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  • Articles  (6)
  • Articles: DFG German National Licenses  (6)
  • Blackwell Science  (6)
  • Geosciences  (6)
  • 1
    Electronic Resource
    Electronic Resource
    Efficient finite-difference modelling of seismic waves using locally adjustable time steps[ Paper pre] (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Conventional finite-difference modelling algorithms for seismic forward modelling are based on a time-stepping scheme with a constant (global) time step. Large contrasts in the velocity model or in the spatial sampling rate cause oversampling in time for some regions of the model. The use of locally adjustable time steps can save large amounts of computation time for certain modelling configurations.  The computation of spatial derivatives across the transition zone between regions of the model with different temporal sampling requires the definition of the wavefield at corresponding time levels on both sides of the transition zone. This condition can be obtained by extrapolation in time, which is inaccurate, or by multiple time integration in the transition zone. The error in the latter solution is of the same order as the conventional time-stepping scheme because both methods are based on the same iteration formula. The technique of multiple time integration simply requires the use of different sizes of time step. It is applicable only for certain factors of variation of the time step.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00110.x
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  • 2
    Electronic Resource
    Electronic Resource
    Short note: A simple nomogram for interpretation due to magnetic horizontal cylinders[ Received ] (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The magnetic anomaly ΔF due to a long horizontal cylinder yields three points related to its extrema. A nomogram is presented for the determination of some source parameters of the causative body.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00109.x
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  • 3
    Electronic Resource
    Electronic Resource
    A shear experiment over the Natih field in Oman: pilot seismic and borehole data[ Received ] (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An experimental multicomponent three-dimensional (3D) seismic survey has been carried out over the Natih field in Oman. This paper describes the small-scale two-dimensional experiment carried out beforehand, and how the results obtained from this pilot were used to assess the feasibility of a nine-component three-dimensional (9C3D) operation as well as to determine the field parameters for the field-scale 3D survey. It also describes the two VSPs and a wireline shear log, acquired in conjunction with the pilot experiment, and the importance of such borehole data for establishing the correct time-to-depth relationship for the seismic data and for providing an independent check on the seismic interpretation. The observation of cusps in the offset VSP indicated the strong anisotropy of the Fiqa shales overlying the Natih reservoir.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00111.x
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  • 4
    Electronic Resource
    Electronic Resource
    Thin-bed AVO effects[ Paper pre] (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Tuning caused by closely spaced impedance boundaries affects seismic amplitudes. At zero-offset the shape of the composite reflected signal approaches the time-derivative of the original pulse as the layer thickness decreases. For layers thinner than half of the tuning thickness, the reflected amplitude is modified by a factor equal to twice the time-thickness of the thin layer. Offset-dependent tuning can be approximated by the time differences between primary reflections. For high-velocity contrasts locally converted waves will also affect the total reflected seismic response. The contribution from intrabed multiples can, in most cases, be ignored. Correction for offset-dependent tuning should be considered before conventional AVO analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00101.x
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  • 5
    Electronic Resource
    Electronic Resource
    Bounds on the complex conductivity of geophysical mixtures[ Received ] (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Following previous work on bounds for complex dielectrics, bounds on the complex conductivity of a mixture of two isotropic components can be developed which are independent of any special assumption concerning the geometry of the mixture. If certain broad restrictions are assumed, such as isotropy of the mixture, then the bounds can be made more restrictive. These bounds reveal the range of the induced polarization response which can be caused by a mixture of two materials of known complex conductivity. The bounds can also be generalized for spectral responses. The bounds are conservative lithologically in the sense that many of the special models corresponding to boundary responses have lithological counterparts.The chief use for the given bounds is to gain insight into the nature of the induced polarization response. It is also possible to use the bounds to estimate the volume fractions of the components. We illustrate how this is done for the case of a general anisotropic medium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00108.x
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  • 6
    Electronic Resource
    Electronic Resource
    Constraining of the zero total surface charge in galvanic modelling (1998)
    Oxford BSL : Blackwell Science
    Geophysical prospecting 46 (1998), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The galvanic problem is frequently solved by a Fredholm integral equation of the second kind based on a single layer source formulation. At higher conductivity contrasts between the model and its surroundings the homogeneous part of the integral equation approaches an eigenvalue equation. With infinite contrast the solution of this limiting integral equation is non-unique, but in the subspace of zero total charge the solution is unique. This mathematical property of the integral equation is reflected in its numerical solution with the result that large numerical errors may appear and convergence of the solution becomes very slow. Errors are, for the most part, related to the computed excess charge generated in the numerical solution. The effect is studied by comparing the results computed from the solution of the integral equation alone with those computed from a particular solution where the requirement of zero total charge is used as a constraint. The model examples clearly show that the use of the constraint condition significantly improves the accuracy of the results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1998.00112.x
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