Shengwen Jin

Wave-equation-based seismic illumination analysis

We present a wave-equation-based method for seismic illumination analysis. A one-way wave-equation-based, generalized screen propagator is used to extrapolate the wavefields from sources and receivers to the subsurface target. A local plane-wave analysis is used at the target to calculate localized,directionalenergyfluxesforbothsourceandreceiver wavefields. We construct an illumination matrix using these energy fluxes to quantify the target illumination conditions. The target geometry information is used to manipulate the illumination matrix and generate different types of illumination measures. The wave-equation-based approach can properly handle forward multiple-scattering phenomena, including focusing/defocusing, diffraction, and interference effects.Itcanbedirectlyappliedtocomplexvelocitymodels. Velocity-model smoothing and Fresnel-zone smoothing are not required. Different illumination measurements derived from this method can be applied to target-oriented or volumetric illumination analyses. This new method is flexible and practical for illumination analysis in complex 2D and 3D velocity models with nontrivial acquisition and target

Seismic Wave Propagation and Scattering in Heterogeneous Crustal Waveguides Using Screen Propagators: I SH Waves

The great advantages of one-way propagation methods, such as the gen- eralized screen propagators (GSP) method, are the fast speed of computation, often several orders of magnitude faster than the full-wave finite difference and finite element methods, and the huge savings in internal memory. In this article, a half- space GSP is formulated for the SH half-space problem. Two versions of the half- space GSP are derived: the wide-angle pseudo-screen and the phase-screen. The Moho discontinuity is treated as parameter perturbations from the crustal back- ground. The validity and limitations of this treatment are discussed. It is shown that half-space screen propagators can accurately propagate guided crustal waves that are composed of small-angle waves with respect to the horizontal direction. Comparisons of numerical results with a wavenumber integration method for flat crustal models and a finite difference algorithm for heterogeneous models show excellent agree- ments. For a model with propagation distance of 250 km, dominant frequency at 0.5 Hz, the GSP method is about 300 times faster than a finite difference algorithm with a similar accuracy. These comparisons demonstrate the accuracy and efficiency of the method. We apply our method to simulate regional wave propagation in different types of complex crustal waveguides including those with small-scale random het- erogeneities. The influence of these heterogeneities on Lg amplitude attenuation and Lg coda formation is significant.

Offset‐domain pseudoscreen prestack depth migration

The double square root equation for laterally varying media in midpoint-offset coordinates provides a convenient framework for developing efficient 3-D prestack wave-equation depth migrations with screen propagators. Offset-domain pseudoscreen prestack depth migration downward continues the source and receiver wavefields simultaneously in midpoint-offset coordinates. Wavefield extrapolation is performed with a wavenumber-domain phase shift in a constant background medium followed by a phase correction in the space domain that accommodates smooth lateral velocity variations. An extra wide-angle compensation term is also applied to enhance steep dips in the presence of strong velocity contrasts. The algorithm is implemented using fast Fourier transforms and tri-diagonal matrix solvers, resulting in a computationally efficient implementation. Combined with the common-azimuth approximation, 3-D pseudoscreen migration provides a fast wavefield extrapolation for 3-D marine streamer data. Migration of the 2-D Marmousi model shows that offset domain pseudoscreen migration provides a significant improvement over first-arrival Kirchhoff migration for steeply dipping events in strong contrast heterogeneous media. For the 3-D SEG-EAGE C3 Narrow Angle synthetic dataset, image quality from offset-domain pseudoscreen migration is comparable to shot-record finite-difference migration results, but with computation times more than 100 times faster for full aperture imaging of the same data volume.

Three-dimensional Illumination Analysis Using Wave Equation Based Propagator

A wave-equation based method is developed for seismic illumination analysis. The approach uses the one-way screen propagator to provide fast and accurate wavefield extrapolation. A local plane wave analysis is used to determine energy propagation directions. The method can avoid the singularity problem usually linked to the high frequency asymptotic method. It provides a practical tool for three-dimensional full-volume illumination analysis in complicated structures. To demonstrate the potential application of this method, numerical examples for the 3D SEG/EAGE salt model are calculated.

Seismic depth migration with pseudo-screen propagator

The generalized screen propagator (GSP) is a fast and accurate method for computing wave propagation in laterally varying velocity media. The method solves the acoustic one-way wave equation by shuffling FFTs between frequency-space and frequency-wavenumber domain. The velocity model is decomposed into a vertically varying reference velocity and a laterally varying perturbation. Downward continuation of the wavefield is performed with a phase shift in a background medium followed by a correction that accommodates the lateral velocity variations. All frequencies of interest are migrated and summed to produce an image at each depth level. The screen propagators are applicable to both poststack and prestack depth migrations. We illustrate the method with the SEG-EAGE salt model and Marmousi data set. The image results show that the pseudo-screen propagator is a big improvement over the phase-screen propagator for steeply dipping events in strong-contrast heterogeneous media.

Prestack depth migration using a hybrid pseudo‐screen propagator

A hybrid pseudo-screen propagator is applied to prestack depth migration for recursive wave eld backpropagation. The propagator is decomposed into a phase-screen term and a large-angle compensation term, the latter is computed by a FD scheme. Downw ardextrapolated waveelds are calculated rst by a phase shift of the primary w ave eld propagating in the bac kground medium. The scattering contributions generated by the velocity heterogeneities are then superposed. Numerical examples show the ability of the method to handle the steep dip events in the case of strong lateral velocity variation and to pro vide a good image when applied to the Marmousi dataset.

Windowed GSP (Generalized Screen Propagators) Migration Applied to SEG-EAEG Salt Model Data

A windowed GSP(Generalized Screen Propagators) migration method is applied to the Amoco synthetic data for the 2D slice A-A’ from the SEG-EAEG salt model to test the accuracy, stability and wide-angle capacity of the method. The migrated image is compared with the popular f-x and Kirchhoff methods and shown better quality than both the latter methods. Not only most of the subsalt structure is reconstructed clearly, but also the nearly vertical reflectors, including the steeply dipping interfaces and the reflecting faults which are missing in the f-x migrated image, are correctly imaged. This demonstrates the potential applicability of the GSP migration method to subsalt structure mappings. However, some migration noises still exist, indicating the necessity of further improvement of the wide-angle capacity of the method.

Iterative imaging for subsalt interpretation and model building

A key factor for imaging geology below complex salt bodies is identifying and defining the shape of the salt geometry. In recent years, a combination of improved imaging algorithms, increased computational speed, and integrated interpretation platforms have made it increasingly possible to use imaging in near real-time as an interactive aid in the salt model interpretation process. The availability of real-time feedback showing the effect of changes in the velocity model on the seismic image and salt interpretation can significantly improve image quality and reduce cycle times for model building and interpretation. In this study, we utilize a range of imaging algorithms that allow a trade-off between complexity and run time. These techniques range from simple vertical stretching to full two-way shot imaging. Fast, simple algorithms implemented as add-ins to the interpretation platform are used for testing incremental changes to the interpretation. Complex algorithms for full volume imaging are implemented...

Energy partition and attenuation of Lg waves by numerical simulations using screen propagators

Abstract Energy partition and attenuation of Lg waves in complex crustal waveguides with both large-scale structures and small-scale random heterogeneities are studied by numerical simulations. A newly developed screen propagator method (half-space generalized screen propagators) is tested and applied to this problem. The screen method is two to three orders of magnitude faster than finite difference (FD) method and uses much less internal memory. The method has no numerical dispersion and can easily incorporate various Q models into the codes. After analyzing different attenuation mechanisms, this paper is concentrated in simulating the leakage attenuation of Lg waves caused by forward large-angle scattering from random heterogeneities, which scatters the guided waves out of the trapped modes and leaking into the mantle. In addition to energy attenuation curves, variations of angular spectra of Lg waves along the path are also shown to give insight on the energy partition and scattering effects. The curve of equivalent Q for leakage attenuation as a function of normalized scale length (ka) of the random heterogeneities agrees well with the scattering theory. The comparisons of the method with wavenumber integration and FD method, and the results of the numerical simulations demonstrate the validity and capacity of the screen propagator method in studying Lg attenuation.

Wave equation GSP prestack depth migration and illumination

A heterogeneous medium with strong velocity contrast and complex subsurface structures presents a great challenge to seismic imaging. Prestack depth migrations based on the wave equation are accurate but very expensive, thereby limiting their use. However, recent advances in efficient 3D wavefield extrapolators and the continued increase in computing performance are making application of full 3D prestack wave equation depth imaging more affordable. In addition, since PreSDM was first employed for subsalt and carbonate imaging, there have been major advances in efficient and accurate 3D wavefield extrapolation technologies. The generalized screen propagator (GSP) is a one-way wave-equation-based wide-angle propagator that can provide high-resolution and high-fidelity subsurface images. GSP neglects up/down reverberations between heterogeneities and correctly handles forward multiple-scattering phenomena including focusing/defocusing, diffraction, and wave-interference effects. The algorithm alternates between the space-domain and wavenumber-domain via the fast Fourier transform (FFT). The operations within each domain are self-adaptive to the complexity of the medium, making this method robust in the presence of strong velocity contrasts. GSP can be directly applied for prestack shot-record depth migration. The forward downgoing wave from the source side and backward extrapolation of the upgoing wave from the receiver side independently use the same propagator. At each depth step, application of a correlation or deconvolution imaging condition determines a depth image. Figure 1 shows a prestack shot-record migration on the Sigsbee2 data set provided by the Smaart JV Project. Figure 1b shows that the top of salt is well imaged by the GSP migration even for the very steep-sided synclinal feature in the top salt surface. The amplitude and phase of the base salt are imaged well and have good continuity, and the diffractors beneath the salt are clearly reconstructed. The image quality in the finite-difference migration (Figure 1c) is generally comparable to the GSP result, but the …