Best Practice Flows › Velocity model workflow › Setting up the velocity model › Checkshot extrapolation
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Checkshot extrapolation

The Checkshot extrapolation method turns well checkshot surveys into a full 3D interval velocity cube. The user divides the section into layers bounded by time horizons and selects a velocity source for each layer (either a constant value or a checkshot-derived curve). The wizard interpolates velocities between wells and extrapolates them into the rest of the grid, optionally filtering the result to remove high-frequency artefacts.

Required inputs

  • Checkshot data — loaded on one or more wells.
  • Layer table — one row per layer, specifying:
  • Top / Bottom of layer — time horizons, time maps, constant time or topography that bound the layer.
  • Layering type — Proportional, Parallel to Top or Parallel to Bottom; controls how the layer is subdivided internally.
  • Velocity definition — Constant (user-entered value) or From checkshot (derived per well from the checkshot curve).

Parameters

  • Datum — elevation reference.
  • Max Depth, Depth Step, Bottom Velocity — output cube extent.
  • Frequency Filter — high-cut filter applied to the checkshot velocity curve (default 25 Hz); smaller values give more aggressive smoothing.
  • Reference Geometry — Maps intersection / Polygon / Seismic 3D, with the corresponding XY Step or IL/XL Each selectors.
  • Use Smooth — enables post-interpolation smoothing, with Smooth traces radius and Smooth samples radius controls.

Workflow

  1. Select Checkshot extrapolation in the velocity model wizard.
  2. Configure Datum, Max Depth, Depth Step, Bottom Velocity, Frequency Filter and Reference Geometry.
  3. Build the layer table by selecting top and bottom bounds, layering type and velocity source for each layer.
  4. Enable post-smoothing if the output appears noisy.
  5. Run the model. Per-well velocities are extracted from the checkshot, spatially interpolated, and written to the output cube.

Output

Interval and average velocity cubes, depth and time maps per layer, plus per-well mismatch logs between the model and the input checkshot (useful for QC).

When to use

  • Projects where checkshot coverage is the primary velocity constraint.
  • Models where geological layering is well understood and can be captured by horizons.
  • Areas needing smooth, extrapolated velocities outside the well control.

When markers or horizons give additional constraints, combine methods via Checkshot + markers or Checkshot + markers + horizons.

See Also