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  • 1
    Electronic Resource
    Electronic Resource
    2.5D full-wavefield viscoacoustic invercion (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 43 (1995), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Full-wavefield inversion for distributions of acoustic velocity, density and Q on a vertical slice through a25D model is implemented for common-source gathers in a cross-hole geometry. The wavefield extrapolation used is 3D, so all geometrical spreading, scattering, reflection, and transmission effects are correctly and automatically compensated for. In order to keep the number of unknowns tractable, application was limited to 2.5D models of known geometry; the latter assurnes a prior step, such as tomography, to fix the layer geometries. With the model geometry fixed, reliable solutions are obtained using synthetic data from only two independent source locations. Solutions from data with noisy and missing traces are comparable to those from noise-free data, but with higher residuals. When the source locations are spatially widely separated, conunon-source gathers may be summed and treated as a single wavefield to yield the same model estimates as when the individual source wavefields are treated separately, at substantially reduced cost. Inversions for full 3D parameter distributions can be handled with the same software, requiring only solution for more unknowns.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1995.tb00295.x
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  • 2
    Electronic Resource
    Electronic Resource
    3D ACOUSTIC REVERSE-TIME MIGRATION (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Acoustic reverse-time finite-difference migration for zero-offset data is extended from two- to three-dimensional media. The formulation is based on the full three-dimensional acoustic wave equation and so has no dip restrictions and it involves extrapolation in a velocity distribution variable in three dimensions. The algorithm is demonstrated by successful migration of synthetic data sets for three models: a point diffractor, an oblique pinch-out, and a dome overlying a planar reflector.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1989.tb02205.x
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  • 3
    Electronic Resource
    Electronic Resource
    3D ACOUSTIC PRESTACK REVERSE-TIME MIGRATION (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 38 (1990), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A prestack reverse-time migration algorithm which operates on common-source gathers, recorded at the Earth's surface, from 3D structures, is conceived, implemented and tested. Reverse-time extrapolation of the recorded wavefield (a boundary-value problem), and computation of the excitation-time imaging condition for each point in a 3D volume (an initial-value problem), are both performed using a second-order finite-difference solution of the full 3D scalar wave equation. The algorithm is illustrated by processing synthetic data for a point diffractor, an oblique wedge, and the French double dome and fault model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1990.tb01872.x
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  • 4
    Electronic Resource
    Electronic Resource
    WEIGHTED STACKING OF SEISMIC DATA USING AMPLITUDE-DECAY RATES AND NOISE AMPLITUDES (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 38 (1990), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The signal-to-noise (S/N) ratio of seismic reflection data can be significantly enhanced by stacking. However, stacking using the arithmetic mean (straight stacking) does not maximize the S/N ratio of the stack if there are trace-to-trace variations in the S/N ratio. In this case, the S/N ratio of the stack is maximized by weighting each trace by its signal amplitude divided by its noise power, provided the noise is stationary. We estimate these optimum weights using two criteria: the amplitude-decay rate and the measured noise amplitude for each trace. The amplitude-decay rates are measured relative to the median amplitude-decay rate as a function of midpoint and offset. The noise amplitudes are measured using the data before the first seismic arrivals or at late record times. The optimum stacking weights are estimated from these two quantities using an empirical equation.Tests with synthetic data show that, even after noisy-trace editing, the S/N ratio of the weighted stack can be more than 10 dB greater than the S/N ratio of the straight stack, but only a few decibels more than the S/N ratio of the trace equalized stack. When the S/N ratio is close to 0 dB, a difference of 4 dB is clearly visible to the eye, but a difference of 1 dB or less is not visible. In many cases the S/N ratio of the trace-equalized stack is only a few decibels less than that of the optimum stack, so there is little to be gained from weighted stacking. However, when noisy-trace editing is omitted, the S/N ratio of the weighted stack can be more than 10 dB greater than that of the trace-equalized stack. Tests using field data show that the results from straight stacking, trace-equalized stacking, and weighted stacking are often indistinguishable, but weighted stacking can yield slight improvements on isolated portions of the data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1990.tb01851.x
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  • 5
    Electronic Resource
    Electronic Resource
    AUTOMATIC EDITING OF NOISY SEISMIC DATA (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 37 (1989), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Seismic data often contain traces that are dominated by noise; these traces should be removed (edited) before multichannel filtering or stacking. Noise bursts and spikes should be edited before single channel filtering. Spikes can be edited using a running median filter with a threshold; noise bursts can be edited by comparing the amplitudes of each trace to those of traces that are nearby in offset-common midpoint space. Relative amplitude decay rates of traces are diagnostic of their signal-to-noise (S/N) ratios and can be used to define trace editing criteria. The relative amplitude decay rate is calculated by comparing the time-gated trace amplitudes to a control function that is the median trace amplitude as a function of time, offset, and common midpoint. The editing threshold is set using a data-adaptive procedure that analyses a histogram of the amplitude decay rates.A performance evaluation shows that the algorithm makes slightly fewer incorrect trace editing decisions than human editors. The procedure for threshold setting achieves a good balance between preserving the fold of the data and removing the noisiest traces. Tests using a synthetic seismic line show that the relative amplitude decay rates are diagnostic of the traces’S/N ratios. However, the S/N ratios cannot be accurately usefully estimated at the start of processing, where noisy-trace editing is most needed; this is the fundamental limit to the accuracy of noisy trace editing.When trace equalization is omitted from the processing flow (as in amplitude-versus-offset analysis), precise noisy-trace editing is critical. The S/N ratio of the stack is more sensitive to type 2 errors (failing to reject noisy traces) than it is to type 1 errors (rejecting good traces). However, as the fold of the data decreases, the S/N ratio of the stack becomes increasingly sensitive to type 1 errors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1989.tb02238.x
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  • 6
    Electronic Resource
    Electronic Resource
    τ-p SEISMIC DATA FOR VISCOELASTIC MEDIA – PART 1: MODELLING (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 39 (1991), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Viscoelastic modelling reveals that the interaction of compressional-wave velocity Cp, compressional-wave quality factor Qp, shear-wave velocity Cs, shear-wave quality factor Qs and Poisson's ratio as a function of time intercept τ and ray parameter p, is complicated; however, distinct, potentially diagnostic behaviours are seen for different combinations of viscoelastic parameters.Synthetic seismograms for three viscoelastic reservoir models show that variations in the Poisson's ratio produce visible differences when compared to the corresponding elastic synthetic seismograms; these differences are attributable to interaction of the elastic parameters with Qp and Qs.When the P-wave acoustic impedance contrast is small, viscoelastic effects become more apparent and more useful for interpretation purposes. The corresponding amplitude and net phase spectra reveal significant differences between the elastic and the viscoelastic responses. When P-wave reflectivities are large, they tend to dominate the total response and to mask the Q reflectivity effects. The attenuation effects are manifested as an amplitude decay that increases with both time and ray parameter.The sensitivity of the computed seismic responses for various combinations of viscoelastic parameters suggests the opportunity for diagnostic interpretation of τ-p seismic data. The interpretation of the viscoelastic parameters can permit a better understanding of the rock types and pore fluid distribution existing in the subsurface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1991.tb00306.x
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  • 7
    Electronic Resource
    Electronic Resource
    τ-p SEISMIC DATA FOR VISCOELASTIC MEDIA - PART 2: LINEARIZED INVERSION (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical prospecting 39 (1991), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: An approach to extraction of viscoelastic parameters from seismic data is implemented and succesfully tested. Viscoelastic inversion is performed using adaptive damping factors to control the sensitivity of the viscoelastic parameters in relation to the τ-p seismic data. A priori information is incorporated through the damping factors as standard deviations of the data and of the viscoelastic model parameters. The stability of the inversion process is controlled by the variation of the damping factors as a function of the residual errors and parameter updates at each iteration.Tests on synthetic and real data show that P- and S-wave quality factors, Qp and Qs, in addition to P- and S-wave velocities and density Cp, Cs and p, can be extracted from τ-p seismic information. Singular value decomposition analysis demonstrates that estimated Qp and Qs values are more affected by the presence of data inaccuracies and noise than are those of Cp and p. Cs and Qs are not uniquely recovered due to the limited contribution of P-S converted waves.Knowledge of the viscoelastic parameters is of particular importance in accurately describing petrophysical properties of rocks and pore fluids existing in the subsurface; this is demonstrated with real data from the Gulf of Mexico.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.1991.tb00307.x
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  • 8
    Electronic Resource
    Electronic Resource
    Parsimonious 3D post-stack Kirchhoff depth migration (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: Parsimonious post-stack migration is extended to three dimensions. By tracing single rays back along each incident wave direction (as determined by a local slant stack at the receivers), the ray tracing can be embedded in the migration. This approach significantly reduces the computer time and disk space needed because it is not necessary to build and save image time maps; 3D migration can be performed on a workstation or personal computer rather than using a supercomputer or cluster.The location of a reflector in the output image is defined by tracing a zero-offset ray to the one-way traveltime (the image condition); the orientation of the reflector is defined as a surface perpendicular to the raypath. The migration impulse response operator is confined to the first Fresnel zone around the estimated reflection point, which is much smaller than the large isochronic surface in traditional Kirchhoff depth migration. Additional efficiency is obtained by applying an amplitude threshold to reduce the amount of data to be migrated. Tests on synthetic data show that the proposed implementation of parsimonious 3D post-stack Kirchhoff depth migration is at least two orders of magnitude faster than traditional Kirchhoff migration, at the expense of slightly degraded migration image coherence. The proposed migration is expected to be a useful complement to conventional time migrations for fast initial imaging of subsurface structures and for real-time imaging of near-offset sections during data acquisition for quality control.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2478.2005.00487.x
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  • 9
    Electronic Resource
    Electronic Resource
    P-to-S conversion for a thin anisotropic zone produced by vertical fracturing[Received D] (1997)
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 45 (1997), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Point-source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil- and gas-filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P-to-S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P-wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode-converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1997.400281.x
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  • 10
    Electronic Resource
    Electronic Resource
    Algorithms for staggered-grid computations for poroelastic, elastic, acoustic, and scalar wave equations[Received A] (1997)
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 45 (1997), S. 0 
    Details
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Heterogeneous wave equations are more complicated numerically than homogeneous wave equations, but are necessary for physical validity. A wide variety of numerical solutions of seismic wave equations is available, but most produce strong numerical artefacts and local instabilities where model parameters change rapidly. Accuracy and stability of heterogeneous equations is achieved through staggered-grid formulations. A new pseudospectral staggered-grid algorithm is developed for the poroelastic (Biot) equations. The algorithm may be reduced to handle the elastic and acoustic limits of the Biot equations. Comparisons of results from poroelastic, elastic, acoustic and scalar computations for a 2D model show that porous medium parameters may affect amplitudes significantly. The use of homogeneous wave equations for modelling of a heterogeneous medium, or of a centred rather than a staggered grid, or of simplified (e.g. acoustic) wave equations when elastic or poroelastic media are synthesized, may produce erroneous or ambiguous interpretations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2478.1997.390275.x
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