The project datum defines the elevation reference and the replacement velocity used when reconciling data acquired or processed at different levels. Every depth- and time-domain calculation in g-Space — including horizon export, velocity modelling, and volumetric calculations — references this datum, so it must be correct before data is loaded or re-processed.

Setting the datum

To configure the project datum, follow these steps:

1. Open Module properties and select the Parameters tab.
2. Set the required values for Replacement Velocity and Datum.
3. Tick the Shift to Datum checkbox so that subsequent imports and exports are referenced to the new datum.

[image: placeholder_Module_properties_Parameters_tab_with_Da.png]

Datum mode (single or multiple)

The project Datum mode controls whether all seismic data share one datum or may each use their own. Open it from the File menu > Configuration, in the Datum mode group:

• Single datum (default) — all seismic data use the same datum and replacement velocity (most common case). Seismic import then runs in a simplified mode with the per-file datum fields hidden.
• Multiple datum — seismic data have different datums or replacement velocities; the seismic import view exposes per-file datum and V0 (replacement velocity) fields.

When to change the datum

• At project creation, to match the datum of the primary seismic survey.
• Before importing horizons or wells whose elevations are referenced to a different datum — use the Shift to Datum option to translate values on the fly.
• When merging data from surveys processed against different replacement velocities.

Changing the datum after data has been loaded causes every datum-dependent object to be recalculated — expect a service-task pass to follow (see Service Tasks).

See Also

• Module properties
• Coordinate and measurement system
• Creating a new project