Conventional Prestack Depth Migration Research Articles

Improving prestack time migration by introducing a new velocity-related parameter: Parameter picking and 3D real data application

Prestack time migration (PSTM), a commonly used tool for seismic imaging, has been widely applied in 3D seismic data processing. However, the conventional PSTM algorithms use only one effective velocity parameter (i.e., root-mean-square [rms] velocity) for each imaging point, which may not be accurate when stronger lateral variations occur in seismic velocities. In this paper, we introduce a new parameter called the velocity variation factor that considers velocity variations in inhomogeneous media to improve PSTM. This new parameter, together with the rms velocity, describes the propagation Green’s function at an imaging point with two effective parameters rather than one effective parameter as in conventional PSTMs. This provides a more accurate traveltime calculation for the wave propagating through media with moderate lateral velocity variation. Unlike the conventional bending-ray PSTM, the additional effective parameter is fully independent of the rms velocities. We estimate the two effective parameters at each imaging point by flattening the neighboring image gathers with a global optimization algorithm. The objective function is built at each imaging point using a selective crosscorrelation-based time shift, which can quantitatively describe the slight bending of events in the local migrated gathers regardless of the quality of the gathers. We estimate the two effective parameters using the very fast simulated annealing algorithm and multiscale approach, thus avoiding the local minimum caused by the noises in the migrated gathers. We apply our two-parameter PSTM to a real 3D land data set to demonstrate its industrial applicability. A comparison of the new imaging result with the conventional prestack depth migration also is presented.

Geomechanical prestack depth migration of the Kraken 3D (Browse Basin, Australia)

Conventional pre-stack depth migration applied to the broadband Kraken 3D Marine Seismic Survey was unable to fully resolve short-wavelength velocity anomalies below the sea floor causing obvious imaging problems and limiting depth conversion and amplitude interpretation. Improved imaging was achieved by initiating tomography using a velocity model built by combining geomechanics with rock physics appropriate for shallow carbonates and mudrocks. 3D gravity modelling using high-resolution bathymetry and compaction trends constitutes a new approach for iteratively building a 3D geomechanical model. Effective stress is derived by applying Terzaghi’s principle within an integration (along depth) involving the model bulk and fluid densities and the vertical component of gravity (all of which may vary spatially, using more refined models). Carbonate and mudrock rock physics models, believed to be appropriate for Neogene sediments along much of the NW Shelf of Australia were derived from abundant core and wireline data acquired during the recent IODP Expedition 356. These models provide the necessary link between effective stress and P-wave velocity with Backus averaging handling the “seismic scale” mixing of different lithologies expected in the Kraken 3D area. Kirchhoff prestack depth migration was revisited from archived preprocessed gathers using the geomechanical model combined with Common reflection angle gathers (CRAM) to initiate tomography. CRAM gathers were very effective in the presence of multiples and complex near surface topography to achieve a better update in tomography. Heavy smoothing of velocities was imposed where sequence stratigraphic interpretation suggests only distal mudstone facies. Tomography in shallower layers was then revisited to restore geologically plausible depth structures and deliver a clear improvement in imaging relative to previous processing efforts.

Tomographic velocity inversion for ADCIGs in areas with a rugged surface

Pre-stack depth migration velocity analysis is one of the key techniques influencing image quality. As for areas with a rugged surface and complex subsurface, conventional prestack depth migration velocity analysis corrects the rugged surface to a known datum or designed surface velocity model on which to perform migration and update the velocity. We propose a rugged surface tomographic velocity inversion method based on angle-domain common image gathers by which the velocity field can be updated directly from the rugged surface without static correction for pre-stack data and improve inversion precision and efficiency. First, we introduce a method to acquire angle-domain common image gathers (ADCIGs) in rugged surface areas and then perform rugged surface tomographic velocity inversion. Tests with model and field data prove the method to be correct and effective.

Curved-wave prestack depth migration

Conventional prestack depth migration (PSDM) based on point-source shot data involves a huge number of time-consuming wave extrapolations. Areal-source prestack data generated by a synthesis procedure can provide an alternative migration scheme that is efficient and accurate. We have developed a curved-wave synthesis method to implement curved-wave PSDM technology. A base-kernel synthesis operator is defined as a relative curved source-excitation datum. A general synthesis operator is constructed by disturbing the illumination directions of a base-kernel operator. The curved wavefields are synthesized by synthesis operators and then migrated by combination with a common-shot depth-migration algorithm. Curved wavefields can be constructed according to the configuration of a geologic structure to obtain a high-quality image of the target. Curved-wave records arising from different curved-source wavefields with different illumination properties are stacked after depth migration to produce an accurate image. A synthesis operator can be designed by simulating the configuration of target structures, or by velocity constraint in a target-oriented way, to efficiently achieve a high-quality image of a complex target area. Characteristic of curved excitation surface and space-variant illumination, the method is more adaptable to a complex medium than the plane-wave technique and more efficient than shot migration. Numerical demonstrations on synthetic and real data have proved effective and given good results.

Target-oriented curved-wave prestack depth migration by controlled illumination

Conventional prestack depth migration (PSDM) based on full prestack data involves many computations for wavefield extrapolation. Areal shot-record technology offers an attractive alternative for efficient PSDM because its synthesis process greatly decreases the amount of prestack data required for migration. Constructing its synthesis operator is key to the image quality of the migrated areal shot record. Curved-wave PSDM technology expands areal shot-record migration. It constructs synthesis operators by defining a complex function as a base kernel of curved wavefields multiplied by a factor of its illumination perturbation related to ray parameter and then synthesizes curved wavefields for migrations. Based on curved-wave-migration theory, we propose an efficient and accurate curved-wave, controlled-illumination method to migrate in a target-oriented way. We construct tar-get-oriented synthesis operators by wave theory, combined with rotation or perturbation of the illuminating direction of a base-kernel synthesis operator. The base-kernel operator is obtained by inverse extrapolation of a predefined source wavefield at the given target level. Different model-constrained synthesis operators are constructed by rotating or perturbing illumination of the kernel operator. Then they are applied to shot records to synthesize curved-wave records. The resulting curved wavefields are entered for depth extrapolation and imaging, combining image results from different curved wavefields to produce the total image. The method controls the illumination of synthesized source wavefields in a target-oriented way directly at the surface and can achieve high-quality images of the target zone with great efficiency. Numerical demonstrations on the standard Marmousi model provide good imaging results of the complicated structure.

Anisotropic prestack depth migration in practice

Conventional prestack depth migration involves building a velocity model for the subsurface using velocities that are assumed to be isotropic. When the earth is anisotropic, it is impossible for this conventional earth model to correctly predict the seismic raypaths and hence to accurately migrate the recorded data.Common problems with conventional prestack depth migration are overcorrected common-image point gathers and depth misties with available well data.Applying anisotropic prestack depth migration involves making some assumptions about the symmetry of the anisotropy and then determining the vertical velocity and the anisotropic parameters, delta and epsilon. These parameters were determined using a combination of well information, higher moveout analysis and migration scans.The results of a successful anisotropic prestack depth migration tie the available well information, have flat common-image point gathers and have higher resolution and improved focussing.

A two‐pass approximation to 3-D prestack migration

A two‐pass approximation to 3-D Kirchhoff migration simplifies the migration procedure by reducing it to a succession of 2-D operations. This approach has proven very successful in the zero‐offset case. A two‐pass approximation to 3-D migration is described here for the prestack case. Compared to the one‐pass approach, the scheme presented here provides significant reduction in computation time and a relatively simple data manipulation scheme. The two‐pass method was designed using velocity independent prestack time migration (DMO‐PSI) applied in the crossline direction, followed by conventional prestack depth migration in the inline direction. Velocity analysis, an important part of prestack migration, is also included in the two‐pass scheme. It is carried out as a 2-D procedure after 3-D effects are removed from the data volume. The procedure presented here is a practical full volume 3-D prestack migration. One of its main benefits is a realistic and efficient iterative velocity analysis procedure in three dimensions. The algorithm was designed in the frequency domain and the computational scheme was optimized by processing individual frequency slices independently. Irregular trace distribution, a feature that characterizes most 3-D seismic surveys, is implicitly accounted for within the two‐pass algorithm. A numerical example tests the performance of the two‐pass 3-D prestack migration program in the presence of a vertical velocity gradient. A 3-D land survey from a fold and thrust belt region was used to demonstrate the algorithm in a complex geological setting. The results were compared with images from other 2-D and 3-D migration schemes and show improved resolution and higher signal content.