Wave Equation Prestack Depth Migration Research Articles

Salt-related structure geologic model of Pre-Caspian Basin and analysis on imaging capability

The Pre-Caspian Basin is one of the most prolific petroliferous basins in Kazakhstan. However, the hydrocarbon reservoir is always located in salt dome and pre-salt structures. The salt-related structure is so complex that it is difficult to obtain the satisfied imaging results. In order to improve the imaging precision and provide authentic amplitude information to reservoir prediction, we should analyze the imaging and amplitude preserving capability between different migration methods. According to actual geologic and reservoir characteristics, we design a typical complex salt-related structure model of the basin. Based on the forward modelling seismic record, we compare the one-way wave-equation pre-stack depth migration (WPDM) and pre-stack reverse-time depth migration (RTM). The results show that RTM can greatly improve the imaging quality of salt dome, but its improvement for the pre-salt structures is small compared with WPDM. Both methods are equally capable of preserving seismic reflection characteristics that affected mainly by the lateral variation of overlying strata.

Analyze the Seismic Attributes in HSX Volcanic Fracture Based on Forward Modeling

This paper uses the Hassan Carboniferous fractured reservoirs as the goal, firstly it analyses the reservoir characteristics of the cracks. On this basis, it uses two-dimensional random fractured media modeling method to build three different fracture models with different fracture parameters (fracture density, dip and speed). Then it uses finite difference wave equation forward and pre-stack depth migration processing of these models, and analyses seismic attribute of the migrated data, finally it finds a variety of seismic attributes sensitive to cracks, lays the foundation for fracture prediction with the seismic multi-attributes.

Beyond receiver functions: Passive source reverse time migration and inverse scattering of converted waves

We present a wave equation prestack depth migration to image crust and mantle structures using multi‐component earthquake data recorded at dense seismograph arrays. TransmittedP and Swaves recorded on the surface are back propagated using an elastic wave equation solver. The wave modes are separated after the reverse‐time continuation of the wavefield from the surface, and subjected to a (cross‐correlation type) imaging condition forming an inverse scattering transform. Reverse time migration (RTM) does not make assumptions about the presence or properties of interfaces – notably, it does not assume that interfaces are (locally) horizontal. With synthetic experiments, and different background models, we show that passive source RTM can reconstruct dipping and vertically offset interfaces even in the presence of complex wave phenomena (such as caustics and point diffraction) and that its performance is superior to traditional receiver function analysis, e.g., common conversion point (CCP) stacking, in complex geological environments.

Recent applications of turning-ray tomography

Recent applications of 2D and 3D turning-ray tomography show that near-surface velocities are important for structural imaging and reservoir characterization. For structural imaging, we used turning-ray tomography to estimate the near-surface velocities for static corrections followed by prestack time migration and the near-surface velocities for prestack depth migration. Two-dimensional acoustic finite-difference modeling illustrates that wave-equation prestack depth migration is very sensitive to the near-surface velocities. Field data demonstrate that turning-ray tomography followed by prestack time migration helps to produce superior images in complex geologic settings. When the near-surface velocity model is integrated into a background velocity model for prestack depth migration, we find that wave propagation is very sensitive to the velocities immediately below the topography. For shallow-reservoir characterization, we developed and applied azimuthal turning-ray tomography to investigate observed apparent azimuthal-traveltime variations, using a wide-azimuth land seismic survey from a heavy-oil field at Surmont, Canada. We found that the apparent azimuthal velocity variations are not necessarily related to azimuthal anisotropy, or horizontal transverse isotropy (HTI), induced by the stress field or fractures. Near-surface heterogeneity and the acquisition footprint also could result in apparent azimuthal variations.

Velocity model building by 3D frequency-domain, full-waveform inversion of wide-aperture seismic data

We assessed 3D frequency-domain (FD) acoustic full-waveform inversion (FWI) data as a tool to develop high-resolution velocity models from low-frequency global-offset data. The inverse problem was posed as a classic least-squares optimization problem solved with a steepest-descent method. Inversion was applied to a few discrete frequencies, allowing management of a limited subset of the 3D data volume. The forward problem was solved with a finite-difference frequency-domain method based on a massively parallel direct solver, allowing efficient multiple-shot simulations. The inversion code was fully parallelized for distributed-memory platforms, taking advantage of a domain decomposition of the modeled wavefields performed by the direct solver. After validation on simple synthetic tests, FWI was applied to two targets (channel and thrust system) of the 3D SEG/EAGE overthrust model, corresponding to 3D domains of [Formula: see text] and [Formula: see text], respectively. The maximum inverted frequencies are 15 and [Formula: see text] for the two applications. A maximum of 30 dual-core biprocessor nodes with [Formula: see text] of shared memory per node were used for the second target. The main structures were imaged successfully at a resolution scale consistent with the inverted frequencies. Our study confirms the feasibility of 3D frequency-domain FWI of global-offset data on large distributed-memory platforms to develop high-resolution velocity models. These high-velocity models may provide accurate macromodels for wave-equation prestack depth migration.

A new approach to seismic interpretation in challenging imaging environments

The structural interpretation of prestack, depth-migrated seismic data has traditionally relied on full-offset stacks, attempting to take advantage of the stacking process to generate a high signal- to-noise ratio. However, in complex velocity environments (for example, subsalt), our observations suggest full-offset stacking can result in a lower signal-to-noise ratio than a partial stack of subsurface angle data. Despite the application of multipathing algorithms such as wave equation PSDM (prestack depth migration) or CRAM (common-reflection-angle migration), the full stack seismic data generally exhibit low signal-to-noise ratios, allowing for multiple valid structural interpretations. (Figure 1). This leads to a variety of common problems when assessing the prospect, well placement, trap/seal, volumes, and the differences in risking a four-way prospect from a three-way prospect.

Illumination Compensation for Wave Equation Migration Shadow

AbstractDue to the effects of complicated subsurface structures and seismic acquisition system, the seismic wave illumination to some subsurface targets may be inhomogeneous, so the seismic acquisition system can not effectively acquire the reflected information from these targets, and migrated images on these targets will have some migration shadows. Based on the windowed Fourier frame expansion of wavefield and Green function, an illumination compensation method for wave equation migration shadow is proposed by using the wave equation migration and illumination analysis in angle domain and combining the wave equation inverse theory. The compensation method can take into account the effects of the seismic acquisition system and wave propagation path, remove the migration shadow on the complicated subsurface structures, and improve the results of wave equation prestack depth migration in complex structure regions.

Explicit prestack depth migration in laterally varying VTI media

We present an efficient and stable method for wave equation prestack depth migration (PSDM) in laterally varying VTI media. The method accurately handles large lateral variations in the Thomsen ani...

3D wavefield extrapolation with optimal separable approximation

An accurate and wide-angle one-way propagator for wavefield extrapolation is an important topic for research on wave-equation prestack depth migration in the presence of large and rapid velocity variations. Based on the optimal separable approximation presented in this paper, the mixed domain algorithm with forward and inverse Fourier transforms is used to construct the 3D one-way wavefield extrapolation operator. This operator separates variables in the wavenumber and spatial domains. The phase shift operation is implemented in the wavenumber domain while the time delay for lateral velocity variation is corrected in the spatial domain. The impulse responses of the one-way wave operator show that the numeric computation is consistent with the theoretical value for each velocity, revealing that the operator constructed with the optimal separable approximation can be applied to lateral velocity variations for the case of small steps. Imaging results of the SEG/EAGE model and field data indicate that the new method can be used to image complex structure.

High Order Generalized Screen Propagator for Wave Equation Prestack Depth Migration

AbstractDifferent from the scattering theory used in the derivation of conventional generalized screen propagator, in this paper, a high order formula of generalized screen propagator for one‐way wave equation is proposed by using the asymptotic expansion of single‐square‐root operator. The high order generalized screen propagator improves not only the calculation precision of conventional generalized screen propagator, but also the suitability of generalized screen propagator to the media with strong lateral‐velocity variation. Applying the high order generalized screen propagator to wave equation prestack depth migration can get a better migration result than that of conventional generalized screen propagator. The higher the order number of generalized screen propagator, the higher the calculation precision, and the more the calculation amount. Using the wave equation prestack depth migration on SEG‐EAGE 2‐D salt dome model as an example, the second‐order generalized screen propagator increases 30% calculation work relative to the conventional (one‐order) generalized screen propagator. The presentation of high order generalized screen propagator is a theoretic development and improvement to the conventional generalized screen propagator.

AVO inversion based on common shot migration

Conventional AVO inversion utilizes the trace amplitudes of CMP gathers. There are three main factors affecting the accuracy of the inversion. First, CMP gathers are based on the hypothesis of horizontal layers but most real layers are not horizontal. Greater layer dip results in a greater difference between the observed CMP gathers and their real location. Second, conventional processing flows such as NMO, DMO, and deconvolution will distort amplitudes. Third, the formulation of reflection coefficient is related to incidence angles and it is difficult to get the relationship between amplitude and incidence angle. Wave equation prestack depth migration has the ability of imaging complex media and steeply dipping layers. It can reduce the errors of conventional processing and move amplitudes back to their real location. With true amplitude migration, common angle gathers abstraction, and AVO inversion, we suggest a method of AVO inversion from common shot gathers in order to reduce the effect of the above factors and improve the accuracy of AVO inversion.

Prestack wave-equation depth migration in VTI media

Wave-equation prestack depth migration (PSDM) naturally handles multi-arrivals, is not limited by the high frequency assumption of Kirchhoff-based PSDM and can, therefore, produce higher quality images in areas of complex geology. In addition, it has been shown that anisotropy, commonly encountered in petroleum exploration, affects image focusing and positioning. Therefore incorporating anisotropy in wave equation PSDM will produce a superior image!

Regularization of illumination in angle domains—a key to true amplitude migration

The turn of the millennium has seen the long expected affordability of 3D prestack depth migration based on wave-equation, or better said, band-limited algorithms. The success of wave-equation PreSDM (prestack depth migration) comes from the fulfillment of two, and only two, promises: correct handling of geometrical spreading and correct handling of multipathing, in complex media. Even then, the migration velocity model needs to be accurate enough to ensure the full effectiveness of this expectedly correct handling of wave propagation. How to get a complex though correct velocity model is still an open issue and is beyond our scope. There are nevertheless capabilities of Kirchhoff migration that wave-equation PreSDM cannot emulate yet, though progress is being made. On the imaging side, Kirchhoff has the well-known capability of imaging steep dips. Kirchhoff migration can also handle velocity models with various types of anisotropy. On the amplitude side, Kirchhoff migration offers better handling of irregular acquisition, better amplitude control through the theory of Beylkin, and better understanding of all illumination and regularization issues. This understanding is needed because the regularization of illumination is the key to a reduction of classic migration artifacts, to an improvement of the image quality, and to the reliability of the migration amplitudes for AVA and 4D processing.

Finite-difference calculation of traveltimes based on rectangular grid

To the most of velocity fields, the traveltimes of the first break that seismic waves propagate along rays can be computed on a 2-D or 3-D numerical grid by finite-difference extrapolation. Under ensuring accuracy, to improve calculating efficiency and adaptability, the calculation method of first-arrival traveltime of finite-difference is derived based on any rectangular grid and a local plane wavefront approximation. In addition, head waves and scattering waves are properly treated and shadow and caustic zones cannot be encountered, which appear in traditional ray-tracing. The testes of two simple models and the complex Marmousi model show that the method has higher accuracy and adaptability to complex structure with strong vertical and lateral velocity variation, and Kirchhoff prestack depth migration based on this method can basically achieve the position imaging effects of wave equation prestack depth migration in major structures and targets. Because of not taking account of the later arrivals energy, the effect of its amplitude preservation is worse than that by wave equation method, but its computing efficiency is higher than that by total Green’s function method and wave equation method.

The Wave‐Equation 3‐D Prestack Depth Migration of SEG/EAGE Salt and Overthrust Model

AbstractThe 3‐D wave equation prestack depth migration is one of the most important steps in the structure imaging and the inversion of elastic parameters of complex medium. It is very difficult to deal with for the implement of this technique relates not only to the innovation of the wavefield extrapolation theory but also large scale calculation. This paper is to get the result of the experiment and completely realize the 3‐D symplectic wave equation prestack depth migration of SEG/EAGE Salt and Overthrust model. We discuss in detail the imaging ability of our symplectic 3‐D prestack depth migration software system with complex geologic structures including the following topics: 1) The choice of imaging parameters , the effect on the migration result of the seismic wavelet and comparison between 2‐D and 3‐D prestack depth migration result of the Salt model. 2) The presentation of the impulse response of the Overthrust model. 3) The imaging result of the two international models indicates that our software system accommodate great transversal velocity variation and have ability of imaging complex geologic structures.

Methods for wave equation prestack depth migration and numerical experiments

In this paper the methods of wave theory based prestack depth migration and their implementation are studied. Using the splitting of wave operator, the wavefield extrapolation equations are deduced and the numerical schemes are presented. The numerical tests for SEG/EAEG model with MPI are performed on the PC-cluster. The numerical results show that the methods of single-shot (common-shot) migration and synthesized-shot migration are of practical values and can be applied to field data processing of 3D prestack depth migration.

Migration Velocity Modeling for Common Imaging Point Gathers by the Wave Equation Method

AbstractThe imaging result of prestack depth migration (Pre‐SDM) is quite sensitive to migration velocity field. Reasonable migration velocity modeling becomes the key to obtain the Pre–SDM imaging with high quality. Firstly, common imaging point (CIP) gathers are extracted by using wave equation Pre‐SDM with high imaging precision. Then, based on the perturbation method, the quantitative relationship between the errors in migration velocity and those in imaged depths is established through a parameterized velocity function and the modified residual curvature analysis (RCA). Finally, migration velocity modeling is realized by using single/multi parameter joint‐iteration inversion. The test on the Marmousi model shows that the method proposed in this paper has a stronger adaptability and better modeling and imaging results for a complex geological body, and it needs only to analyze and control the main reflectors through 3 to 4 iterations to meet the required precision.

PSDM 2, The Sequel

The first special section on prestack depth migration (PSDM) was published in May 2001. The papers provided a synopsis of the state of PSDM at that time. In fact, some issues limiting greater employment of PSDM described in that special section are still prevalent today—i.e., poor data quality, improper geologic models and velocity fields, inaccurate processing algorithms, anisotropy, near surface effects, lack of true amplitude preservation, and relatively high computing costs to run the full wave equation PSDM algorithms. However, the industry has since made great strides to resolve these issues because of the rapid growth in computing power and development of more efficient migration algorithms. It was due to these advances that TLE decided to revisit this depth imaging technology and quickly publish a sequel.