Prestack Depth Migration Method Research Articles

Thermohaline structures across the Luzon Strait from seismic reflection data

A legacy seismic section across the Luzon Strait was reprocessed using the pre-stack depth migration method to reveal thermohaline structures in the water column. Distinct finescale and mesoscale features can be seen from the seismic image. Vertically, reflective patterns are associated with three water layers: upper layer, intermediate layer, and deep layer. The upper layer can be divided into three areas by two opposing weak-to-transparent zones from west to east with individual thicknesses: ∼400, 550, and 700 m. They are interpreted as the South China Sea upper water, the Kuroshio water, and the Pacific upper water, respectively, separated by two Kuroshio frontal zones. Internal waves are ubiquitous in the Kuroshio water fluctuating in different amplitudes and wavelengths laterally. The finestructure shows the western Kuroshio front zone is composed of three subparallel blanking zones with an average eastward dipping angle of 2.3°. They are regarded as the well-mixed frontal interfaces developed in and along a multi-frontal frontal zone. The transparent reflection of the intermediate water above the Hengchun is suggestive of a well-mixed layer by the near-bottom turbulence. In the Pacific intermediate water near the eastern mouth of the Bashi Channel, there is an inverted crescent-shaped structure, which is likely associated with the intrusion of the South China Sea intermediate water through the Bashi Channel. These imaged features are consistent with the observed oceanographic phenomena in the region.

Kinematics of shot-geophone migration

Recent analysis and synthetic examples have shown that many prestack depth migration methods produce nonflat image gathers containing spurious events, even when provided with a kinematically correct migration velocity field, if this velocity field is highly refractive. This pathology occurs in all migration methods that produce partial images as independent migrations of data bins. Shot-geophone prestack depth migration is an exception to this pattern: each point in the prestack image volume depends explicitly on all traces within the migration aperture. Using a ray-theoretical analysis, we have found that shot-geophone migration produces focused (subsurface-offset domain) or flat (scattering-angle domain) image gathers, provided there is a curvilinear coordinate system defining pseudodepth with respect to which the rays carrying significant energy do not turn, and that the acquisition coverage is sufficient to determine all such rays. Although the analysis is theoretical and idealized, a synthetic example suggests that its implications remain valid for practical implementations, and that shot-geophone prestack depth migration could be a particularly appropriate tool for velocity analysis in a complex structure.

Plane wave shot PSDM and controlled illumination techniques

The Pre-Stack Depth Migration (PSDM) method based on wavefield continuation is the most reliable method for imaging complex structure in the subsurface, although there are large computational costs and poorly adaptive geometry. Plane wave shot migration is another method to perform exact wave equation prestack imaging with high computational efficiency and without the migration aperture problem. Moreover, wavefield energy can be compensated at the target zone by controlled illumination. In this paper, plane wave shot PSDM was implemented by the control of the plane down-going wavefield and selection of number and range of the raypaths in order to optimize the imaging effect. In addition, controlled illumination techniques are applied to enhance the imaging precision of interesting areas at different depths. Numerical calculation indicates that plane wave shot imaging is a rapid and efficient method with less computational cost and easy parallel computation compared to the single-square-root operator imaging for common shot gathers and doublesquare-root operator imaging for common midpoint gathers.

True-amplitude prestack depth migration

Most current true-amplitude migrations correct only for geometric spreading. We present a new prestack depth-migration method that uses the framework of reverse-time migration to compensate for geometric spreading, intrinsic [Formula: see text] losses, and transmission losses. Geometric spreading is implicitly compensated by full two-way wave propagation. Intrinsic [Formula: see text] losses are handled by including a [Formula: see text]-dependent term in the wave equation. Transmission losses are compensated based on an estimation of angle-dependent reflectivity using a two-pass recursive reverse-time prestack migration. The image condition used is the ratio of receiver/source wavefield amplitudes. Two-dimensional tests using synthetic data for a dipping-layer model and a salt model show that loss-compensating prestack depth migration can produce reliable angle-dependent reflection coefficients at the target. The reflection coefficient curves are fitted to give least-squares estimates of the velocity ratio at the target. The main new result is a procedure for transmission compensation when extrapolating the receiver wavefield. There are still a number of limitations (e.g., we use only scalar extrapolation for illustration), but these limitations are now better defined.

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...

One-way wave equation seismic prestack forward modeling with irregular surfaces

Mathematical geophone (MG) and equal-time stacking (ETS) principles are used to implement seismic prestack forward modeling with irregular surfaces using the one-way acoustic wave-equation. This method receives seismic primary reflections from the subsurface using a set of virtual MGs. The receivers can be located anywhere on an irregular observing surface. Moreover, the ETS method utilizes the one-way acoustic wave equation to easily and quickly image and extrapolate seismic reflection data. The method is illustrated using high single-noise ratio common shot gathers computed by numerical forward modeling of two simple models, one with a flat surface and one with an irregular surface, and a complex normal fault model. A prestack depth migration method for irregular surface topography was used to reproduce the normal fault model with high accuracy.

Double Square Root Equation Migration Methods of Narrow Azimuth Seismic Data

After reviewing the basic principle of the migration methods based on the double square root (DSR) equation, we introduce a fast algorithm of angle domain imaging to facilitate migration velocity analysis (MVA). Then we discuss two approaches to migrate the narrow azimuth seismic data. One has restriction to the input data set, the other has limitation of the wave propagation direction. The numerical tests show that the DSR equation prestack depth migration methods, such as common azimuth migration, have a promising prospect of application in areas with strong lateral variations of velocity where bsalt, reef or burred hills exist.

Anti-aliasing by interpolation in pre-stack wave-equation migration

Spatial aliasing is an important issue in seismic migration. In this paper, we discuss anti-aliasing schemes in the wavefield continuation which is a wave-equation-based migration method. We describe the difference in interpolation applied pre and post the imaging condition, and compare the effectiveness of the linear interpolation and the sinc function interpolation. Our conclusion is that a five-sample sinc function interpolation applied before the imaging condition is an optimal anti-aliasing scheme according to theoretical and application analysis. We then apply the 3D Born approximation pre-stack depth migration method with optimal anti-aliasing interpolation to a 80 km2 seismic dataset. The migration result shows that the method we propose is better than a conventional wave equation method (not applied anti-spatial aliasing) in the definition of reflection events.

Factorization Synthesized‐Shot Prestack Depth Migration in Helical Coordinate System

AbstractBased on the synthesized‐shot prestack depth migration, a new highly efficient method for the synthesized‐shot prestack depth migration is proposed. It is a hybrid method which implements wavefield extrapolation with factorization in the helical coordinate system. The wavefield extrapolation is divided into two explicit solving processes: one is a causal process, and the other is an anticausal process. Such explicit solving processes in the helical coordinate system can enhance the wavefield extrapolation efficiency. Moreover, based on the phase encoding principle, the synthesized‐wavefield corresponding to multiple ray parameters is encoded and stacked. Then calculations are implemented with the MPI parallel algorithm based on ray parameters. Thus the calculation efficiency is improved further. After deriving the relevant formulae and analyzing the computation cost quantitatively, numerical calculations for the Marmousi complex model are presented, including the comparisons between them. The imaging results show that the method presented in this paper has advantages of high precision and good efficiency. So it can be applied to field data processing.

Along‐strike variations of crustal reflectivity related to the Andean subduction process

This paper presents the results of Kirchhoff prestack depth migration applied to two onshore deep seismic reflection data sets (ANCORP'96 and PRECORP'95). The prestack depth migration method was implemented in 3D (ANCORP) and in 2D (PRECORP), respectively, from topography. Selective stacking of the migrated depth slices yielded a 3D depth volume (ANCORP) and a 2D depth section (PRECORP), respectively. An additional “offline stacking” provided a final 370 km long 2D depth section (ANCORP). In this paper a 180 km long depth section will be presented. At the beginning of the ANCORP section a double reflection zone is observed, which is associated with the upper and lower boundary of the oceanic crust. Except for some areas a nearly complete image of the Nazca reflector at depths between 60–90 km has been obtained. Moreover, one Bright Spot in each section has been detected: the Quebrada Blanca Bright Spot (QBBS, ANCORP) and the Calama Bright Spot (CBS, PRECORP). The reprocessing yielded two new aspects regarding these structures: Besides the west dip of the QBBS, which was already known from standard poststack processing results, a 3D analysis of the ANCORP data set shows an additional north‐dipping trend of the QBBS. Furthermore, the CBS is discovered for the first time.

Wave Equation Based Pre‐Stack Depth Migration Using Generalized Screen Propagator

AbstractComputational efficiency and accuracy of seismic wave propagators are major concerns for 3‐D pre‐stack depth migration. Generalized screen propagator (GSP) is a dual‐domain one‐way propagator, which neglects reverberations between heterogeneities but correctly handles the forward multiple‐scattering including focusing, diffraction, refraction, and interference of seismic waves. Downward continuation of the wavefield using GSP is performed with a phase shift in a background medium followed by a wide‐angle correction that accommodates strong lateral velocity variations. In this paper, we implement GSP in double square root equation and propose an efficient and accurate offset domain pre‐stack depth migration method for 2‐D common offset and 3‐D common azimuth seismic data. Combined with common angle imaging (CAI) condition, offset domain GSP migration produces high quality images at a number of angles (or offset ray parameter values). Since offset ray parameter is proportional to angle of incidence at the reflector, CAI gathers provide a complete set of tools for migration velocity updating and amplitude‐versus‐offset (AVO) analysis.

Prestack Depth Migration Method Based on Quasi‐Linear Born Approximation

AbstractIn this paper, a prestack depth migration method based on quasi‐linear Born approximation is carried out in frequency‐wavenumber domain and frequency‐space domain, and is used to process common shot gather data of the Marmousi model. Through comparison with the Split‐Step Fourier migration method, Phase‐Screen migration method and the prestack depth migration method with stable Born approximation, it is apparent that the prestack depth migration method based on quasi‐linear Born approximation produces better migration results with improved handling of lateral velocity variations.

Depth imaging with multiples

Depth imaging with multiples is a prestack depth migration method that uses multiples as the signal for more accurate boundary mapping and amplitude recovery. The idea is partially related to model‐based multiple‐suppression techniques and reverse‐time depth migration. Conventional reverse‐time migration uses the two‐way wave equation for the backward wave propagation of recorded seismic traces and ray tracing or the eikonal equation for the forward traveltime computation (the excitation‐time imaging principle). Consequently, reverse‐time migration differs little from most other one‐way wave equation or ray‐tracing migration methods which expect only primary reflection events. Because it is almost impossible to attenuate multiples without degrading primaries, there has been a compelling need to devise a tool to use multiples constructively in data processing rather than attempting to destroy them. Furthermore, multiples and other nonreflecting wave types can enhance boundary imaging and amplitude recovery if a full two‐way wave equation is used for migration. The new approach solves the two‐way wave equation for both forward and backward directions of wave propagation using a finite‐difference technique. Thus, it handles all types of acoustic waves such as reflection (primary and multiples), refraction, diffraction, transmission, and any combination of these waves. During the imaging process, all these different types of wavefields collapse at the boundaries where they are generated or altered. The process goes through four main steps. First, a source function (wavelet) marches forward using the full two‐way scalar wave equation from a source location toward all directions. Second, the recorded traces in a shot gather march backward using the full two‐way scalar wave equation from all receiver points in the gather toward all directions. Third, the two forward‐ and backward‐propagated wavefields are correlated and summed for all time indices. And fourth, a Laplacian image reconstruction operator is applied to the correlated image frame. This technique can be applied to all types of seismic data: surface seismic, vertical seismic profile (VSP), crosswell seismic, vertical cable seismic, ocean‐bottom cable (OBC) seismic, etc. Because it migrates all wave types, the input data require no or minimal preprocessing (demultiple should not be done, but near‐surface or acquisition‐related problems might need to be corrected). Hence, it is only a one‐step process from the raw field gathers to a final depth image. External noise in the raw data will not correlate with the forward wavefield except for some coincidental matching; therefore, it is usually unnecessary to do signal enhancement processing before the depth imaging with multiples. The input velocity model could be acquired from various methods such as iterative focusing analysis or tomography, as in other prestack depth migration methods. The new method has been applied to data sets from a simple multiple‐generating model, the Marmousi model, and a real offset VSP. The results show accurate imaging of primaries and multiples with overall significant improvements over conventionally imaged sections.

Prestack Depth Migration of Deep Reflection Data at Chinese Continental Scientific Drilling Site

AbstractApplication of prestack depth migration methods has some difficulties owing to low signal‐to‐noise ratio and wide recording coverage of deep seismic reflection data. We have developed a depth migration method suitable for processing deep seismic data. It includes the following procedures. The upwind finite‐difference method is used for calculation of traveltime. Velocity analysis with high resolution employs a combination of co‐variance analysis, joint inversion of RMS velocity and reflection two‐way times, and a Kirchhoff prestack depth migration scheme sequentially. After using these velocity analysis methods on semblance gathers, the finite‐difference reverse depth migration is applied for imaging the crust. This prestack depth migration scheme has been used to process 2D deep seismic reflection data obtained in Donghai County for site selection of Chinese Continental Scientific Drilling Program. The resultant migration produces a good‐resolution profile for imaging the crustal structures and a velocity model for locating lithology of the ultrahigh pressure metamorphic rocks.

Ray equation migration of wide-angle reflections in Dabie orogenic zone

Ray equation migration of wide-angle reflections in 2-D medium is one kind of Kirchhoff prestack depth migration method. Based on ray theory, this method can be used extensively in 2-D inhomogeneous medium, and shows its advantage in wide-angle reflection study. After calculating ray fields, we can get the wave fields of sources and receivers by interpolation, and the intensity (or amplitude) Ap corresponding to the depth point P at travel-time tP=tSP+tPR can be found in seismograms. Ray equation migration is completed by transforming AP according to the specific relation, migrating it to the depth point, then calculating the ray fields of lots of sources and receivers in the same way and finally stacking the outcomes. Numerical calculation has yielded satisfactory results. Data processing of the Zhuangmu-Yuexi-Huangmei-Zhanggondu profile passing through the Dabie orogenic zone provides a structural form of M-discontinuity along the profile. The result shows that the high pressure metamorphic rock zone of South Dabie rock mass is related to the uplifting of M-discontinuity here. "Mountain root" exists under North Dabie rock mass, which conforms with gravity isostasy theory. The Xiaotian-Mozitan fault is a suture zone as a result of the collision of North China plate with Yangtze plate, and extends through M-discontinuity. The abyssal fault near Liu’an is the extended eastern section of the Luonan-Minggang fault, which is also confirmed here.

Prestack plane‐wave Kirchhoff migration in laterally varying media

Conventional Kirchhoff migration methods are applied to seismograms in the offset‐time (x − t) domain. We describe the theory, numerical details, and examples of a prestack depth migration method in the plane‐wave domain that is valid in laterally inhomogeneous media. The theory is based on a Kirchhoff‐Helmholtz formulation of the wavefield and uses plane‐wave‐transformed shot gathers for migration. We use geometric ray theory for the source wavefield continuation operator and a plane‐wave expansion of the receiver wavefield in the integrand of the Kirchhoff‐Helmholtz integral. The source and receiver plane‐wave traveltimes are computed at each grid point in the subsurface using a finite‐difference approximation of the eikonal equation with appropriate initial and boundary conditions. We developed an efficient technique to compute imaging time by a combination of these two times. The technique allows us to design algorithms for migrating shot gather or constant ray‐parameter sections efficiently. We evaluate the efficiency and accuracy of the algorithm with three sets of synthetic data examples with varying degrees of complexity. We also compare the performance of the parallel algorithms using Parallel Virtual Machine (PVM). Migration of a marine data set results in excellent images of a mud volcano and the top of an accretionary prism of an active continental margin on the Nicoya Peninsula of Costa Rica.

Using Layer Replacement to Improve Velocity Analysis Beneath Near-Surface Reefs

Exploration in the Timor Sea is hampered by the presence of many near-surface limestone reefs. Reef structures form complex velocity anomalies because of their internal velocity structure and because they produce a rapidly varying sea-floor profile. Distortion of seismic data recorded over such reefs comprises event pull-up and poor imaging. Conventional common mid-point processing is not adequate to interpret prospective targets below the reefs. Seismic distortion associated with reefs can be reduced using replacement techniques of varying complexity. Wave-equation datuming is an accurate method but computationally expensive, and replacement statics is an efficient method but only approximate. All replacement methods require an estimate of the velocity structure inside the reef itself, although this is not normally attempted in production processing. The seismic image beneath surface reefs can be improved by integrating layer replacement with velocity analysis. The seismic data are first modified by replacing the water velocity with an average velocity of the reef. This involves two steps: downward continuing to the sea floor, and then upward continuing to the surface. A sufficiently accurate value for an average velocity can usually be obtained using conventional velocity analysis. This approach removes most of the velocity inhomogeneity associated with the water/reef interface and renders subsequent velocity analysis more accurate. A model of a typical reef from Browse Basin, which includes strong lateral velocity inhomogeneity on the reef flanks, shows that velocity analysis post-replacement gives much more accurate RMS velocities inside and immediately below the reef. Events below the reef are imaged more faithfully. Conventional velocity analysis is not accurate enough to give interval velocities for the reef because the CMP method cannot image such strong lateral velocity changes. The same model can be analyzed using reflection tomography based on the algebraic reconstruction method (ART). ART is sufficiently accurate to give useful estimates of the interval velocities within the model reef. Comparing results before and after replacement indicates that replacing the water layer prior to ray tracing gives closer estimates of the high-velocity reef flanks and reduces velocity smearing across the sea floor to water interface. Estimates of the interval velocities inside the reef are important for accurate imaging. These can be used to continue downward through the reef structure for a second pass of wave-equation replacement, or for applying a variety of pre-stack depth migration methods.

Efficient <i>x-f</i> depth migration of shot records: practical aspects

In the case of a subsurface with a complex structure, it may be necessary to perform depth migration in the prestack domain. In this paper a prestack depth migration method is discussed which belongs to the class of single shot record migration techniques (Van der Schoot et al. 1989).