Duryodhan Epili

3-D prestack Kirchhoff depth migration: From prototype to production in a massively parallel processor environment

Portable, production-scale 3-D prestack Kirchhoff depth migration software capable of full-volume imaging has been successfully implemented and applied to a six-million trace (46.9 Gbyte) marine data set from a salt/subsalt play in the Gulf of Mexico. Velocity model building and updates use an image-driven strategy and were performed in a Sun Sparc environment. Images obtained by 3-D prestack migration after three velocity iterations are substantially better focused and reveal drilling targets that were not visible in images obtained from conventional 3-D poststack time migration. Amplitudes are well preserved, so anomalies associated with known reservoirs conform to the petrophysical predictions. Prototype development was on an 8-node Intel iPSC860 computer; the production version was run on an 1824-node Intel Paragon computer. The code has been successfully ported to CRAY (T3D) and Unix workstation (PVM) environments.

Interactive demultiple in the post-migration depth domain

In efforts to further improve final migrated images we have developed a new methodology for post-migration multiple removal in the migration depth domain. The typical input for the prediction phase of this process is a 3D depth migration volume and the corresponding velocity field. A post-stack Wavefield Extrapolation (WFE) based multiple prediction is used to identify/confirm the multiple events in the migration depth domain. Once multiple events are identified, an effective and efficient demultiple procedure called interactive demultiple is applied to remove the residual multiple from the final migration. The key ingredient of this new interactive demultiple methodology is the attribute-based subtraction. We will describe the main steps of this methodology, and demonstrate its effectiveness by showing some field data applications.

Wave-equation Based Residual Multiple Prediction And Elimination In Migration Depth Domain As an Aid to Seismic Interpretation

We have developed a new methodology for predicting and removing multiples in the migration depth domain based on wavefield extrapolation and attribute based subtraction. The input for the prediction part is a 3D prestack depth migration volume and the corresponding velocity field. The output is the predicted multiple model (surface related or inter-bed) in the migration depth domain. The residual multiple removal technique combines the multiple prediction part with the recently developed attribute-based subtraction. Applications to both marine and land data have proven this methodology to be very effective in further reducing the residual multiples in the final migration images.

Simulation and Imaging of GPR Data Scattered by Reinforcing Bars in a Concrete Bridge Deck

Ground Penetrating Radar (GPR) is a non‐destructive method that can be used to locate reinforcing bars in concrete structures such as bridge decks. Provided that the frequency used is sufficiently high, and the antenna radiation pattern adequately illuminates the features of interest, reliable images can be obtained. Both 2‐D and 3‐D GPR data were collected over a bridge deck before and after resurfacing with a layer of reinforced concrete. Numerical simulation of the 2‐D data shows the best results that can be expected. Migration of both the 2‐D and 3‐D data enhances and focuses image details by collapsing diffractions, and moving reflectors to their correct spatial positions.Migrated images provide a realistic picture for interpretation. Comparing the images with photographs and blueprints shows good correspondence; some minor deviations (e.g., in spacing of reinforcing bars) of the actual configuration as produced by the contractors, from the specifications, are detected.

3-D WARRP Design For Exploration Targets

Summary An experimental velocity/depth model with a complex subsurface is defined from the existing data base and used to evaluate 3-D Wide Angle Reflection/Refraction (WARRP) techniques. The velocities in the model increase from approximately 4900 ft/s (1.5 km/s) near the surf ace to 8850 ft/s (2.7 km/s) in the intermediate layer and to 13100 ft/s (4.0 km/s) in the deeper layer with large contrasts at depths of 2460 ft (.75 km) and 6550 ft/s (2 km) approximately. Previous 2-D seismic profiles in geographical areas having properties similar to the model can image the near surface layers, but failed to provide an interpretable image for the deeper structure. Recently, wide-angle data have been acquired over poor record areas, and the data show significant improvements by which seismic data interpretation becomes possible with some degree of confidence. With this in mind, several stationary WARRP acquisition strategies over the structurally complex, heterogeneous 3-D velocity model are investigated to verify the suitability of the data for turning wave tomography and wide-angle migration. The turning wave analysis involves the subsurface ray coverage, and the wide-angle analysis involves the target illumination with selective offsets. Modeling indicates the available turning ray and wide-angle reflected arrivals with sufficient energy for accomplishing both data processing. Three such design scenarios (orthogonal, brick, and uniform source-receiver spreads) are f ound suitable for this evaluation, although, in actual practice economic considerations may bias the preference of one over the other.

Improved Subsalt Imaging Using TTI Anisotropy And Reverse Time Migration Scans

We present an advanced velocity model building and imaging methodology that resulted in significant enhancements in defining the salt flanks with overhangs and subsalt structures. The key technologies used were (a) True Azimuth Multiple Elimination (TAME), (b) Tilted Transverse Isotropy (TTI) model building (FAN), (c) TTI Reverse Time Migration (RTM), (d) RTM Delayed Imaging Time (DIT) scans and (e) post-migration multiple attenuation.

Depth focusing reflection tomography with application to prestack depth migration

Reflection tomography can determine velocity models and reflector depth, but has limitations: (1) it is unstable in the presence of strong lateral velocity changes; (2) it is strongly dependent on the starting reference model; (3) it requires constraints or a priori information, and (4) it is computationally inefficient. In order to overcome limitations, three strategies have been combined in Depth Focusing Reflection Tomography (DFRT). It uses (1) a smaller parameterization at the early inversion stage (variable parameterization), (2) consistent depth perturbation as a criterion for the extremum problem (depth focusing concept), and (3) a systematic exploring process instead of a random search to find a global extremum (exploring process).

Anisotropic tomography of the Glenn Pool crosswell data

The Glenn Pool oil field is located on the North Eastern Platform of Oklahoma, extending into Tulsa and Creek Counties. Crosswell seismic tomography is used as a geophysical method for reservoir imaging and description. Initial processing of the crosswell data indicated the presence of seismic anisotropy (Vertical Transverse Isotropy, VTI). The anisotropy is characterized through Thomsen’s Epsilon and Delta parameters. An anisotropic ray tracing scheme is developed to compute travel times honoring these parameters. A systematic velocity inversion procedure is developed based on the weighted residual back projection technique and a common domain vertical velocity is obtained. The results are very encouraging and are in excellent agreement with the geologic description from well log, core, and outcrop studies.