How do I export a vtk cube as 3D SEG-Y?

 
You need 3D Seismics. Check that box in the Plugin options and Apply.
 
If the command Convert to Seismic 3D still does not appear when right-clicking your Cube , try Schema Command > Reset to Default or add the command manually.

·         To create a Seismic Cube from a Structured Points Cube: right-click the Cube  > Convert to Seismic 3D
·         The resulting Seismic Cube can be exported to SEG-Y by right-click > Export > SEG-Y
·         To export a single in-line (or cross line) from a Seismic Cube  you must first create a Seismic Line
·         To create a Seismic Line : Right-click the Seismic Cube  > Seismic Subset 3D; check the Slice check-box and select correct in-line (cross line)
·         The resulting Seismic Line can now be exported to SEG-Y
 
In the past, exporting to Kingdom via SEG-Y, we’ve experienced problem with the Kingdom importer when it comes to 8-bit sample data sets. The revised SEG-Y standard specifies data sample format code for 8-bit data that Kingdom did not recognize. You need to set the sample format manually (but maybe that’s not be a problem anymore).