23 metres away from primary target interval, page-22

Measured depth and tvdrkb or tvdrt are measured from the kellybushings or rotary table on the rig floor, which is X metres above ground level (and varies for different rig types). Ground level is usually Y metres above sea level and depends on local topography.
Therefore when talking about depth to a reservoir etc you always correct it to a datum (the same value) which is usually sea level.
So TVDRT = TVDSS + Y + X

Hope that helps