Change the default check shot source elevation, which by default is the top of the sonic log, to align with the seismic time zero. The check shot data is typically at the KB whereas seismic data is at the highest elevation above the seismic survey to account for variations in surface topography.

Specify a velocity that matches the speed of the subsurface material found in the surface elevations. This correction is applied to surface valleys to correct the subsurface readings for distortions caused by varying surface topography. The change in time is the difference between the check shot time datum and the value in the Time Datum box divided by the datum velocity.
GeoSyn calculates a check shot correction without reference to this value, then as a final step, adds or subtracts section to correct the new time datum.

Select either a Spline curve or Linear correction type.

Select from the following:
Drift Curve A record of the transit correction applied to each sample of the original active sonic. Inspect this log for large drift corrections.
Check Shot Transit Curve A record of the velocity field observed by the check shot. This log should be a rough approximation of the sonic log and is particularly useful for spotting datum errors, such as an incorrect well KB or check shot depth datum.
Create New Tops List for Check Shots to import the check shot depth time pairs as tops. You can then check the position of these tops in time on the synthetic against the check shot time values. These times differ if the synthetic is moved to a new datum.