Dense Anisotropic (TTI) Angle Gathers from RTM for High-Fidelity Model Refinement

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Refine structural interpretation

Reverse time migration (RTM) has increasingly become the high-end final migration of choice in areas of the greatest structural and velocity complexity. However, this has created additional demands on the accuracy of the velocity models used to create these images. The new challenge is in understanding how best to use RTM in the model building and refining process. RTM images help in refining structural interpretation, but it is necessary to move into the prestack domain—via dense anisotropic angle gathers—to take full advantage of RTM’s benefits in this area.

The benefits of dense anisotropic RTM angle gathers include

  • more reliable input to subsalt velocity updates (tomography or scans) compared to wavefield extrapolation (e.g., Shot WEM) or ray-based (e.g., Kirchhoff or Beam) methods
  • supplementing image volumes for interpretation and geobody geometry definition, such as salt overhangs
  • the ability to improve stack quality (reducing uncertainty in top-salt, salt-flank, and base-salt interpretation) compared to image-only RTM volumes by
    • controlled muting to eliminate more noise (e.g., top-salt refractions) before stacking
    • reducing residual moveout on gathers before stacking, by using residual flattening techniques such as REVEAL residual event alignment for enhanced inversion and structural interpretation
    • providing additional constraints to automated model updating processes such as Full Waveform Inversion (FWI). WesternGeco now offers dense anisotropic (TTI) RTM angle gathers for high-fidelity depth velocity model refinement.

    Generate angle gathers valid for all dips

    Generating dense gathers for every point in the image volume allows more control in velocity model updating, as well as the ability to perform optimized muting and gather flattening to enhance final stack image quality.

    Correctly handling seismic anisotropy enables events to be positioned in “true depth” both laterally and vertically, as well as generating flatter gathers. Our implementation handles both VTI and TTI modes of anisotropy.

    Unlike other methods that rely on an underlying assumption of near-flat events, our implementation derives angle gathers that are valid for all dips, including steeply dipping or over-turned events such as salt overhangs.

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    RTM: The High-End Final Migration of Choice

    The red line on the stack data indicates the location of the RTM angle gathers shown on the right. This example is from the Green Canyon area of the Gulf of Mexico.
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