Pre-stack Migration Research Articles

Moderately Deep Seismic Reflection Imaging of the Ulleung Basin, East Sea using Post- and Pre-Stack Migration

국내 울릉분지의 탄화수소 자원 탐사 및 지질구조 기초연구 등의 목적으로 일련의 탄성파 탐사가 수행되었다. 성공적인 탄성파 해석을 위해 지하 구조를 정확하게 묘사하는 탄성파 단면이 필요하다. 울릉분지의 기존 연구사례에서 소개된 탄성파 단면들은 영상화되는 영역이 천부에 국한된 경우가 다수이다. 본 연구에서는 울릉분지의 지하구조를 5.0 s 하부까지 영상화하기 위하여 참 반사 보정을 수행하였다. 시간 영역 참 반사 보정을 적용하여 부정합면을 포함하는 심부 지하구조의 연속성이 향상됨을 확인하였다. 참 반사 보정 결과 퇴적층서와 기반암의 경계가 명확히 구분되었고 에돌이 잡음이 감소하였다. 겹쌓기-전 참 반사 보정단면은 반사 신호의 진폭이 강조되어 음향 기반암 매질 특성을 반영하였다. 연구 결과 도출된 시간 영역 참 반사 보정 탄성파 단면은 울릉분지 퇴적 발달양상을 연구하는데 도움이 될 것이다.

Advanced methods for seismic imaging when mapping hydrocarbon traps associated with salt domes

This paper is the first in a series of tutorials on the usage of modern migration methods for seismic imaging in areas of intense salt tectonics. In the paper, various types of hydrocarbon traps associated with salt domes in the Dnieper-Donets depression have been analyzed. Moreover, classification of methods for seismic imaging has been given. With examples from other sedimentary basins, the benefits of pre-stack migration, a consideration of multipathing of seismic energy and the usage of converted waves in imaging of near stock objects have been shown.

Prestack exploding reflector modelling and migration for anisotropic media

ABSTRACTThe double‐square‐root equation is commonly used to image data by downward continuation using one‐way depth extrapolation methods. A two‐way time extrapolation of the double‐square‐root‐derived phase operator allows for up and downgoing wavefields but suffers from an essential singularity for horizontally travelling waves. This singularity is also associated with an anisotropic version of the double‐square‐root extrapolator. Perturbation theory allows us to separate the isotropic contribution, as well as the singularity, from the anisotropic contribution to the operator. As a result, the anisotropic residual operator is free from such singularities and can be applied as a stand alone operator to correct for anisotropy. We can apply the residual anisotropy operator even if the original prestack wavefield was obtained using, for example, reverse‐time migration. The residual correction is also useful for anisotropic parameter estimation. Applications to synthetic data demonstrate the accuracy of the new prestack modelling and migration approach. It also proves useful in approximately imaging the Vertical Transverse Isotropic Marmousi model.

Source–receiver two‐way wave extrapolation for prestack exploding‐reflector modelling and migration

ABSTRACTMost modern seismic imaging methods separate input data into parts (shot gathers). We develop a formulation that is able to incorporate all available data at once while numerically propagating the recorded multidimensional wavefield forward or backward in time. This approach has the potential for generating accurate images free of artiefacts associated with conventional approaches. We derive novel high‐order partial differential equations in the source–receiver time domain. The fourth‐order nature of the extrapolation in time leads to four solutions, two of which correspond to the incoming and outgoing P‐waves and reduce to the zero‐offset exploding‐reflector solutions when the source coincides with the receiver. A challenge for implementing two‐way time extrapolation is an essential singularity for horizontally travelling waves. This singularity can be avoided by limiting the range of wavenumbers treated in a spectral‐based extrapolation. Using spectral methods based on the low‐rank approximation of the propagation symbol, we extrapolate only the desired solutions in an accurate and efficient manner with reduced dispersion artiefacts. Applications to synthetic data demonstrate the accuracy of the new prestack modelling and migration approach.

Pre-stack depth migration for improved imaging under seafloor canyons: 2D case study of Browse Basin, Australia

In the Browse Basin, as in many areas of the world, complex seafloor topography can cause problems with seismic imaging. This is related to complex ray paths, and sharp lateral changes in velocity. This paper compares ways in which 2D Kirchhoff imaging can be improved below seafloor canyons, using both time and depth domain processing.In the time domain, to improve on standard pre-stack time migration (PSTM) we apply removable seafloor static time shifts in order to reduce the push down effect under seafloor canyons before migration. This allows for better event continuity in the seismic imaging. However this approach does not fully solve the problem, still giving sub-optimal imaging, leaving amplitude shadows and structural distortion. Only depth domain processing with a migration algorithm that honours the paths of the seismic energy as well as a detailed velocity model can provide improved imaging under these seafloor canyons, and give confidence in the structural components of the exploration targets in this area. We therefore performed depth velocity model building followed by pre-stack depth migration (PSDM), the result of which provided a step change improvement in the imaging, and provided new insights into the area.

3D PS-wave imaging with elastic reverse-time migration

One of the important features of elastic reverse-time migration is that it is a depth domain prestack migration method based on a vectorial wavefield. PP- and PS-wave images can be easily generated by implementing an imaging condition to pure wave modes separated with divergence and curl operators in isotropic media. In the 3D case, however, the curl calculation will generate a 3C vector S-wave, which makes it impossible to generate a scalar PS image with the crosscorrelation imaging condition. Therefore, we first analyzed the polarity distribution of a 3D S-wave and concluded that the separated S-wave keeps perpendicular to a plane determined by the particle motion direction and the propagation direction of the S-wave. This specific plane is the raypath plane when the S-wave is excited by an incident P-wave. The normal direction of the raypath plane can be calculated by computing the vector product of the propagation directions of source and receiver wavefields. Then, we used this normal direction as the reference direction of the S-wave to scalarize the S-wave. Scalarization can be achieved by assigning a positive sign if the vector S-wave is equidirectional with the reference direction or a negative sign if the vector S-wave is contradirectional with the reference direction. Additionally, the scalar S-wave has the same absolute value as the vector S-wave. Next, the P-wave and the scalar S-wave can be applied to an imaging condition to generate a scalar PS image. Numerical examples with true velocity models and those in the presence of velocity error and data noise are presented to demonstrate the potential of the proposed method.

3D seismic processing and interpretation from 2D seismic data: Application in environmentally sensitive areas of the Neuquén Basin, Argentina

The increasing environmental and regulatory restrictions for recording 3D seismic, along with urban constraints, are limiting the acquisition of new 3D seismic information in some areas. A low-cost methodology can provide a pseudo-3D seismic data set. Geologic complexity is the most important condition to understand to assess the quality of the pseudo-3D volume created. These data can help to reduce many structural uncertainties that can be solved geometrically or at least outlined, giving real improvement in evaluation of prospective areas or in delineation of fields and generating new drilling strategies. In addition, the generated 3D data can be used as input for seismic-attribute calculation that otherwise would not have been possible because of discrete 2D information. The technique basically consists of conducting prestack migration using 24 offset planes, limiting with a structure control, and using the spatial propagation of the migration operator. The results of this data treatment show a homogeneous and consistent distribution of each processing attribute.

Converted-wave imaging technology and application for complex structures

Throughout the early years of the 21st century, multi-component seismic surveying has developed rapidly with larger capacity recording systems and digital three-component sensors. These technologies capture the seismic wavefield more completely than conventional P-wave surveys and allow extra information to be obtained from a converted-wave or ‘PS’ image. However, converted-wave seismic data is much more challenging to acquire and process than conventional P-wave data. Most existing multi-component surveys to date have been deployed in areas with geologically simple structures. This paper presents a case study of a 250 km2 3D three-component seismic survey located in an area with complex structures and faults. The complexity of the converted-wave data obtained in this survey has rarely been seen before, bringing huge technology challenges to the processing and interpretation. After several early failures, an extremely good image of the converted-wave has been obtained by combining a variety of processing algorithms. The structural features seen remain consistent with that of the P-wave image, providing a good foundation for subsequent inversion and reservoir analysis. This article describes the main techniques used, focusing in particular on three of the most challenging steps: converted-wave statics, anisotropy correction, and pre-stack time migration. We hope that this workflow will provide a technical guide, or reference for multi-component seismic data processing for complex structures.

Application of pre-stack reverse time migration based on FWI velocity estimation to ground penetrating radar data

Abstract Reverse-time migration (RTM) is used for subsurface imaging to handle complex velocity models including steeply dipping interfaces and dramatic lateral variations and promises better imaging results compared to traditional migration method such as Kirchhoff migration algorithm. RTM has been increasingly used in seismic surveys for hydrocarbon resource explorations. Based on the similarity of kinematics and dynamics between electromagnetic wave and elastic wave, we develop pre-stack RTM method and apply it to process ground penetrating radar (GPR) data. Finite-difference time domain (FDTD) numerical method is used to simulate the electromagnetic wave propagation including forward and backward extrapolations, the cross-correlation imaging condition is used to obtain the final image. In order to provide a velocity model with relatively higher accuracy as the initial velocity model for RTM, we apply a full waveform inversion (FWI) in time domain to estimate the subsurface velocity structure based on reflection radar data. For testing the effectiveness of the algorithm, we have constructed a complex geological model, common-offset radar data and common-shot profile (CSP) radar reflection data are synthesized. All data are migrated with traditional Kirchhoff migration method and pre-stack RTM method separately, the migration results from pre-stack RTM show better coincidence with the true model. Furthermore, we have performed a physical experiment in a sandbox where a polyvinyl chloride (PVC) box is buried in the sand, the obtained common-offset radar data and common-shot radar data are migrated by using Kirchhoff migration method and pre-stack RTM algorithm separately, the pre-stack RTM result shows that RTM algorithm could get better imaging results.

Data-driven wavefield focusing and imaging with multidimensional deconvolution: Numerical examples for reflection data with internal multiples

Standard imaging techniques rely on the single scattering assumption. This requires that the recorded data do not include internal multiples, i.e., waves that have bounced multiple times between reflectors before reaching the receivers at the acquisition surface. When multiple reflections are present in the data, standard imaging algorithms incorrectly image them as ghost reflectors. These artifacts can mislead interpreters in locating potential hydrocarbon reservoirs. Recently, we introduced a new approach for retrieving the Green’s function recorded at the acquisition surface due to a virtual source located at depth. We refer to this approach as data-driven wavefield focusing. Additionally, after applying source-receiver reciprocity, this approach allowed us to decompose the Green’s function at a virtual receiver at depth in its downgoing and upgoing components. These wavefields were then used to create a ghost-free image of the medium with either crosscorrelation or multidimensional deconvolution, presenting an advantage over standard prestack migration. We tested the robustness of our approach when an erroneous background velocity model is used to estimate the first-arriving waves, which are a required input for the data-driven wavefield focusing process. We tested the new method with a numerical example based on a modification of the Amoco model.

The possibilities of compressed-sensing-based Kirchhoff prestack migration

An approximate subsurface reflectivity distribution of the earth is usually obtained through the migration process. However, conventional migration algorithms, including those based on the least-squares approach, yield structure descriptions that are slightly smeared and of low resolution caused by the common migration artifacts due to limited aperture, coarse sampling, band-limited source, and low subsurface illumination. To alleviate this problem, we use the fact that minimizing the L1-norm of a signal promotes its sparsity. Thus, we formulated the Kirchhoff migration problem as a compressed-sensing (CS) basis pursuit denoise problem to solve for highly focused migrated images compared with those obtained by standard and least-squares migration algorithms. The results of various subsurface reflectivity models revealed that solutions computed using the CS based migration provide a more accurate subsurface reflectivity location and amplitude. We applied the CS algorithm to image synthetic data from a fault model using dense and sparse acquisition geometries. Our results suggest that the proposed approach may still provide highly resolved images with a relatively small number of measurements. We also evaluated the robustness of the basis pursuit denoise algorithm in the presence of Gaussian random observational noise and in the case of imaging the recorded data with inaccurate migration velocities.

Heat Flow Calculation Based on Seismic Data

Heat flow calculation is a reliable method to estimate the vibration about temperature, main factors of the existence of marine gas hydrates below seafloor. It would increase the accuracy of resources volume estimating and reduce cost of exploration significantly. Depth of Bottom Simulating Reflectors (BSRs), known as the base of gas hydrate stability zone (GHSZ), is a critical variable in this calculation. It should be recognized and mapped using the good quality three-dimensional (3D) pre-stack migration seismic data. By introducing heat flow derived from the depths of BSRs, this method would improve the resolution of the profiles and the quality of imaging and can be used in the specific areas.

ガボン共和国沖合Nguma鉱区へのExtended Elastic Impedanceの適用例

Since farming in to an offshore exploration block of Elf Gabon (presently Total Gabon) in 1974, MPDC Gabon Co., Ltd. (subsidiary of Mitsubishi Corporation) has been acting E&P business in Gabon. The Nguma permit is located in northern offshore Gabon approximately 65 km southwest from Port Gentil city and covers an area of 1,199 km2 (presently reduced to 600 km2).The turbidite sandstone in several geological periods is expected as possible reservoir in this area, and the main target is the Batanga sandstones in the Maastricht period. This facies is missing in the some wells and possibly to be distributed unevenly, and grasping Batanga sandstone distribution is one key point of this exploration.The distribution of reservoir sandstone was predicted using Extended Elastic Impedance (hereafter referred as EEI). EEI can approximate reflectivity or impedance to several elastic parameters (such as bulk modulus, Lamé parameter, rigidity modulus, and poison ratio) by a projection of acoustic impedance (AI) and gradient impedance (GI) data (AVO attributes). Gamma ray (GR) is the most suitable well log item to indicate sandstone reservoir and this prediction is first target for this study. However it is not tied to elastic modulus directly like as water saturation. Therefore it may not be necessarily derived from AVO attributes, and then the feasibility evaluation for applying EEI in this area was applied by taking empirical correlation.In data processing, EEI inversion with its proper (AVO friendly) pre-conditioning was applied on 3D seismic bin gather after pre-stack time migration. This EEI inversion derived pseudo GR and it has fine consistency with real GR at not only control well but also new well. In interpretation, turbidite sandstone distribution was predicted and delineated using EEI data.

Unexpected HTI velocity anisotropy: a wide-azimuth, low fold, 3D seismic processing case study

This case-study demonstrates seismic processing in the presence of Horizontal Transverse Isotropic (HTI) velocity anisotropy encountered in a low-fold land 3D survey in New Zealand. The HTI velocity anisotropy was unexpected, being suspected only after the initial poor stack response compared to vintage 2D sections in the area, and the sparse 3D design made it difficult to identify. The paper shows how anisotropy was singled out from other possible causes, such as geometry errors. We discuss the key steps of the processing flow incorporated to deal with the HTI anisotropy to attain a high quality final processed volume. In particular we show data examples after the application of azimuthally dependant NMO velocities, along with pre-stack HTI migration. Examples are shown which demonstrate the preservation of the HTI anisotropy before and after 5D trace interpolation. Maps and vertical profiles of 3D attributes are used to demonstrate the magnitude and direction of the HTI velocity field, which varies 5% to 10% between the fast and slow horizontal directions. These observations coincide with the local stress state deduced from borehole break-out studies. We conclude that the fast velocity direction corresponds to the present maximum horizontal stress direction. Finally the paper summarises the implications for processing wide azimuth 3D data in this area and suggests improvements for future 3D survey design. This paper was originally published in the Proceedings of the 23rd International Geophysical Conference and Exhibition, which was held from 11–14 August 2013 in Melbourne, Australia.

Corrigendum to “Acoustic VTI modeling and pre-stack reverse-time migration based on the time-space domain staggered-grid finite-difference method” [J. Appl. Geophys. 90 (2013) 41–52

Corrigendum to “Acoustic VTI modeling and pre-stack reverse-time migration based on the time-space domain staggered-grid finite-difference method” [J. Appl. Geophys. 90 (2013) 41–52

Data‐driven Green's function retrieval and application to imaging with multidimensional deconvolution

An iterative method is presented that allows one to retrieve the Green's function originating from a virtual source located inside a medium using reflection data measured only at the acquisition surface. In addition to the reflection response, an estimate of the travel times corresponding to the direct arrivals is required. However, no detailed information about the heterogeneities in the medium is needed. The iterative scheme generalizes the Marchenko equation for inverse scattering to the seismic reflection problem. To give insight in the mechanism of the iterative method, its steps for a simple layered medium are analyzed using physical arguments based on the stationary phase method. The retrieved Green's wavefield is shown to correctly contain the multiples due to the inhomogeneities present in the medium. Additionally, a variant of the iterative scheme enables decomposition of the retrieved wavefield into its downgoing and upgoing components. These wavefields then enable creation of a ghost‐free image of the medium with either cross correlation or multidimensional deconvolution, presenting an advantage over standard prestack migration.

Case study: Successful application of 3D depth processing in Eromanga Basin, Queensland

The Eromanga Basin is a large Mesozoic sedimentary basin in Central and Northern Australia. It covers an area of 1,000,000 km2 and is the location of important onshore petroleum and natural gas deposits in Australia including the nation's largest onshore oilfield at Jackson. We performed a pre-stack time migration and later on a pre-stack depth migration sequence on a 300sqkm block in the basin with the objective to map out intricate formations for detection of areas with high potential for gas hydrocarbon. The basin contains an ancient meteorite crater (astrobleme) which distorts seismic images, and is made of sandstone, siltstone, mudstone, coal and shale. The oil and gas producing interval is located around 1,200m below the surface. Vibroseis seismic data was processed through an amplitude-preserved and surface-consistent sequence followed by offset-class regularisation and pre-stack migration. Refraction and residual statics were applied to correct for near surface time distortions. The time migrated data illustrates high signal-to-noise ratio with generally flat rock beddings with some noticeably complicated formations at known horizons, forming strong velocity boundaries. These formation oddities generally appear from Cadna-Owie ( C ) Formation and deeper; the Top C horizon is of significance for exploration as the top of the unit approximates a distinctive seismic reflector and is mappable over the entire basin. During depth-velocity modelling it was revealed that there are significant velocity variations (~500m/s) at Top C and other boundaries. Time imaging was not able to image these intervals properly. Depth migration improved seismic image quality especially in fault shadow zones, and areas with complex seismo-geological settings. The benefits of depth imaging on the Eromanga Basin data were demonstrated by significant uplift in structural conformity and increased confidence in interpretation results.

CRS stack based seismic imaging - a case study from St Ives mining camp, Kambalda, Western Australia

Seismic exploration in hard rock environments is challenging due to complex geological conditions that reduce the signal-to-noise (S/N) ratio of seismic data and, thus, an accurate velocity model recovery likelihood. This limits the application of conventional imaging methods, e.g., normal moveout (NMO), dip moveout (DMO) stacking or pre-stack migration. The common reflection surface (CRS) stacking technique can be used in complex environments to increase the fold and therefore the signal quality and produces reliable stacked sections with high resolution. On the other hand, CRS stacking approach does not depend on a velocity model. The CRS stacking approach was applied to 2D and 3D datasets acquired across the St Ives mining camp in Kambalda, Western Australia located within the core of a regional-scale Kambalda Dome. The main objective of this study is to review the application of the CRS approach at the St Ives mining camp in Kambalda, Western Australia. Stacked 2D seismic sections and a 3D seismic volume obtained using the CRS approach are superior to those obtained by conventional DMO/NMO processing. They are characterized by a higher S/N ratio and improved continuity of seismic reflection events. Parameters (wavefield attributes) estimated using the CRS approach have a clear geophysical interpretation and will be used for building velocity models. The target area and the existing faults and fractures were imaged clearly and the high grade of tectonic displacement necessary to ensure a sufficiently large production rate was verified. The CRS approach is now adopted as a part of the standard processing flow for hard rock seismic.

Unexpected HTI velocity anisotropy: a wide-azimuth, low fold, 3D seismic processing case study

This case-study demonstrates seismic processing in the presence of Horizontal Transverse Isotropic (HTI) velocity anisotropy encountered in a low-fold land 3D survey in New Zealand. The HTI velocity anisotropy was unexpected, being suspected only after the initial poor stack response compared to vintage 2D sections in the area, and the sparse 3D design made it difficult to identify. The paper shows how anisotropy was singled out from other possible causes, such as geometry errors. We discuss the key steps of the processing flow incorporated to deal with the HTI anisotropy to attain a high quality final processed volume. In particular we show data examples after the application of azimuthally dependant NMO velocities, along with pre-stack HTI migration. Examples are shown which demonstrate the preservation of the HTI anisotropy before and after 5D trace interpolation. Maps and vertical profiles of 3D attributes are used to demonstrate the magnitude and direction of the HTI velocity field, which varies 5% to 10% between the fast and slow horizontal directions. These observations coincide with the local stress state deduced from borehole break-out studies. We conclude that the fast velocity direction corresponds to the present maximum horizontal stress direction. Finally the paper summarises the implications for processing wide azimuth 3D data in this area and suggests improvements for future 3D survey design. Key words: Anisotropy, HTI, 3D Seismic, Stress.

A comparative overview of velocity-independent imaging’s methods

SUMMARY Relatively poor performance of full pre-stack migration in hard rock seismic exploration is related to our inability to produce accurate velocity field which, is caused by complicated structures, highly variable reflectivity, and low signal to noise ratio. That is why the estimation of velocities becomes a significant problem in hard-rock environment. Consequently we wish to use imaging approaches which are loosely dependent or completely independent of the knowledge of rock velocities. To get there, we review and analyse different velocityindependent imaging techniques developed over nearly past three decades. However, seismic imaging in complicated geological conditions often requires more than just one parameter such as accurate velocity field. Improvements of signal to noise ratio and data regularisation are typically necessary to enable construction of high resolution, high quality images. In such data, pre-conditioning should enable enhanced performance of velocity-less imaging techniques since their performance is critically dependent on input S/N ratio. The final approach, then, involves merging several techniques such are denoising, wavefield reconstruction/interpolation and velocityindependent tools together to improve image quality in hard-rock environment.