Prestack Time Migration
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Non-zero offset Kirchhoff prestack time migration uses ray traced travel times and allows prestack time migration using a laterally varying, v(x,y,z), velocity field. It is assumed that the lateral velocity variation is not strong enough to require depth migration. This approach effectively combines NMO, DMO and migration into a single imaging step. The algorithm runs on common offset data and is available in 2D and 3D. The migration code used is the same as that used for prestack depth-migration; the only difference is in the travel-times used to drive the migration and weights and anti-aliasing filters appropriate for time migration.
There are no assumptions about the trace recording azimuths. The algorithm honours the input positions of the data and outputs at trace bin-centres. Anti-alias filters and amplitude scaling are applied. Migration amplitude weights are based on Bleistein’s Kirchhoff inversion formulae, as described by Zhang et al (2002), which also includes description of the anti-aliasing protection. Spherical spreading corrections are applied within the algorithm.
The ray-tracing assumes a local v(z) behaviour so that the migration operator is symmetric but varies spatially. The ray-tracing implicitly incorporates all high-order v(z) ray-bending moveout terms. Both VTI and HTI anisotropy options are available. For VTI, interval values of the anisotropic parameter eta are input; they may be space and time varying as appropriate.
An iterative approach to velocity model building is used.
- Selected CMPs or target lines are output using the initial velocity model. This is done using the same migration code as is used for the main volume output.
- A new velocity field is picked from this migrated output.
- Interval velocities are clipped and smoothed consistent with the time-migration assumptions. The smoothing radius is usually related to the migration aperture.
- The target output is re-migrated using the new field to check for convergence prior to the full volume migration.
“UTMOST” Kirchhoff Pre-stack Time Migration (Note clear fault imaging)
Phase Shift Pre-stack Time Migration