Huazhong Wang

3D Kirchhoff prestack time migration in average illumination-azimuth and incident-angle domain for isotropic and vertical transversely isotropic media

Conventional offset domain prestack migration tends to bring ambiguity and migration artifacts because it smears energy from different angles at the image point. To avoid this, prestack depth migration implementations in angle domain have been investigated in the past decades. As an efficient imaging tool, angle domain Kirchhoff prestack time migration is still useful and was proposed recently. However, existing algorithms cannot handle ray bending and anisotropy correctly. Practically, azimuth analysis for fractured reservoirs should be carried out after migration for most geological settings. Unfortunately, the existing migration algorithm implicitly involves some kind of binning to source-receiver azimuth, which may not be the real wave-path azimuth, especially for side-scattering or out-of-plane waves. In this paper, we present an algorithm for 3D Kirchhoff prestack time migration in average illumination azimuth and incident angle domain, which matches true wave path naturally and more accurately. To handle ray bending and vertical transversely isotropy, we propose several approaches to estimate two-way traveltime and the corresponding angular attributes through extended offset-to-angle mapping. Based upon these approaches, our 3D prestack time migration can provide high-quality common-image gathers for amplitude variation with incident angle and/or amplitude variation with offset and azimuth analyses, even in media with slight to moderate lateral heterogeneity. The 2D and 3D synthetic examples prove the validity of our methods.

A Strategy For Source Wavefield Reconstruction In Reverse Time Migration

The RTM imaging condition require that the source and receiver wavefield be available simultaneously. The source wavefield can be fully reconstructed backwards in time if proper initial and boundary values are chosen. The wavefield on the boundaries and their neighbors can be stored during the forward modeling step, serving as the boundary values for the backward propagation. However, it still requires large amount storage of data. An alternative is approximately reconstructing the source wavefield only with the wavefield on the boundaries. Although it will lose spatial accuracy near the boundaries, the recovered wavefield in terms of reconstruction ratio is acceptable and the total amount of data storage is further reduced. Numerical experiments with both the constant-density acoustic equation and the pseudo-acoustic equation in TI media demonstrate that the recovered wavefield with this strategy would be acceptable for RTM.

3D Traveltime Computation From Rugged Topography In VTI/TTI Media

Summary A 3D dynamic programming approach to first-arrival traveltime computation is extended to anisotropic velocity models with rugged topography, which is necessary for 3D kirchhoff integral migration or near-surface tomography in the piedmont area in southwestern China. The traveltime computation method based on Fermat’s principle uses simple calculus techniques and a systematic mapping scheme to determine first-arrival times on a uniform grid, which has no limitation on large velocity contrast and space-varying anisotropic parameters. In order to get the minimum traveltime a set of nonlinear equations is solved on each grid point, where the wavefront velocity ( 1/ ( ) group S  ) is determined by group angle. This set of nonlinear equations can be expressed on the assumption that the wavefront velocity in VTI/TTI (vertical symmetry transversely isotropic/tilted symmetry transversely isotropic) media can be approximated by a truncated Fourier-type cosine series. The numerical example on VTI/TTI model demonstrates that the method is correct and effective.

Kirchhoff PSDM angle-gather generation based on the traveltime gradient

Angle-domain common-image gathers (ADCIGs) are the basic data in migration velocity analysis (MVA) and amplitude variation with angle (AVA) analysis. We propose a common-angle gather-generating scheme using Kirchhoff PSDM based on the traveltime gradient field. The scheme includes three major operations: (1) to calculate the traveltime field of the source and the receiver based on the dynamic programming approach; (2) to obtain the reflection angle according to the traveltime gradient field in the image space; and (3) to generate the ADCIGs during the migration process. Because of the computation approach, the method for generating ADCIGs is superior to conventional ray-based methods. We use the proposed ADCIGs generation method in 3D large-scale seismic data. The key points of the method are the following. (1) We use common-shot datasets for migration, (2) we load traveltimes based on the shot aperture, and (3) we use the MPI and OpenMp memory sharing to decrease the amount of input and output (I/O). Numerical examples using synthetic data suggest that the ADCIGs improve the quality of the velocity and the effectiveness of the 3D angle-gather generation scheme.

Application of sparse time-frequency decomposition to seismic data

The Gabor and S transforms are frequently used in time-frequency decomposition methods. Constrained by the uncertainty principle, both transforms produce low-resolution time-frequency decomposition results in the time and frequency domains. To improve the resolution of the time-frequency decomposition results, we use the instantaneous frequency distribution function (IFDF) to express the seismic signal. When the instantaneous frequencies of the nonstationary signal satisfy the requirements of the uncertainty principle, the support of IFDF is just the support of the amplitude ridges in the signal obtained using the short-time Fourier transform. Based on this feature, we propose a new iteration algorithm to achieve the sparse time-frequency decomposition of the signal. The iteration algorithm uses the support of the amplitude ridges of the residual signal obtained with the short-time Fourier transform to update the time-frequency components of the signal. The summation of the updated time-frequency components in each iteration is the result of the sparse time-frequency decomposition. Numerical examples show that the proposed method improves the resolution of the time-frequency decomposition results and the accuracy of the analysis of the nonstationary signal. We also use the proposed method to attenuate the ground roll of field seismic data with good results.

Frequency Grouping Coding Scheme For Blended Source Imaging

Summary The efficiency of common shot migration can be greatly enhanced by coding migration scheme. However, coding migration scheme introduces the crosstalk noise which can degrade the quality of migration images. In this paper, we present a matrix representation for both frequency dependent coding migration and frequency independent coding migration. A new coding scheme called frequency grouping coding scheme is presented by analyzing the matrix representation. This coding migration scheme makes use of the wavefield similarity of the adjacent frequency to suppress the crosstalk introduced by coding migration scheme. Conventionally, the crosstalk suppression is conducted on a single coding correlation matrix or a single frequency coding correlation matrix. However, this method uses the summation of multi coding correlation matrixes to suppress the corsstalk. This coding scheme could either be used as a single-experiment strategy or a multi-experiments strategy. Numerical tests prove the feasibility of this coding scheme.

Least square migration with Hessian in the local angle domain

Summary Analysis of directional character of Hessian operator leads to two new schem es for computing inverse Hessian in angle domain. The methods modify the original explicit Hessian formula into angle domain. The implementations avoid the huge inversion problems. Due to the need of stability, damping factors are used in the wavenumber domain. Tests on the SEG/EAGE salt model show good compensation results.

Target‐oriented efficient illumination imaging and acquisition design

Target-oriented efficient illumination imaging and acquisition design are investigated by combining controlled illumination and local plane wave decomposition. Based on the plane source synthesis and the local plane wave decomposition, the target-oriented illumination-controlled areal source is synthesized and analyzed, and ranges of efficient illumination sources and directions can be selected. Under the guidance of the range information, the targetoriented efficient illumination source imaging and acquisition design can be implemented. For the target layer of Marmousi model with complex overlaying structures, the above targetoriented efficient illumination source imaging method is applied, and the results show the feasibility and great potential of the method.

Frequency Domain Wave Equation Based Angular Hessian For Amplitude Correction

We present an angular Hessian decomposition method based on the wave equation in the frequency domain in order to do the amplitude correction for the prestack depth migration. By the explicit and fast wave equation seismic forward modeling using either the one-way wave-equation or the full-wave equation, we can get the Green’s function. After the local angle domain decomposition, we introduce the angular Hessian directly produced by the angular components of the Green’s function. This implementation avoids the huge inversion calculation on the space-domain exact Hessian in the least-squares migration. Dip-angle domain common image gathers are produced by this method. We apply the angular Hessian operator to the SEG/EAGE salt2d data set, where better subsalt images with higher resolution and dip-angle domain common image gathers are obtained. We also compare the angular Hessians using the Green’s function generated by the oneway wave equation method and the full-wave equation method using a simple salt model.

Target-oriented curved-wave prestack depth migration by controlled illumination

Conventional prestack depth migration (PSDM) based on full prestack data involves many computations for wavefield extrapolation. Areal shot-record technology offers an attractive alternative for efficient PSDM because its synthesis process greatly decreases the amount of prestack data required for migration. Constructing its synthesis operator is key to the image quality of the migrated areal shot record. Curved-wave PSDM technology expands areal shot-record migration. It constructs synthesis operators by defining a complex function as a base kernel of curved wavefields multiplied by a factor of its illumination perturbation related to ray parameter and then synthesizes curved wavefields for migrations. Based on curved-wave-migration theory, we propose an efficient and accurate curved-wave, controlled-illumination method to migrate in a target-oriented way. We construct tar-get-oriented synthesis operators by wave theory, combined with rotation or perturbation of the illuminating direction of a base-kerne...