Multicomponent Migration Research Articles

2D anisotropic multicomponent Gaussian-beam migration under complex surface conditions

Elastic-wave migration in anisotropic media is a vital challenge, particularly for areas with irregular topography. Gaussian-beam migration (GBM) is an accurate and flexible depth migration technique, which is adaptable for imaging complex surface areas. It retains the dynamic features of the wavefield and overcomes the multivalued traveltimes and caustic problems of Kirchhoff migration. We have extended the GBM method to work for 2D anisotropic multicomponent migration under complex surface conditions. We use Gaussian beams to calculate the wavefield from irregular topography, and we use two schemes to derive the down-continued recorded wavefields. One is based on the local slant stack as in classic GBM, in which the PP- and PS-wave seismic records within the local region are directly decomposed into local plane-wave components from irregular topography. The other scheme does not perform the local slant stack. The Green’s function is calculated with a Gaussian beam summation emitted from the receiver point at the irregular surface. Using the crosscorrelation imaging condition and combining with the 2D anisotropic ray-tracing algorithm, we develop two 2D anisotropic multicomponent Gaussian-beam prestack depth migration (GB-PSDM) methods, i.e., using the slant stack and nonslant stack, for irregular topography. Numerical tests demonstrate that our anisotropic multicomponent GB-PSDM can accurately image subsurface structures under complex topography conditions.

A numerical study on chloride migration in cracked concrete using multi-component ionic transport models

This paper presents a numerical study on the mechanism of chloride migration in cracked concrete. Unlike most of existing work, this study utilises multi-component ionic transport models to reflect the influence of ionic interactions by coupling the mass conservation and Poisson’s equations. To provide a better understanding of the influences caused by multi-component migration and concrete cracking, two categories of geometry samples are displayed and discussed respectively. Finally, through a comparatively overall numerical exploration, which includes external voltage, ionic interactions, heterogeneous nature, binding effect and multiple cracks arrangement, a series of important transport features, which cannot be revealed previously from existing cracked concrete models, are highlighted.

Multiwave velocity analysis based on Gaussian beam prestack depth migration

Prestack depth migration of multicomponent seismic data improves the imaging accuracy of subsurface complex geological structures. An accurate velocity field is critical to accurate imaging. Gaussian beam migration was used to perform multicomponent migration velocity analysis of PP- and PS-waves. First, PP- and PS-wave Gaussian beam prestack depth migration algorithms that operate on common-offset gathers are presented to extract offset-domain common-image gathers of PP- and PS-waves. Second, based on the residual moveout equation, the migration velocity fields of P- and S-waves are updated. Depth matching is used to ensure that the depth of the target layers in the PP- and PS-wave migration profiles are consistent, and high-precision P- and S-wave velocities are obtained. Finally, synthetic and field seismic data suggest that the method can be used effectively in multiwave migration velocity analysis.

Implementation of elastic reverse-time migration using wavefield separation in the frequency domain

SUMMARY Considerable effort has been devoted to the migration of multicomponent data in elastic media with wavefield separation techniques being the most successful. Most of this work has been carried out in the time domain. In this paper, we formulate a multicomponent migration technique in the frequency domain. Reverse-time migration can be viewed as the zero-lag cross-correlation between virtual source and back-propagated wavefields. Cross-correlating the Helmholtz decomposed wavefields rather than directly correlating the vector displacement fields results in sharper, more interpretable images, contaminated by fewer crosstalk artefacts. The end products are separate P and S wave (and if desired, PS and SP) migration images. We test our migration algorithm on synthetic seismic data generated using the SEG/EAGE salt-dome, Overthrust and Marmousi-2 models. We correctly image the location and shape of the target zone for oil exploration using these data sets. Furthermore, we demonstrate that our new migration technique provides good images even when the initial velocity model is only approximate.

Multi-Component Migration in the Gas Phase of Soil: Comparison between Results of Experiments and Simulation by Dusty Gas Model

Fick's law has been used to simulate gas systems within soil, although this law can be applied only to binary gas systems and the effect of the Knudsen diffusion is not considered. By contrast, the dusty gas model can be applied to multi-component gas systems with Knudsen diffusion. Blanc's law is a simplified version of the dusty gas model, but it can be used only for gas systems in which the tracer gas is dilute. Although the dusty gas model is superior to other methods for simulating gas systems within soil, it is not generally used because of its complexity. Numerical techniques such as the Eulerian-Lagrangian method for solving the advection-dispersion equation can be used to simulate the migration of chemical substance in the water or gas phases of soil within the range of every Péclet number. We derived the compound diffusion coefficient and compound velocity from the dusty gas model and formulated the advection-diffusion equation with these values by using the characteristic finite element scheme. Results of the model developed here were consistent with the results of column experiments conducted in this study, and the precision of the developed model was verified.

Multicomponent prestack depth migration by scalar wavefield extrapolation

We present a new multicomponent prestack depth migration methodology based on successive application of conventional scalar wave equation migration. We do not separate the data into PP‐ and PS‐waves; rather, we migrate each x‐, y‐, and z‐component of the data using both P and S propagation velocities, followed by polarization filtering in the depth domain. By generating intermediate images in the depth domain, we can account for polarity reversals of the PS reflection for all dips. Since the polarization angles are calculated from the data, it is straightforward to accommodate anisotropic effects (quasi‐P and quasi‐S) into multicomponent migration.The multicomponent migration results in our synthetic examples demonstrate that even for a single shot gather, we can obtain clean PP‐ and PS‐wave images over complex structures and resolve the problem of PS‐wave polarity reversals.

Prestack migration using arrival angle information

When prestack depth migration is applied to field data, migration artifacts or false images resulting from limited observation geometry or aperture often make it difficult to interpret the migrated images. The occurrence of artifacts is especially a problem on VSP data that usually have poor geometry for migration. To reduce such unwanted artifacts in migration images, I propose here that information about incident angles or polarization directions of scattered waves at a receiving point obtained from multicomponent data be incorporated into the extrapolation of wavefields in migration. In this method, a given arrival of a wave from a subsurface scatterer is extrapolated in the direction focused on its incident direction at a receiving point to improve the migration image of the scatterer. This focusing is achieved by a projection of observed multicomponent scattered data onto the expected polarization direction at a receiving point for a subsurface scatterer. Numerical examples using simple 2-D and 3-D homogeneous models show that the multicomponent migration presented here can largely decrease migration artifacts or false images. Numerical experiments with more realistic models reveal that multicomponent migration can provide interpretable migration images even for very limited observation geometry such as a single source offset VSP, where single‐component migration fails. Additional tests indicate that this multicomponent migration method can be useful even in case where the observed data contain noise and a background velocity model deviates from the true velocity structure.

MULTICOMPONENT COMMON‐RECEIVER GATHER MIGRATION OF SINGLE‐LEVEL WALK‐AWAY SEISMIC PROFILES1

AbstractSeismic data are usually separated into P‐waves and S‐waves before being put through a scalar (acoustic) migration. The relationship between polarization and moveout is exploited to design filters that extract the desired wavetype. While these filters can always be applied to shot records, they can only be applied to a triaxial common‐receiver gather in special cases since the moveout of scattered energy on the receiver gather relates to path differences between the surface shots and the scatterer while the polarization is determined by the path from scatterer to downhole geophone. Without the ability to separate wavefields before migration, a ‘vector scalar’ or an elastic migration becomes a necessity.Here the propagation of the elastic wavefield for a given mode (e.g. P‐S) is approximated by two scalar (acoustic) propagation steps in a ‘vector scalar’ migration. ‘Vector’ in that multicomponent data is migrated and 'scalar’ in that each propagation step is based on a scalar wave equation for the appropriate mode. It is assumed that interaction between the wavefields occurs only once in the far‐field of both the source and receiver. Extraction of the P, SV and SH wavefields can be achieved within the depth migration (if one assumes isotropy in the neighbourhood of the downhole receiver) by a projection onto the polarization for the desired mode. Since the polarization of scattered energy is only a function of scatterer position and receiver position (and not source position), the projection may be taken outside the migration integral in the special case of the depth migration of a common‐receiver gather. The extraction of the desired mode is then performed for each depth migration bin after the separate scalar migration of each receiver gather component.This multicomponent migration of triaxial receiver gathers is conveniently implemented with a hybrid split‐step Fourier‐excitation‐time imaging condition depth migration. The raytracing to get the excitation‐time imaging condition also provides the expected polarization for the post‐migration projection. The same downward extrapolated wavefield can be used for both the P‐P and P‐S migrations, providing a flexible and efficient route to the migration of multicomponent data.The technique is illustrated on a synthetic example and a single‐level Walk‐away Seismic Profile (WSP) from the southern North Sea. The field data produced images showing a P‐P reflector below the geophone and localized P‐P and P‐S scatterers at the level of the geo‐phone. These scatterers, which lie outside the zone of specular illumination, are interpreted as faults in the base Zechstein/top Rotliegendes interface.