Migration
Spectrum offers a wide range of imaging solutions to suit the technological and financial requirements of our clients. The application of each technique is preceded by intensive testing of all parameters to ensure effective use of these powerful processing algorithms.
Time MigrationFull 3D migration on common offset volumes is achieved using either a phase shift or FXY algorithm. Post stack demigration is not available using this route, however it is possible to remove NMO from the gathers and run a new velocity analysis on migrated gathers prior to stacking.
Pre-stack Kirchhoff time migration is available within the SPA processing package as a proprietary module on our high-speed PC cluster. This migration uses the double square root equation for definition of diffraction curves and does not assume zero offset. The result is properly imaged gathers suitable for AVO analysis. The process copes with lateral velocity variations, vertical velocity variations and anisotropy. An advantage of the technique is that it is immune to inconsistent and sparse unaliased trace sampling and can interpolate missing traces.
The migration software runs on the P.C. cluster to perform a full 2D or 3D pre-stack Kirchhoff time migration that is effective at focussing energy in its correct position without compromising amplitude and phase information. The program builds travel time calculations using a double square root equation thus allowing time and space variant velocity variations. The shifted samples are phase corrected and scaled for distance moved before summation in the correct position. An option is available to use a curved ray calculation, as opposed to straight ray and to include an eta anisotropy term.
Intensive tests are performed before the production run to determine sufficient dip and aperture parameters.
Impulse Response - Anisotropic (black) vs Isotropic (colour)
Spectrum has provided pre-stack depth migration since 1996 and has since achieved global experience and many satisfied customers. Our depth capabilities include 3D as well as 2D application with processing algorithms run on our high-speed multi-node clusters to ensure fast turnaround. Remote viewing of model-building for all clients is also provided as a standard service.
Projects include the world’s largest 2D PSDM project - over 100,000 km of seismic data in the US Gulf of Mexico. This involved 450 separate lines with events tying throughout.
Depth Velocity MethodsThe key to any successful pre-stack depth migration project is having an accurate velocity field. There are many ways to derive an interval velocity field and all are available at Spectrum:
- Dix Conversion. An existing time migration velocity field can be converted to depth interval velocities via the Dix Equation. This is usually an initial step and is modified after further analysis.
- Coherency Inversion. Typically used in areas of abrupt vertical velocity changes, this method utilizes existing interpretation along known horizons.
- Gradient. Typically used in areas of generalized velocity increases with depth, such as sedimentary basins.
- Velocity Insertion. Putting in polygons/volumes of anomalous velocity changes, such as salt bodies, diapirs, etc.
- Tomography. Used to update depth interval velocity fields, both horizon tomography and grid tomography can be utilized.
- Residual Correction. Kirchhoff migrated depth gathers can be “flattened” before stack to improve stack resolution.
There are two main pre-stack depth migration algorithms, both available at Spectrum:
- Kirchhoff. The most commonly used PSDM algorithm. Typically gives excellent results for steep dips and outputs depth gathers for further analysis.
- Wave Equation. Takes advantage of multi-path ray-tracing, this method can improve results sub-salt.
Decisions as to which velocity method to use and which migration algorithm to use is decided jointly between the client and Spectrum.
The figures below show a velocity analysis and migrated results from offshore Brazil.
The velocity analysis shows a single Kirchhoff migrated depth gather with its related semblance analysis and stacked section. Note that the gathers can be analyzed to determine if the migration velocity was too fast or too slow. Thus, the velocity field can be updated to a more accurate velocity field for subsequent migrations.
The two stack sections show a comparison between Kirchhoff pre-stack time migration and Kirchhoff pre-stack depth migration. The area is Santos Basin, offshore Brazil. Note that the top and base salt structures are much more clearly defined in the depth section than on the time section. Even sub-salt sediment becomes more interpretable on the depth section. The ability to accurately define salt structures in depth migration is one of the main benefits of pre-stack depth migration.
