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The Shift Velocities to Datum module transforms a velocity model (Vrms, average, or interval) from its current elevation reference — typically the actual topography surface — to a new flat datum elevation. This is a required step in workflows where seismic data has been datumed to a floating datum and the velocity model must be referenced to that same datum for consistent NMO correction, migration, or statics computation.
For each trace in the velocity gather, the module uses the source and receiver elevation values stored in trace headers (header words 41, 53, 210) to determine the topographic surface elevation. It then shifts each trace's time axis so that time zero corresponds to the specified datum depth rather than the topographic surface. The near-surface zone between the original surface and the datum is filled with a constant replacement velocity (V0). The output is a velocity gather referenced to the datum elevation, compatible with datum-corrected seismic data.
The input velocity gather in post-stack format (one velocity trace per CMP bin). The gather must be in VistaGatherSettingsVelocity format with source and receiver elevation values stored in the trace headers. The number of traces in the gather must equal the number of CMP bins, and the dataset must have been passed through the binning step prior to running this module. The vertical axis represents two-way travel time referenced to the original topographic surface.
The target datum elevation, in metres, to which the velocity model time axis will be shifted (default: -1). When set to -1, the module uses the actual topographic surface elevation of each CMP bin as the datum, effectively leaving the reference unchanged. For Vrms velocity type, this special value causes source and receiver datum fields to be set to the local topography. For Average and Interval velocity types the datum value must be greater than the topographic surface elevation at every bin (i.e. above the ground for a standard positive-up coordinate system). Set this to the processing datum elevation that matches the datum used during seismic data preparation.
The constant replacement velocity (m/s) used to fill the near-surface interval between the original topographic surface and the datum (default: -1). When set to -1, the module takes the velocity from the first sample of each trace (the shallowest velocity pick) as the replacement velocity for that trace. This is equivalent to assuming that the near-surface velocity is the same as the first interpreted velocity layer. Set this to the actual near-surface or replacement velocity used in your statics workflow when that velocity is known; for example, 1500 m/s is commonly used for marine surveys and 2000–2500 m/s for land surveys with strong weathering corrections.
Specifies the type of velocity values contained in the input gather, which determines the mathematical relationship used to shift the time reference (default: Vrms). Three options are available:
Vrms — Root-mean-square velocity (used for NMO correction). The module applies the Vrms combination formula: the output Vrms at each time sample is computed from the replacement velocity (V0) over the near-surface interval and the original Vrms from the topography onward, weighted by their respective two-way times. Use this option when the input gather contains stacking or RMS velocities.
Average — Average velocity (depth divided by two-way time). The module applies a depth-weighted average combination of V0 over the near-surface interval and the original average velocity from the topography onward. Use this option when the input gather contains average velocity picks for depth conversion. The datum value must be above the topographic surface.
Interval — Interval velocity (instantaneous layer velocity). The module applies the depth-based shift algorithm from the GShiftVelocitiesToDatumFilter. Use this option when the input gather contains interval velocities computed by Dix inversion or from a depth velocity model.
The output velocity gather with the same number of traces and samples as the input, but with the time axis shifted so that time zero corresponds to the specified datum elevation. The near-surface samples (above the original topography) are filled with the replacement velocity V0. The output datum elevation fields in the trace headers are updated to reflect the new reference. Connect this output to NMO correction, migration, or depth conversion modules that require a datum-referenced velocity model.