Kirchhoff Prestack Time Migration Research Articles

Velocity analysis of simultaneous-source data using high-resolution semblance—coping with the strong noise

Direct imaging of simultaneous-source (or blended) data, without the need of deblending, requires a precise subsurface velocity model. In this paper, we focus on the velocity analysis of simultaneous-source data using the normal moveout-based velocity picking approach.We demonstrate that it is possible to obtain a precise velocity model directly from the blended data in the common-midpoint domain. The similarity-weighted semblance can help us obtain much better velocity spectrum with higher resolution and higher reliability compared with the traditional semblance. The similarity-weighted semblance enforces an inherent noise attenuation solely in the semblance calculation stage, thus it is not sensitive to the intense interference. We use both simulated synthetic and field data examples to demonstrate the performance of the similarity-weighted semblance in obtaining reliable subsurface velocity model for direct migration of simultaneous-source data. The migrated image of blended field data using prestack Kirchhoff time migration approach based on the picked velocity from the similarity-weighted semblance is very close to the migrated image of unblended data.

GPU acceleration of amplitude-preserved Q compensation prestack time migration

Amplitude-preserved Q compensation prestack time migration (Q migration) is a new method that evolved from prestack Kirchhoff time migration (PKTM). Five algorithms are developed for Q migration on graphics processing units (GPUs). First, the principle of Q migration is briefly introduced. Second, one parallel strategy, namely, imaging domain parallel strategy, is proposed to accelerate Q migration on a single GPU by developing GPU algorithm. Results show that the imaging domain parallel strategy with the corresponding algorithm is superior to the CPU algorithm in several aspects, i.e., faster computing speed, shorter computing time, and higher computational efficiency. Third, based on the imaging domain parallel strategy, two division methods, namely, seismic data division method and velocity data division method, are presented to optimize the performance of Q migration on multi-GPUs and four algorithms are implemented by using Message Passing Interface (MPI)+Compute Unified Device Architecture (CUDA) and multi-thread+CUDA. An optimal algorithm is determined by comparing the performance of four algorithms. Results demonstrate that the optimal algorithm has the shortest computing time, which is 3.85 times shorter than that of a single GPU when four GPUs are all involved in computation and 300 times shorter than that of a 4-core central processing unit (CPU). Finally, a parallel computing framework on GPU cluster is established, which consists of imaging domain parallel strategy, seismic data division method and MPI+CUDA. This framework is suitable for all prestack time migration (PSTM) methods and has a short computing time and high speedup ratio on GPU cluster.

Seismic imaging of subsurface structure using tomographic migration velocity analysis: a case study of South China Sea data

Seismic migration algorithms are often used to reveal subsurface structures and can be roughly divided into stacking methods, pre-stack time migration methods and pre-stack depth migration methods. In theory, pre-stack depth migration methods can provide the most accurate subsurface images. However, these methods are sensitive to the accuracy of the migration velocity model, and thus the creation of a precise migration velocity model is one of the most important steps in seismic processing. We applied the tomographic travel-time inversion and the method of pre-stack Kirchhoff depth migration-based migration velocity analysis (MVA) to certain South China Sea data, in which we iteratively inverted the precise, high-resolution migration velocity model. We selected travel-time differences from the offset-domain common image gathers of the pre-stack Kirchhoff depth migration and inverted the travel-time differences for velocity updating. After several iterations, both the precise migration velocity model and the corresponding pre-stack depth migration image were obtained. The comparisons with Normal Move-out stacking and pre-stack Kirchhoff time migration confirm that the tomographic MVA and corresponding pre-stack Kirchhoff depth migration provide the best image quality and highest-resolution subsurface images from the South China Sea data.

Prestack time imaging algorithm with simultaneous velocity estimation in hard rock environments

Reflection seismic imaging faces several difficulties in hard rock environments. One of them is the estimation of the propagation velocity of seismic waves. Therefore, imaging algorithms that do not require prior construction of a velocity model seem promising for such environments. In this paper we illustrate an application of prestack time migration, which does not require an input velocity model, to hard rock conditions, and we demonstrate its effectiveness on synthetic data. This approach is based on an estimation of local event slopes (horizontal slownesses) in common-shot and common-receiver gathers and a subsequent calculation of the migration attributes (migration velocity, vertical traveltime and horizontal coordinates of the migrated reflection point). These attributes allow us to derive all the information needed to construct a time-migrated image. We also use the obtained migration velocities as an input velocity model for Kirchhoff prestack time migration (PSTM) and compare the results of the proposed approach with a conventional Kirchhoff migration using as an input the picked NMO velocity model. This application to a hard rock synthetic model illustrates the potential of the presented migration algorithm for imaging in hard rock seismic exploration. We believe that this approach can be used in hard rock seismic processing workflows as an automatic tool to obtain an input velocity model for the Kirchhoff PSTM.

A joint high-resolution processing method and its application for thin inter-beds

Seismic processing characterizing thickness and borders of thin inter-beds has gradually evolved from post-stack migration to pre-stack migration, and the latter considers both vertical and lateral resolutions. As the key processing methods for improving vertical and lateral resolution, conventional deconvolution and pre-stack time migration (PSTM) are not simply dominated by the estimation and compression of the wavelet because of its instability. Therefore, considering the variations of wavelet frequency before, during and after PSTM can obtain good common reflection point (CRP) gathers and imaging profiles of thin inter-beds. Based on the frequency characteristics of the wavelet before, during and after PSTM, a joint high-resolution processing method for thin inter-beds is proposed in this paper, including inverse Q filtering for high-frequency compensation before PSTM, optimum weighting Kirchhoff PSTM for preserving high-frequencies during PSTM, and wavelet harmonizer deconvolution for consistent processing and frequency-band broadening after PSTM. An application to real data characterized by mudstone beds in the Oriente Basin proved that the joint high-resolution processing method is effective for determining the thickness and borders of thin inter-beds and is favorable for subsequent reservoir prediction and seismic inversions.

Azimuth-preserved local angle-domain prestack time migration in isotropic, vertical transversely isotropic and azimuthally anisotropic media

Conventional prestack migration does not preserve local directional information of the seismic waves at the image points. New attempts such as sectored migration of azimuth-limited or common-offset-vector data only concern source-receiver azimuth and offset on the surface, which can be poor representation of subsurface wavepath direction. Moreover, they could result in inaccurate imaging because they do not account for the energy propagation between azimuths or offset-vectors. In the past decade, local angle-domain seismic imaging has been highly advocated to avoid migration artifacts and to improve velocity estimation in complex media. Considering prestack time migration (PSTM) is still widely used in seismic imaging and seismic data preconditioning for amplitude variations with offset or incident-angle (AVO/AVA) analysis, fracture detection, and reservoir characterization, we present an azimuth-preserved local angle-domain Kirchhoff PSTM approach for such purposes. We apply a seismic imaging condition in 3D local angle domain and use extended superposition of impulse responses retaining subsurface angular attributes, which are evaluated through the incident and scattering phase slowness vectors using classical-diffraction moveout equations in isotropic, vertical transversely isotropic (VTI) and azimuthally anisotropic media. Two-dimensional synthetic examples demonstrate what the migrated results look like in local angle domain. A wide-azimuth synthetic example with horizontal transversely isotropy (HTI) proves the necessity of azimuthal migration for reliable imaging and azimuthal analysis when azimuthal anisotropy exists in the overburden. Real data examples show the advantages of imaging in subsurface angle domain for properly focusing and revealing azimuth- and angle-dependent variations of residual moveout and migrated amplitudes.

The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin, China

Conventional seismic exploration method based on post-stack data usually fails to identify the distribution of fractured and caved carbonate reservoirs in the Tarim Basin, so the rich pre-stack information should be applied to the prediction of carbonate reservoirs. Amplitude-preserved seismic data processing is the foundation. In this paper, according to the feature of desert seismic data (including weak reflection, fast attenuation of high frequency components, strong coherent noises, low S/N and resolution), a set of amplitude-preserved processing techniques is applied and a reasonable processing flow is formed to obtain the high quality data. After implementing a set of pre-stack amplitude-preserved processing, we test and define the kernel parameters of amplitude-preserved Kirchhoff PSTM (pre-stack time migration) and subsequent gathers processing, in order to obtain the amplitude-preserved gathers used to the isotropic pre-stack inversion for the identification of caved reservoirs. The AVO characteristics of obtained gathers fit well with the synthetic gathers from logging data, and it proves that the processing above is amplitude-preserved. The azimuthal processing techniques, including azimuth division and binning enlargement, are implemented for amplitude-preserved azimuthal gathers with the uniform fold. They can be used in the anisotropic inversion to detect effective fractures. The processing techniques and flows are applied to the field seismic data, and are proved available for providing the amplitude-preserved gathers for carbonate reservoir prediction in the Tarim Basin.

Accelerating Time and Depth Seismic Migration by CPU and GPU Cooperation

In this paper, we discuss the porting strategy and the achieved performance of Petrobras production code that implements Kirchhoff prestack time and depth seismic migration algorithms on a cluster of 64 GPUs and 256 CPU cores. The time migration of a real-world survey was 20 times faster in the full cluster with GPUs than in the full cluster without GPUs. The introduction of GPUs accelerated the depth migration of another real-world survey by a factor of 5. In both cases, a large fraction of the achieved gains were due to the porting strategy that privileges CPU-GPU cooperation instead of pure GPU speed. Another strategy that contributed to the high gains was GPU overloading by multiple MPI processes. To cope with the heterogeneous load of real-world data, we developed a dynamic load balancing scheme that significantly speeds up computation.

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-pathazimuth, 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.

Prestack Kirchhoff time migration of 3D coal seismic data from mining zones

Conventional seismic data processing methods based on post-stack time migration have been playing an important role in coal exploration for decades. However, poststack time migration processing often results in low-quality images in complex geological environments. In order to obtain high-quality images, we present a strategy that applies the Kirchhoff prestack time migration (PSTM) method to coal seismic data. In this paper, we describe the implementation of Kirchhoff PSTM to a 3D coal seam. Meanwhile we derive the workflow of 3D Kirchhoff PSTM processing based on coal seismic data. The processing sequence of 3D Kirchhoff PSTM includes two major steps: 1) the estimation of the 3D root-mean-square (RMS) velocity field; 2) Kirchhoff prestack time migration processing. During the construction of a 3D velocity model, dip moveout velocity is served as an initial migration velocity field. We combine 3D Kirchhoff PSTM with the continuous adjustment of a 3D RMS velocity field by the criteria of flattened common reflection point gathers. In comparison with post-stack time migration, the application of 3D Kirchhoff PSTM to coal seismic data produces better images of the coal seam reflections.

Computing prestack Kirchhoff time migration on general purpose GPU

This paper introduces how to optimize a practical prestack Kirchhoff time migration program by the Compute Unified Device Architecture (CUDA) on a general purpose GPU (GPGPU). A few useful optimization methods on GPGPU are demonstrated, such as how to increase the kernel thread numbers on GPU cores, and how to utilize the memory streams to overlap GPU kernel execution time, etc. The floating-point errors on CUDA and NVidia's GPUs are discussed in detail. Some effective methods that can be used to reduce the floating-point errors are introduced. The images generated by the practical prestack Kirchhoff time migration programs for the same real-world seismic data inputs on CPU and GPU are demonstrated. The final GPGPU approach on NVidia GTX 260 is more than 17 times faster than its original CPU version on Intel's P4 3.0G.

Method of Prestack Time Migration of Seismic Data of Mountainous Regions and Its GPU Implementation

AbstractThe geological structure in the mountainous regions of western China is quite complicated, and the elevation varies violently. At present, the main approach for seismic data imaging is the prestack Kirchhoff time migration, however, violent variation of surface elevation makes the datum plane selection of prestack time migration very difficult. This article proposed a new prestack time migration method through modifying the routine travel time computation, this method can greatly raise the imaging effect in mountainous region with obvious variation of surface elevation. In addition, this paper also introduced application of GPU in prestack time migration, by which the prestack time migration has been optimized and implemented, and the computation efficiency of single NVIDIA Tesla C1060 GPU can be more than 70 times faster than single core CPU with main frequency of 2.5 GHZ when without the I/O problem.

Effect of errors in the migration velocity model of PS-converted waves on traveltime accuracy in prestack Kirchhoff time migration in weak anisotropic media

We investigated the effect of errors in the migration-velocity model of PS-converted waves on the traveltime calculated in prestack Kirchhoff time migration in weak anisotropic media. The prestack-Kirchhoff-time-migration operator contains four parameters: the PS-converted-wave velocity, the vertical velocity ratio, the effective velocity ratio, and the anisotropic parameter. We derived four error factors that correspond to those parameters. Theoretical and numerical analyses of the error factors show them all to be inversely proportional to the velocity and to traveltime. Traveltime errors for shallow events usually are larger than for deep events. Error in PS-converted-wave velocity causes the largest traveltime error, and error in the vertical velocity ratio causes the smallest traveltime error. For a small horizontal-distance/depth ratio, the error in the effective velocity ratio affects traveltime more than does the anisotropic-parameter error. However, the anisotropic-parameter error affects traveltime more when the horizontal-distance/depth ratio is larger. Traveltime errors caused by errors in effective velocity ratio and the anisotropic parameter mainly stem from the converted-S-wave raypath of the PS-converted waves. To save processing time and cost, PS-wave velocity can be estimated accurately without an accurate vertical velocity ratio, effective velocity ratio, and anisotropic parameter. These findings are useful for understanding PS-wave behavior and for PS-wave imaging in anisotropic media.

Velocity model updating in prestack Kirchhoff time migration for PS converted waves: Part II – Application

ABSTRACTWe have developed a practical approach for updating the velocity of PS converted waves based on the inverse normal‐moveout common‐image‐point gather obtained from prestack Kirchhoff time migration. We have integrated all the steps involved in updating the migration velocity model into an interactive tool and have applied this approach to a real seismic data set from the Alba Field in the North Sea. Based on experience in handling the real data, we discuss various practical aspects of updating the velocity model, including: what kind of initial velocity model should be used; which parameters in the velocity model should be updated; and how to update them. Application of prestack Kirchhoff time migration to the data set using the updated velocity model produces an improved image of the Alba Field.

Velocity model updating in prestack Kirchhoff time migration for PS converted waves: Part I – Theory

ABSTRACTA velocity model updating approach is developed based on moveout analysis of the diffraction curve of PS converted waves in prestack Kirchhoff time migration. The diffraction curve can be expressed as a product of two factors: one factor depending on the PS converted‐wave velocity only, and the other factor depending on all parameters. The velocity‐dependent factor represents the hyperbolic behaviour of the moveout and the other is a scale factor that represents the non‐hyperbolic behaviour of the moveout. This non‐hyperbolic behaviour of the moveout can be corrected in prestack Kirchhoff time migration to form an inverse normal‐moveout common‐image‐point gather in which only the hyperbolic moveout is retained. This hyperbolic moveout is the moveout that would be obtained in an isotropic equivalent medium. A hyperbolic velocity is then estimated from this gather by applying hyperbolic moveout analysis. Theoretical analysis shows that for any given initial velocity, the estimated hyperbolic velocity converges by an iterative procedure to the optimal velocity if the velocity ratio is optimal or to a value closer to the optimal velocity if the velocity ratio is not optimal. The velocity ratio (VP/VS) has little effect on the estimation of the velocity. Applying this technique to a synthetic seismic data set confirms the theoretical findings. This work provides a practical method to obtain the velocity model for prestack Kirchhoff time migration.

Imaging beneath gas clouds using 3D prestack Kirchhoff time migration of PS-converted waves—A case study from the North Sea

We present a practical approach to imaging structures beneath gas clouds using PS-converted waves. In this approach, we developed methods to build the migration velocity model and to perform 3D prestack Kirchhoff time migration on a PC cluster and improved the processing flow for PS-converted waves. The technique was successfully applied to a 3D/4-C marine data set acquired in August 2001 and made available to us by Kerr-McGee North Sea (UK).

Multicomponent OBC (4C) imaging over Pamberi, offshore Trinidad

Results from the P-P and P-Sv prestack time migration of a 2.5D four-component (4C) OBC seismic swath test acquired in 2004 over the Pamberi area in Block Lower Reverse L offshore Trinidad, are presented. The 4C swath acquisition consisted of twelve 100-m spaced source lines, shot into two parallel 15-km OBC receiver lines, separated by 400 m. A description of the processing applied to the recorded multicomponent (4C) seismic data, through curved-ray anisotropic Kirchhoff prestack time migration for both the compressional P-wave and mode-converted PSv-wave, is provided. Mode-converted shear-wave data acquired from 4C surveys allow for imaging where conventional seismic data are perturbed due to the presence of shallow gas and/or fluid in the pore spaces of the rock. Furthermore, mode-converted shear waves propagate with a different raypath than that of the compressional wave, thereby providing an alternative illumination of the subsurface target. As both the compressional P-wave and mode-converted S-wave record independent measurements of the same subsurface, more reliable rock properties can be uniquely determined, allowing for improved reservoir characterization and lithology prediction. A conventional streamer 3D survey was acquired over the Pamberi area in 2003, but failed to image or resolve adequately the target reflectors comprising the reservoir under the main growth fault. Therefore, the purpose of the 4C swath test was to evaluate the potential of long-offset multicomponent technology for resolving stratigraphic interpretation in complex areas with challenging geology. The processed images from both the compressional wave and mode converted shear waves are shown to illustrate the improvement in resolution at the reservoir prospect level which were not clearly evident on the previous conventional towed streamer 3D data volume.

Pre-stack time migration and velocity analysis methods with common scatter-point gathers

The pre-stack time migration and velocity analysis methods on common scatter-point (CSP) gathers are based on pre-stack Kirchhoff time migration. The concept of equivalent offset provides a new idea for the research of seismic pre-stack migration and velocity estimation. In this paper, according to the theory of the equivalent offset, we derive the travel-time equation of CSP gathers, introduce the related pre-stack time migration method and discuss velocity analysis methods based on the CSP gathers, and point out the advantages of CSP methods over conventional methods. Processing of a model example and field seismic data show the validity of CSP pre-stack time migration and the velocity analysis methods based on it.

Effect of regularization in the migration of time-lapse data

With 4D seismic studies the aim is to look at signal changes in the reservoir which can improve our understanding of the dynamic reservoir system and enable us to optimize production. For a successful study, noise must be minimized as the magnitude of the signal related to the 4D effect is generally weak and the same order of magnitude as the noise. There are many sources of non-repeatability in marine 4D seismic studies from differences in acquisition parameters, differences in environmental factors, and also from data processing effects. When we compare time-lapse datasets, this nonrepeatable noise will remain and impede the interpretation of the data. Pre-stack Kirchhoff Time Migration (PreSTM) is a processing technique utilized in seismic exploration and reservoir monitoring. Such a migration scheme is attractive as it allows the implementation of spatially and temporally varying velocity fields to migrate the data. It is only suitable for datasets with smooth velocity contrasts as any sharp velocity changes (e.g., shallow surface heterogeneities) would leave residual energy visible as a migration smile. Even with smooth velocity variations, another form of migration noise comes from the sub-optimal interference of the migration operator. As the PreSTM implementation uses a symmetrical operator (not the case for Pre-stack Depth Migration), the best possible constructive and destructive interference requires square bins with the midpoint position of the traces at the bin centre. In addition, the traces in a single offset volume should be mono-azimuth (or contain an equally distributed azimuth content for each bin) and mono-offset. When this is not the case residual migration noise will remain as an imprint of the acquisition pattern. In a 4D study where each dataset has slightly different acquisition parameters, each vintage will also have its own pattern of migration noise. This noise is uncorrelated between vintages and appears as energy on the difference section. The use of weighting schemes inside the PreSTM algorithm can help reduce noise by normalizing the migration output. However, regularizing the data on input optimizes the constructive and destructive interference, this handles amplitudes on seismic events as well as attenuating migration smiles. In this paper we consider the level of migration noise from sub-optimal recording positions in a constant velocity medium for a Kirchhoff PreSTM algorithm. In addition, we outline some processing techniques to reduce the level of migration noise.

The effects of migration velocity errors on traveltime accuracy in prestack Kirchhoff time migration and the image of PS converted waves

We analyze prestack PS migration images and their focusing sensitivity to errors in the computed PS traveltimes. The key analysis tool is a formula that defines PS traveltimes errors as explicit functions of velocity model errors. The most important factors in this formula are the PS velocity and the P-to-S velocity ratio. Analysis shows that the error in PS traveltime for shallow events is usually larger than that for deep events for a given error in the velocity model. Also the PS traveltime is affected more severely by errors in the PS-velocity model than in the P-to-S velocity ratio. The effect of traveltime errors increases with dip angle of reflectors. Numerical analysis shows that, for a fixed scatterpoint, the effect of the PS-wave velocity error is several times larger than the effect of the error in the P-to-S velocity ratio. Examples from field data show that the PS-wave velocity must be estimated accurately with errors less than 1% in order perfectly flatten the events in common-image-point (CIP) gathers. In contrast, an error in the PS-velocity ratio of up to several percent is allowed. This suggests that for acceptable PS-wave migration, only the PS-velocity model and a rough estimate of the P-to-S velocity ratio is needed. This finding is useful for processing PS-wave data because it is difficult and time consuming to estimate the velocity ratio accurately from the real data. This finding is also useful for our understanding of PS-wave behavior and for PS-wave imaging.