A velocity model describes how seismic velocity varies with position and depth across the survey. g-Space uses it to convert interpreted data between the time and depth domains, to build depth maps and cubes, and to support depth imaging and structural interpretation. This chapter describes how to launch the velocity modeling wizard and summarises the eight model-building methods it offers.

Launching the wizard

1. Open the Velocity model bar in the ribbon.
2. Click Create depth velocity. The Create velocity model wizard opens.
3. On the first page, choose one of the velocity model types listed under Velocity models. A preview image illustrates the selected method.
4. Fill in the parameters and any layer or tie tables that appear for the chosen method, then click Run to build the model, or Save to workflow to keep the settings as a workflow task.

The new model is added to the depth velocity model set and becomes available for time-to-depth conversion and QC. Existing models can be brought in instead with Import velocity model.

Running the wizard or saving it to a workflow

The buttons along the bottom of the wizard decide what happens to the settings you entered:

• Back and Next — move between the pages of the wizard.
• Run — build the model now. You are asked for a name for the new velocity model, and if a model with that name already exists you are asked whether to overwrite it.
• Save to workflow — keep the current settings as a Create velocity model task instead of building the model now. The wizard closes, you are asked for the name of the velocity model the task will produce, and the task is added to the current workflow under that name.
• Close — close the wizard without building a model.

A saved task stores the complete wizard setup — the selected velocity model type and every parameter entered for it. Reopen the task from the Workflows wizard to review or change it: the Create velocity model wizard opens again with the saved values and keeps the same buttons. Save to workflow then writes the changes back into the same task instead of adding a second one, while Run builds the model straight away from the values on screen.

When the workflow itself is executed, the task builds the velocity model under its stored name without prompting. A velocity model that already carries that name is replaced without a confirmation question, so give the task a distinct name when earlier results must be kept.

Velocity model methods

The wizard offers the following methods, ordered from the simplest to the most data-intensive:

• Constant — applies a single interval velocity to the whole output cube.
• Constant layered — assigns a constant velocity to each layer, where layers are bounded by horizons, time maps, constant times or topography.
• Markers + horizons — derives interval velocities from well marker-to-horizon ties and interpolates them between wells.
• Grid velocity by markers + horizons — uses the same marker-to-horizon ties but writes the result on a bin grid.
• Checkshot extrapolation — builds a layered cube from well checkshot surveys and extrapolates it across the grid.
• Checkshot + markers — combines checkshot depth-time calibration with well-marker depth control.
• Checkshot + markers + horizons — combines checkshots, well markers and time horizons in a single well-tied model.
• Velocity volume — builds an interval velocity cube from a velocity volume that is already in the project, in the time or in the depth domain.

See Also

• Velocity model bar
• Velocity model QC
• Import velocity model
• Workflows