Re: How to apply depth shift on check-shot data
Our transit times are OK. Only depth corresponding each shot needs to be shifted -15 m for each single-shot. I am corious if we can ONLY shift depth and keep transit time?
A lot depends on what data you are looking at.
The checkshots usually have corrections applied to them for various things before they are used. For example, if you are dealing with an offshore well then the airguns will be lowered over the side of the rig, some distance away (horizontally and vertically) from the depth reference location (which might be the Kelly Bushing, or the drill floor); these corrections are all applied to the transit times. This is usually done before you get the data, however if you look on the raw logs from the checkshots you will see the full range of corrections applied to the transit times, usually in a table with some comments.
By contrast, the depths in a checkshot are usually supplied as "Measured Depth" and not altered - that is depth along the hole - which is usually the corrected version of the "logger's depth"; this is sometimes given as MDRKB (measured Depth Relative to KB) or MDRDF (relative to drill floor); the formation tops should are also given with the same depth reference, as will the wireline logs.
The correction that is usually done to both times and depths is to convert to TVDSS (True Vertical Depth Sub Surface) - this includes correcting the depth value to the same reference datum as the seismic (the Seismic Reference Datum, which is often Mean Sea Level - which may result in "negative" two-way-time values on land), as well as corrections for any deviation of the borehole from the vertical.
If you are using (interpretation) software then how to manage this varies; you can usually specify whether the checkshots you have are TVDSS or MDRKB, as well as the height of the KB above the SS reference, and whether the SS reference is Seal Level (MSL) or Ground Level (GL, and hence tied to the elevation of the well head)
So - whether the transit times are still valid if you subtract 15m off the depths in a checkshot file rather depends on the times (raw, or corrected, and which corrections are applied) the depths (whether they are MDRKB, TVDSS, or relative to some other datum) and what you want to use them for.
The general case would be that if you just remove 15m from the depths, the transit times will not be valid. I would also be VERY careful to document what you are doing and why in the checkshot file - having multiple versions of "checkshot" files that are slightly different with no description as to what was done will become very confusing in a few years time....
Here's a nice reference for the difference depth measurements used :http://en.wikipedia.org/wiki/Depth_in_a_well
Note the picture uses MDRRT - as the rig in question has a rotary table in place of the kelly bushing - however it shoes the differences nicely.
It also shows that sometimes LAT - lowest Astronomical tide - is used as a datum plane for the seismic measurements, in place of Mean Sea Level (MSL)