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  • 1
    Electronic Resource
    Electronic Resource
    Broadband constant-coefficient propagators (2005)
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Science Ltd
    Geophysical prospecting 53 (2005), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The phase error between the real phase shift and the Gazdag background phase shift, due to lateral velocity variations about a reference velocity, can be decomposed into axial and paraxial phase errors. The axial phase error depends only on velocity perturbations and hence can be completely removed by the split-step Fourier method. The paraxial phase error is a cross function of velocity perturbations and propagation angles. The cross function can be approximated with various differential operators by allowing the coefficients to vary with velocity perturbations and propagation angles. These variable-coefficient operators require finite-difference numerical implementation. Broadband constant-coefficient operators may provide an efficient alternative that approximates the cross function within the split-step framework and allows implementation using Fourier transforms alone. The resulting migration accuracy depends on the localization of the constant-coefficient operators. A simple broadband constant-coefficient operator has been designed and is tested with the SEG/EAEG salt model. Compared with the split-step Fourier method that applies to either weak-contrast media or at small propagation angles, this operator improves wavefield extrapolation for large to strong lateral heterogeneities, except within the weak-contrast region. Incorporating the split-step Fourier operator into a hybrid implementation can eliminate the poor performance of the broadband constant-coefficient operator in the weak-contrast region. This study may indicate a direction of improving the split-step Fourier method, with little loss of efficiency, while allowing it to remain faster than more precise methods such as the Fourier finite-difference method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.2005.00474.x
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    Paper (German National Licenses)
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  • 2
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    Effect of main frequencies on characterizing fault damage zones using forward modeling and attribute of variance (2020)
    Society of Exploration Geophysicists
    Details
    Publication Date: 2020-10-12
    Description: Faulting processes have created large damage zones with complex structures in the field; however, estimating the width and geometry of such fault structures in the subsurface is challenging due to a lack of data. Seismic attributes (e.g., coherence and variance) from seismic surveys have been used for the characterization of faults, but most cases do not detail the effectiveness of this approach. By using forward modeling and the associated seismic attributes of variance, four fault models of idealized damage zones are characterized and the frequency effect is evaluated on the width estimation of fault damage zones in the subsurface. The main results indicate that (1) the general geometric pattern of damage zones could be identified by using simulated amplitude and seismic variance with main frequencies of 10, 25, and 40 Hz; (2) the estimated widths of damage zones at a low frequency of 10 Hz are larger (up to twofold) than those at frequencies of 25 and 40 Hz; for large damage zones (〉400 m), the width is best estimated by a frequency of 25 Hz; and (3) scattering noise and diffraction around the fault are found in data at a high frequency of 40 Hz, which results in width overestimation of the damage zones by approximately 17%. The internal structures are difficult to distinguish as scattering noise and chaotic reflections dominate seismic signals. More factors that may influence the accuracy of damage zone width estimation via seismic attributes, include the bedding thickness, fracture density, and velocity. An in-depth understanding of this approach is useful in the application of seismic variance to characterize fault damage zones that may significantly control the fluid migration in the subsurface.
    Print ISSN: 2324-8858
    Electronic ISSN: 2324-8866
    Topics: Geosciences
    Published by Society of Exploration Geophysicists
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  • 3
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    Effects of randomly orienting penny-shaped cracks on the elastic properties of transversely isotropic rocks (2020)
    Society of Exploration Geophysicists
    Details
    Publication Date: 2020-10-21
    Description: Fractured reservoirs, as one kind of unconventional reservoirs, have great potential for oil and gas development, and their accurate characterization requires the development of rock-physics models that better simulate real fractured rocks. However, current models focus mainly on the elastic properties of rocks with aligned cracks, while the effects of randomly orienting cracks in transversely isotropic (TI) rocks are poorly studied even though such conditions are frequently encountered in the earth. To address this problem, we have derived models for the elastic properties of rocks with a TI background permeated by 3D inclined cracks and randomly orienting cracks. Then, based on the developed models, we comprehensively study the effects of the two inclination angles (i.e., the dip angle between the cracks and the isotropic plane and the rotation angle between the cracks and the plane normal to the isotropic plane, respectively) of 3D inclined cracks on the elastic properties of TI rocks. We determine that the two angles have significant influences on the elastic coefficients and hence the elastic velocities, and that their influences on the elastic properties are varying in different directions. We further investigate the effects of crack density and aspect ratio of randomly orienting cracks on the elastic properties of the fractured rocks with a TI background. The results show that the increasing crack density and crack aspect ratio reduce the elastic coefficients and velocities for rocks with randomly orienting cracks, in which the relations between compressional-wave velocities and the crack properties (i.e., crack density and crack aspect ratio) are obtained to aid the interpretation of the acquired acoustic exploration data. The proposed new models can greatly improve the modeling capability for the elastic properties of rocks with a TI background permeated by inclined and randomly orienting cracks.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists
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  • 4
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    Fast multifrequency focal beam analysis for 3D seismic acquisition geometry (2012)
    Society of Exploration Geophysicists (SEG)
    Details
    Publication Date: 2012-03-01
    Description: A method for the efficient computation of multifrequency focal beams for 3D seismic acquisition geometry analysis has been developed. By computing them for all the frequency components of seismic data, single-frequency focal beams can be extended to multifrequency focal beams. However, this straightforward method involves considerable computer time and memory requirements, especially in complex media settings. Therefore, we propose a rapid 3D multifrequency focal beam method in which only a few single-frequency focal beam computations are followed by a number of smart interpolations. The 3D wavefield extrapolation in the focal beam analysis is conducted by the combined applications of a 3D degenerate Fourier migrator and a 3D Born-Kirchhoff interpolation operator, a process that reduces the computational cost for complex media. The multifrequency focal beam analysis is applied to a 3D model from an oil field of China, demonstrating how spatial sampling differences affect seismic imaging.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists (SEG)
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  • 5
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    Anisotropic finite-difference algorithm for modeling elastic wave propagation in fractured coalbeds (2012)
    Society of Exploration Geophysicists (SEG)
    Details
    Publication Date: 2012-01-01
    Description: The simulation of wave propagations in coalbeds is challenged by two major issues: (1) strong anisotropy resulting from high-density cracks/fractures in coalbeds and (2) numerical dispersion resulting from high-frequency content (the dominant frequency can be higher than 100 Hz). We present a staggered-grid high-order finite-difference (FD) method with arbitrary even-order (2L) accuracy to overcome the two difficulties stated above. First, we derive the formulae based on the standard Taylor series expansion but given in a neat and explicit form. We also provide an alternative way to calculate the FD coefficients. The detailed implementations are shown and the stability condition for anisotropic FD modeling is examined by the eigenvalue analysis method. Then, we apply the staggered-grid FD method to 2D and 3D coalbed models with dry and water-saturated fractures to study the characteristics of the 2D/3C elastic wave propagation in anisotropic media. Several factors, like density and direction of vertical cracks, are investigated. Several phenomena, like S-wave splitting and waveguides, are observed and are consistent with those observed in a real data set. Numerical results show that our formulae can correlate the amplitude and traveltime anisotropies with the coal seam fractures.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists (SEG)
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  • 6
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    Rytov Series Approximation for Rough Surface Scattering (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-02-01
    Description: A Rytov series approximation for rough surface scattering is presented for an analytical description of the close relation of topographic statistics and topographic scattering. The Rytov series approximation is not subject to the stringent restrictions that apply to the Born series approximation. Numerical calculations of the Rytov series approximation are conducted for several benchmark models. Comparisons with the full-waveform numerical solution and the Born series approximation are made for all examples to investigate the ranges of validity of the Rytov series approximation. The first-order Rytov approximation ignores multiple scatterings between any two surface points. In general, it has been considered valid for the large-scale roughness components. The high-order Rytov approximation accounts for multiple scattering between surface points and, therefore, becomes a realistic method for multiscale surfaces. Tests with the Gaussian/semicircular convex topographies and two randomly rough topographies show that the Rytov series approximation improves the Born series approximation in both amplitude and phase. For the two sharp edges in the semicircular convexity model, the fourth-order Rytov approximation is required to account for strong wave fluctuations. For general rough surfaces without infinite gradients and extremely large surface heights, the second-order Rytov approximation might be sufficient to guarantee the accuracy of rough surface scattering.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 7
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    Compensation for transmission losses based on one-way propagators in the mixed domain (2012)
    Society of Exploration Geophysicists (SEG)
    Details
    Publication Date: 2012-05-01
    Description: Most true-amplitude migration algorithms based on one-way wave equations involve corrections of geometric spreading and seismic Q attenuation. However, few papers discuss the compensation of transmission losses (CTL) based on one-way wave equations. Here, we present a method to compensate for transmission losses using one-way wave propagators for a 2D case. The scheme is derived from the Lippmann-Schwinger integral equation. The CTL scheme is composed of a transmission term and a phase-shift term. The transmission term compensates amplitudes while the wave propagates through subsurfaces. The transmission term is a function of the vertical wavenumbers of two adjacent heterogeneous screens. The phase-shift term is a Fourier finite-difference (FFD) propagator implemented in a mixed domain via Fourier transform. The transmission term can be flexibly incorporated into the conventional phase-shift migration algorithm, i.e., FFD, at every depth step. We analyze the effects of frequency, lateral velocity contrast, and vertical velocity ratio on the accuracy of the presented formulae. Numerical examples from a flat model and a fault model with lateral velocity variations are presented to demonstrate the ability of the proposed scheme for compensation of transmission losses.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
    Published by Society of Exploration Geophysicists (SEG)
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  • 8
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    Boundary-reflected waves and ultrasonic coda waves in rock physics experiments (2016)
    Springer
    Details
    Publication Date: 2016-12-01
    Print ISSN: 1672-7975
    Electronic ISSN: 1993-0658
    Topics: Geosciences , Physics
    Published by Springer
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  • 9
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    Computation of elastic properties of 3D digital cores from the Longmaxi shale (2016)
    Springer
    Details
    Publication Date: 2016-06-01
    Print ISSN: 1672-7975
    Electronic ISSN: 1993-0658
    Topics: Geosciences , Physics
    Published by Springer
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  • 10
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    Insights Into Layering in the Cratonic Lithosphere Beneath Western Australia (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-02-01
    Description: The characteristics of internal lithospheric discontinuities carry crucial information regarding the origin and evolution of the lithosphere. However, the formation and mechanisms of the midlithosphere discontinuity (MLD) are still enigmatic and controversial. We investigate the midlithospheric discontinuities beneath the Archean Western Australian Craton, which represents one of the oldest continents on the globe, using a novel receiver-based reflectivity approach combined with other geophysical information comprising tomographic P and S wave velocity, radial anisotropy, electrical resistivity, and heat flow data. The MLD is rather shallow with a depth of 68–82 km. Multiple prominent discontinuities are observed in the lithospheric mantle using constructed high-frequency (0.5–4 Hz) P wave reflectivities. These multiple discontinuities coincide well with the broad-scale reduction of relative P and SV wave velocities at the top of the graded transition zone from the lithosphere to the asthenosphere. Strong radial anisotropy in the upper lithosphere mantle tends to be weak across the MLD, which might reflect quasi-laminar lithospheric heterogeneity behavior with a horizontal correlation length that is greater than its vertical correlation length. Broad-scale electrical resistivity variations show little coherence with the MLD. Given these various geophysical observations, the upper lithosphere exhibits rigid and elastic properties above the MLD, while the lower lithosphere tends to be ductile and rheological or viscous. A model comprising quasi-laminar lithospheric heterogeneity could effectively represent the MLD characteristics beneath the Archean continent. ©2018. American Geophysical Union. All Rights Reserved.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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