If the source project has a SRD (Seismic Reference Datum) that is different from the Petrel SRD then the following data types will be shifted during import:
In order to know how much to shift the time values by you are prompted to supply a replacement velocity. This should be the linear average velocity for the interval between the two SRDs.

A default replacement velocity can be specified in the Match
Merge Option settings. The default value will be used and the replacement
velocity window will not appear during import.