Welcome to Petroleum Geology Forums

This is a free online community that aims to bring petroleum professionals and geologists together and share valuable knowledge. Registration is easy so become a member now for instant free access.
  • Petroleum Geologists can stay up to date with industry related topics and exchange ideas and concepts.
  • Upstream Oil and Gas Consultants get a chance to share their expertise and gain exposure to land future projects.
  • Geology students and graduates can join the discussion and get into contact with potential future employees.

  >> Register Now





Post new topic Reply to topic  [ 4 posts ] 
How to apply depth shift on check-shot data 
Author Message

Joined: Fri Jun 06, 2014 6:44 am
Posts: 2
Post How to apply depth shift on check-shot data
hi All,

I am wondering if it is possible to change depth of recorded check-shot data (each stack or shot) after recording finished? I heard it is possible to change it while processing. I would appreciate if someone could help me.

Many Thanks,

Regards
-mehmet


Fri Jun 06, 2014 7:12 am

Joined: Sat Feb 15, 2014 7:33 am
Posts: 31
Post Re: How to apply depth shift on check-shot data
aras1227 wrote:
hi All,

I am wondering if it is possible to change depth of recorded check-shot data (each stack or shot) after recording finished? I heard it is possible to change it while processing. I would appreciate if someone could help me.

Many Thanks,

Regards
-mehmet


Do you mean checkshots or a Vertical Seismic Profile?

Checkshots link the depth to the seismic transit time from the surface; if you start to mess with the depths or times you are destroying the measured relationship.

You could shift things around in a way that preserved the travel times by converting from (depth,time) pairs to (depth ,interval velocity) pairs and then changing the depths (you could do the same with the times too - however I'd be cautious about doing this.


Sun Jun 08, 2014 4:11 am

Joined: Fri Jun 06, 2014 6:44 am
Posts: 2
Post 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?


Sun Jun 08, 2014 1:38 pm

Joined: Sat Feb 15, 2014 7:33 am
Posts: 31
Post Re: How to apply depth shift on check-shot data
aras1227 wrote:
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....

EDIT :

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)


Sun Jun 08, 2014 5:17 pm
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 


Related topics 
 Topics   Author   Replies   Views   Last post 
There are no new unread posts for this topic. How to estimate Sonic velocity above first logged depth?

Rashied

2

1083

Sun Dec 21, 2014 9:11 pm

GuyM View the latest post

There are no new unread posts for this topic. Seismic Depth Imaging in Structurally Complex setting

crolorpret

1

1649

Mon May 21, 2012 2:12 am

seismic dunedain View the latest post

There are no new unread posts for this topic. Stochastic Inversion of pre-stack data

rajesh_madan61

0

819

Wed Feb 19, 2014 9:05 am

rajesh_madan61 View the latest post

There are no new unread posts for this topic. Best practices for seismic data acquisition in highly undula

Ramesh Srivastava

0

144

Wed May 31, 2017 8:28 am

Ramesh Srivastava View the latest post

 



Search for:
Jump to:  


Content on EPGeology.com is intended for personal use only and to supplement, not replace, professional judgment. EPGeology.com disclaims any and all liability for your use of its content. As most of our content is supplied by our users we can not check copyright, and stress that copyright remains at the original owner. If you suspect copyright infringement please use the contact form to report it.
Contact || © EPGeology.com. || Powered by phpBB Asteroid Mining

phpBB SEO