Depth reference:The point in a well from which depth is measured.
Alternatively, the depth reference is the point at which the depth is defined as being zero. it is typically the top of the kelly bushing or the level of the rig floor on the rig used to drill the well.
The depth measured from that point is the measured depth (md) for the well.
Even when the drilling rig has been removed, all subsequent measurements and operations in the well are still tied in to the same depth reference.
However, for multiwell studies, the depths are normally shifted to the permanent datum (generally mean sea level).
The depth reference and its elevation above the permanent datum are recorded on the log heading.
In some contexts, the term may refer to any point from which depth is measured.