly reflective basement are the target of the two approximately 2.5 km deep fully cored scientific boreholes in central Sweden that are part of the project COSC (Collisional Orogeny in the Scandinavian Caledonides). Thus, a continuous 5 km tectonostratigraphic profile through the Caledonian nappes into Baltica’s basement will be recovered. The first borehole, COSC-1, was successfully drilled in 2014 and revealed a thick section of the seismically highly reflective Lower Seve Nappe. The Seve Nappe Complex, mainly consisting of felsic gneisses and mafic amphibolites, appears to be highly anisotropic. To allow for extrapolation of findings from core analysis and downhole logging to the structures around the borehole, several surface and borehole seismic experiments were conducted. Here, we use three long offset surface seismic profiles that are centred on the borehole COSC-1 to image the structures in the vicinity of the borehole and below it. We applied Kirchhoff pre-stack depth migration, taking into account the seismic anisotropy in the Seve Nappe Complex. We calculated Green’s functions using an anisotropic eikonal solver for a VTI (transversely isotropic with vertical axis of symmetry) velocity model, which was previously derived by the analysis of VSP (Vertical Seismic Profile) and surface seismic data. We show, that the anisotropic results are superior to the corresponding isotropic depth migration. The reflections appear significantly more continuous and better focused. The depth imaging of the long offset profiles provides a link between a high-resolution 3-D data set and the regional scale 2-D COSC Seismic Profile and complements these data sets, especially in the deeper parts below the borehole. However, many of the reflective structures can be observed in the different data sets. Most of the dominant reflections imaged originate below the bottom of the borehole and are situated within the Precambrian basement or at the transition zones between Middle and Lower Allochthons and the basement. The origin of the deeper reflections remains enigmatic, possibly representing dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.〈/span〉
    Print ISSN: 2051-1965
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 8
    Publication Date: 2019
    Description: 〈span〉〈div〉SUMMARY〈/div〉A remarkably well preserved representation of a deeply eroded Palaeozoic orogen is found in the Scandinavian Caledonides, formed by the collision of the two palaeocontinents Baltica and Laurentia. Today, after 400 Ma of erosion along with uplift and extension during the opening of the North Atlantic Ocean, the geological structures in central western Sweden comprise far transported allochthonous units, the underlying Precambrian crystalline basement, and a shallow west-dipping décollement that separates the two and is associated with a thin layer of Cambrian black shales. These structures, in particular the Seve Nappes (upper part of the Middle Allochthons), the Lower Allochthons and the highly reflective basement are the target of the two approximately 2.5 km deep fully cored scientific boreholes in central Sweden that are part of the project COSC (Collisional Orogeny in the Scandinavian Caledonides). Thus, a continuous 5 km tectonostratigraphic profile through the Caledonian nappes into Baltica’s basement will be recovered. The first borehole, COSC-1, was successfully drilled in 2014 and revealed a thick section of the seismically highly reflective Lower Seve Nappe. The Seve Nappe Complex, mainly consisting of felsic gneisses and mafic amphibolites, appears to be highly anisotropic. To allow for extrapolation of findings from core analysis and downhole logging to the structures around the borehole, several surface and borehole seismic experiments were conducted. Here, we use three long offset surface seismic profiles that are centred on the borehole COSC-1 to image the structures in the vicinity of the borehole and below it. We applied Kirchhoff pre-stack depth migration, taking into account the seismic anisotropy in the Seve Nappe Complex. We calculated Green’s functions using an anisotropic eikonal solver for a VTI (transversely isotropic with vertical axis of symmetry) velocity model, which was previously derived by the analysis of VSP (Vertical Seismic Profile) and surface seismic data. We show, that the anisotropic results are superior to the corresponding isotropic depth migration. The reflections appear significantly more continuous and better focused. The depth imaging of the long offset profiles provides a link between a high-resolution 3-D data set and the regional scale 2-D COSC Seismic Profile and complements these data sets, especially in the deeper parts below the borehole. However, many of the reflective structures can be observed in the different data sets. Most of the dominant reflections imaged originate below the bottom of the borehole and are situated within the Precambrian basement or at the transition zones between Middle and Lower Allochthons and the basement. The origin of the deeper reflections remains enigmatic, possibly representing dolerite intrusions or deformation zones of Caledonian or pre-Caledonian age.〈/span〉
    Print ISSN: 2051-1965
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
    Publication Date: 2016-10-09
    Description: This paper demonstrates the capabilities of Fresnel Volume Migration (FVM) for 3-D single-component seismic data in a crystalline environment. We show its application to the ISO89-3D data set, which was acquired in 1989 at the German continental deep drilling site (KTB) near Windischeschenbach (Southeast Germany). A key point in FVM is the derivation of the emergent angle for the recorded wavefield. This angle is used as the initial condition of the ray-tracing-algorithm within FVM. In order to limit the migration operator to the physically relevant part of a reflector, it is restricted to the Fresnel-volume around the backpropagated ray. We discuss different possibilities for an adequate choice of the used aperture for a local slant-stack algorithm using the semblance as a measure of the coherency for different emergent angles. Furthermore, we reduce the number of used receivers for this procedure using the Voronoi diagram, thereby leading to a more equal distribution of the receivers within the selected aperture. We demonstrate the performance of these methods for a simple 3-D synthetic example and show the results for the ISO89-3D data set. For the latter, our approach yields images of significantly better quality compared to previous investigations and allows for a detailed characterization of the subsurface. Even in migrated single shot gathers, structures are clearly visible due to the focusing achieved by FVM.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 10
    Publication Date: 2020-02-12
    Description: As support for the COSC drilling project (Collisional Orogeny in the Scandinavian Caledonides), an extensiveseismic survey took place during September and October 2014 in and around the newly drilled 2.5 km deepCOSC-1 borehole. The main aim of the COSC project is to better understand orogenic processes in past andrecently active mountain belts. For this, the Scandinavian Caledonides provide a well preserved case of Paleozoiccollision of the Laurentia and Baltica continental plates. Surface geology and geophysical data provide knowledgeabout the geometry of the Caledonian structure. The reflectivity geometry of the upper crust was imaged byregional seismic data and the resistivity structure by magnetotelluric methods. The crustal model was refined byseismic pre-site surveys in 2010 and 2011 to define the exact position of the first borehole, COSC-1.The completely cored COSC-1 borehole was drilled in Central Sweden through the Seve Nappe Complex, a partof the Middle Allochthon of the Scandinavian Caledonides that comprises units originating from the outer marginof Baltica. The upper 2350 m consist of alternating layers of highly strained felsic and calc-silicate gneissesand amphibolites. Below 1710 m the mylonite content increases successively and indicates a high strain zoneof at least 800 m thickness. At ca. 2350 m, the borehole leaves the Seve Nappe Complex and enters underlyingmylonitised lower grade metasedimentary units of unknown tectonostratigraphic position.The seismic survey consisted of three parts: a limited 3D-survey, a high resolution zero-offset VSP (verticalseismic profile) and a multi-azimuthal walkaway VSP (MSP) experiment with sources and receivers along threesurface profiles and receivers at seven different depth levels of the borehole. For the zero-offset VSP (ZVSP) ahydraulic hammer source was used and activated over a period of 20 s as a sequence of impacts with increasinghit frequency. The wave field was recorded with 3-component geophones and a receiver spacing of 2 m over thewhole borehole length.As first pre-processing steps, the three component VSP data were decoded and vertically stacked. Afterwards, theshots were merged to get a continuous shot gather. A horizontal rotation was performed, based on the S-wavearrivals.The rotated ZVSP-data show a high signal-to-noise ratio and good data quality. Signal frequencies up to 150 Hzwere observed. On the vertical component, clear direct P-wave arrivals are visible. Several P-wave reflectionsoccur below 1600 m logging depth. On both horizontal components, clear direct S-wave arrivals are visible afterrotation what suggests that the penetrated rock is anisotropic. In addition, several PS-converted waves can beidentified.In order to integrate the borehole data into the 3D surface seismic data, further processing concentrated only on theP-waves. First, deconvolution was applied to sharpen the signals and to suppress multiples. Then the wave field wasseparated into upgoing and downgoing components by median filtering. Finally, a corridor stack was generated us-ing the upgoing wave field in order to allow correlation with the borehole logging data and the surface seismic data.
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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