Add zones above the sonic log to lower it to the same depth as the seismic section (time zero). These zones can be hidden in the main display but still exert their effect on the main display. The added zones are helpful when doing AVO analysis instead of using the Log Editor to manually add visible sections to the top of the log.
To change time datum properties
1. From the Edit menu, click Time Datum (
).
The Time Datum Properties dialog box opens. If the model was built by importing data from Kingdom, a zone is automatically created to account for the difference between the top of seismic section and the top of the sonic log.
2. To the right of the Layers label, click Add to create a default zone with a thickness of zero to the top of the Add Layered Section... pane of the Time Datum Properties dialog box.
3. Hover the cursor over fields in the newly created row until the cursor appears as follows: 
, click to type details for a given field, and then press ENTER or click a different location in the pane.
4. Select from the following options:
Tie depth point to time - click to display the Tie to Datum dialog box where you correlate a sonic log depth to a time point on the seismic section. This tie point is honored throughout GeoSyn, for example, whey you export SEGY data.
Convert to log data - click to convert the otherwise invisible section to a visible part of the log in the main display.