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  • 1
    Electronic Resource
    Electronic Resource
    Calculation of the seismic first-break time field and its ray path distribution using a minimum traveltime tree algorithm (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 grid search method, analogous to Huygens’ Principle, is employed to find the first-break seismic traveltime field in a seismic model. By defining a set of directions in space, a dynamic directed graph (digraph) containing the first-break time information can be constructed during the minimum traveltime tree-searching process. The dynamic di-graph has far fewer edges than the global graph suggested by previous researchers. An efficient sorting algorithm (heapsort) is adapted to the traveltime data structure by employing an indirect heap strategy. The computational speed can thus be improved several times over other approaches. The minimum traveltime tree algorithm is an efficient and flexible method to simultaneously calculate the first-break time field and the corresponding ray paths. It produces a robust and global result in comparison with traditional ray tracing methods. Later seismic phases can also be handled by imposing constraints on the ray paths. A subgrid technique is better than the normal grid technique in terms of accuracy and efficiency.The construction of the digraph uses the local directional information of ray-paths and therefore local anisotropic information can be naturely incorporated. The first-break time field and its ray paths in an anisotropic model can be calculated as easily as those for an isotropic model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb06989.x
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  • 2
    Electronic Resource
    Electronic Resource
    Finite-difference simulation of P–SV-wave propagation: a displacement-potential approach (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A new finite-difference formulation is presented for the simulation of P-SV-wave propagation in heterogeneous, isotropic media. In this approach, the wave equations are separated as two sets of equations: one for the displacement fields and the other for potential fields. These two sets of equations involve only first-order spatial derivatives. In terms of potentials, the P-SV-wavefield can be split into P- and S-wave potential fields, which may open an opportunity of simulating P- and S-wave propagation in different ways. By assuming constant density and shear modulus, both P- and S-wave potentials can be expressed in the form of scalar wave equations. Thus, the Lindman ‘free space' boundary condition (Lindman 1975) for scalar waves can be used. An improvement can be made by adding a dissipation zone to absorbing boundaries. For a 2-D model, four equations are used, which is one less than Virieux's (1986) velocity-stress approach. Therefore, the new algorithm is more efficient and requires less computer memory. The calculation is naturally performed in a staggered-grid manner which gives an excellent result for both internal discontinuities and model edges.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00115.x
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  • 3
    Electronic Resource
    Electronic Resource
    Relative-error-based non-linear inversion: application to seismic traveltime tomography (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 121 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The aim of geophysical inversion is to infer information about the subsurface model parameters subject to certain constraints and plausibility considerations. It is achieved through a systematic adjustment of model parameters along a solution path to the minimum of an objective function. The objective function is related to both the observations (data) and the a priori information about the subsurface model parameters (model). When different statistics are assumed for the data and the a priori information about the model, different objective functions are constructed and different solution models become inevitable. Traditionally, data residuals are used directly in constructing the objective function. If the residuals are statistically proportional to the data magnitudes, a least-squares (LS) method often leads to a solution model biased towards (i.e. which emphasizes) observations of large magnitude, whereas a least absolute deviation (LAD) method can reduce this data-magnitude-dependent effect. An alternative is to formulate the objective function in terms of relative differences. It completely eliminates the possible solution bias due to the intrinsic magnitude-dependent discrepancies in the objective function. The solution path is altered in such a way that data residuals are reduced in a more uniform manner. The idea is illustrated through a seismic traveltime tomographic inversion. Traveltime residuals are often correlated with ray-path lengths. Greater residuals of long ray paths dominate the objective function in the traditional tomographic formulation. The solution path is thus strongly affected by those long ray paths, and short ray paths play a reduced role in the inversion. The final solution is biased toward longer ray paths. The new alternative formulation eliminates this bias by exploiting the relative-error measure to give equal weights to data of the same quality regardless of their ray-path lengths.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb06431.x
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  • 4
    Electronic Resource
    Electronic Resource
    Multiple suppression by 2D filtering in the parabolic τ–p domain: a wave-equation-based method (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 44 (1996), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The filter for wave-equation-based water-layer multiple suppression, developed by the authors in the x-t, the linear τ-p, and the f-k domains, is extended to the parabolic τ-2 domain. The multiple reject areas are determined automatically by comparing the energy on traces of the multiple model (which are generated by a wave-extrapolation method from the original data) and the original input data (multiples + primaries) in τ-p space. The advantage of applying the data-adaptive 2D demultiple filter in the parabolic τ-p domain is that the waves are well separated in this domain. The numerical examples demonstrate the effectiveness of such a dereverberation procedure. Filtering of multiples in the parabolic τ-p domain works on both the far-offset and the near-offset traces, while the filtering of multiples in the f-k domain is effective only for the far-offset traces.Tests on a synthetic common-shot-point (CSP) gather show that the demultiple filter is relatively immune to slight errors in the water velocity and water depth which cause arrival time errors of the multiples in the multiple model traces of less than the time dimension (about one quarter of the wavelet length) of the energy summation window of the filter. The multiples in the predicted multiple model traces do not have to be exact replicas of the multiples in the input data, in both a wavelet-shape and traveltime sense. The demultiple filter also works reasonably well for input data contaminated by up to 25% of random noise. A shallow water CSP seismic gather, acquired on the North West Shelf of Australia, demonstrates the effectiveness of the technique on real data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1996.tb00154.x
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