Pre-Stack Time Migration

In general, with increasing structural complexity, post-stack migration becomes less accurate, and the need for a pre-stack time migration (PSTM) arises. Pre-stack imaging becomes necessary where conflicting dips (or ‘stacking conflicts’) occur in the data, which results in inaccurate velocity analysis. The stacked section in this case is no longer being a good representation of a zero-offset section. Kirchhoff PSTM has several benefits over the “common zero offset” approach, primarily in its ability to improve structural imaging and provide correctly imaged pre-stack data for subsequent AVO analysis. Kirchhoff migration is a summation process, which is only performed for input traces that are within an aperture radius of the output point. The aperture radius is a time-variant function of the specified dip-limits. For each input trace and output sample, travel-times are computed to determine the proper input sample to sum. Input sample values are scaled and filtered before summing. Anti-alias filters as a function of dip and midpoint distance are applied to the data during migration.

SUMMIG is a 2D/3D full curved ray Kirchhoff migration which can work on pre stack and post stack data. This migration can create numerous types of output such as migrated stacks, percentage migrated stacks, migrated gathers and percentage migrated gathers. SUMMIG handles both VTI and HTI anisotropy.

Some features are listed below:

  • Migration from Floating Datum or Topography
  • Migration Aperture & Angle Control
  • Anti-Alias Filtering
  • Various Anisotropy Options – VTI (layering) & HTI (fracturing)
  • Various Input Options – CDP gathers, Offset Planes & COV Gathers
  • Variable Output Grids
  • Migration Output to Stack or Gathers
  • Output both Constant and Variable Offset Planes

SUMMIG has various move-out options:

  • Topography – travel time is calculated from the surface
  • High order from topography – travel time is calculated from surface using 4th or 6th order move-out
  • Anisotropy from topography – travel time is calculated from surface using the equation described in Alkhalifah and Tsvankin

VELANAL is a standalone interactive velocity analysis tool for picking both Isotropic and Anisotropic (VTI) velocities. It is possible to input percentage migrated stacks and percentage migrated gathers. User can also run a series of Eta scans to analyze and flatten the hockey stick effect seen at far offsets due to Vertical Transverse Isotropy (VTI). This spatially and temporally varying field can then be included in the migration to correct for these far offset effects.

HAMMER is a standalone interactive velocity analysis tool for picking Anisotropic (HTI) velocities. The program first identifies the major horizons and estimates an isotropic velocity that best flattens the data. The program then estimates residual move-out on the horizon by a short window cross correlation between the trace and a near offset stack trace. These residual shifts are then fed into a least squares curve fitting algorithm that estimates the fast and slow velocities and their azimuth for that specific horizon. These attributes are calculated independently for each horizon and for each control point.