# Browsing by Subject "Seismic reflection method--Data processing"

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Item Imaging of R3 profile of Chicxulub offshore seismic data using prestack split-stip Fourier migration in the plane wave domain(2008) Aljadher, Ali Mansour; Stoffa, Paul L., 1948-Show more Sixty-five million years ago, a bolide approximately 10 km in diameter traveling over 20 km/sec collided with earth in the Yucatan Peninsula leaving behind the wide multi-ring Chicxulub crater. Two-dimensional (2D) marine seismic reflection data were acquired in 1996 and 2005 to image the crustal deformation. Radial line R3, a 100 km seismic reflection profile, was processed using a conventional seismic data processing flow (McDonald, MS Thesis, 2006). In this study, line R3 is processed using a different scheme using prestack split-step Fourier migration in the plane wave domain. This new seismic imaging of the R3 data collapses the scattered waves, moves the temporal reflection events to their true structural position in depth and increases the signal to noise ratio. The field shot gathers are contaminated with low-frequency guided waves due to the shallow water column and the hard water bottom offshore Yucatan as well as the Scholte waves propagating along the seafloor interface. A 2D normal derivative operator was applied to remove this coherent noise for NMO corrected data. This multichannel filtering approach attempts to reveal the horizontal or nearly-horizontal reflections while non-horizontal evenets such as groundroll and Scholte waves are attenuated. Before migration of the reflections, the seismic shot gathers were mapped from the offset-time (X-T) domain to the vertical delay time, τ, and the horizontal ray parameter, p, or simply τ-p domain. In the τ-p domain, predictive deconvolution often works better since multiples are periodic and hence easier to remove and this usually gives better results than applying the deconvolution in the original offset-time (X-T) domain. Moreover, groundroll and Scholte waves are mapped to points in the τ-p domain and there can then be readily excluded for the imaging, improving the signal to noise ratio of the final depth section. For depth migration, a good velocity model is required to image the data to the correct position and depth. Thus, an optimized velocity model was used for prestack plane wave migration. Prestack depth migration was applied directly on the transformed τ-p gathers that are sorted into constant ray parameter sections. Each plane wave component, i.e. constant p value, was imaged separately and prestack-migrated common-image-gathers (CIGs) are collected. They are in the depth and ray parameter z-p domain, at each shot position. The migrated and stacked results are obtained by stacking a selected range or all the traces in each CIG to generate the final image. Residual depth versus p "moveout" is then used to refine the interval velocity of the depth section. The result of this new processing is an improved image in depth of the crater which is important to understanding the actual structural geometry of this large impact event. The improved image can give a greater confidence in both the geologic structure and the velocity model than time migration since the events are now in their true spatial positionShow more Item Least squares inversion of field seismic data for an elastic 1-D Earth(1993) Wood, Warren Theodore, 1962-; Stoffa, Paul L., 1948-Show more Elastic 1-D inversion of seismic reflection data is successfully applied to common midpoint field data of varying offset ranges. The inversion, actually iterative forward modeling, operates on plane wave seismic data and yields P-wave velocity, (Vₚ), density, and S-wave velocity, (Vₛ) for each of many (typically hundreds) equal time thickness layers. The problem is viewed in terms of a multi-dimensional optimization problem where the error function is defined as the square of model and data misfit. The error is minimized by the Newton method which requires slope (sensitivity of each datum to each model parameter) and curvature (full Hessian) of the error function at each of several iterations. The algorithm is tested on realistic synthetic data, and when applied to conventional common midpoint data is very helpful in determining the nature of a bottom simulating reflector in the Carolina Trough off shore South Carolina. The algorithm is also applied to deep water data acquired in the Nankai Trough offshore Japan as part of a tectonic study of the accretionary prism. Here the waveform inversion results from 8 very wide aperture expanding spread profiles (ESPs), and 610 conventionally acquired common midpoint gathers are used to generate sediment property cross sections across the accretionary wedge deformation front, highlighting the vertical and lateral sediment property changes.Show more Item Practical seismic inversion(1994) Simmons, James L., Jr.; Backus, Milo M., 1932-Show more Seismic data contain information regarding the phase and amplitude of reflected events. Variations in amplitude, traveltime, and waveform as a function of offset are controlled by changes in the subsurface elastic properties. Analysis of prestack seismic data provides the opportunity to distinguish between changes in compressional-wave velocity, shear-wave velocity, and density, in the context of an isotropic, locally one-dimensional earth. A practical approach to prestack seismic inversion is developed and applied to a portion of a real data set. As a data preprocessing step, a predictive deconvolution algorithm is devised which incorporates the angle, time, and spatial dependence of the reverberation period into the deconvolution operator. An impedance model is obtained by use of a matched filter which represents the data as a superposition of simple-interface and thin-layer reflections. The inversion results are used to substantiate a modification to the statistically estimated seismic wavelet for a nonwhite reflectivity spectrum. One-dimensional velocity estimation is cast as a linear inverse problem. An initial velocity model, which is parameterized as a superposition of cubic B-splines, is adjusted to account for the residual moveout of selected events. The velocity analysis is fully automated. Residual moveout estimates are obtained implicitly without picking, and the resulting velocity function is guaranteed to be smooth. Primaries-only ray tracing, in which the linearized approximation to the Zoeppritz equations describes the reflection coefficients, serves as the forward modeling algorithm. The linear prestack inversion is based on the three-term linearized approximation to the Zoeppritz equations. This expression is reformulated so that one term incorporates the apriori relationships between the elastic properties and the other two represent perturbations from the apriori assumptions. Effects of thin layering, the seismic wavelet, and normal moveout stretch are incorporated into the Frechet derivatives. A single-iterate maximum-likelihood solution estimates the model parameter perturbations relative to the smooth starting model. Real data results illustrate the importance of a judicious selection of the data and model covariance matrices. Known hydrocarbon accumulations are detected as perturbations relative to the apriori assumptions.Show more