I have Petrel 2011.2 version.
I am trying to upload checkshots for a well and i am not sure how it is supposed to be written.
my first value is 0 (twt) correlating to 12.85 (MD). how do i know if 12.85 means below seafloor or above seafloor. my kelly bushing is 12.65 m. And do i need to upload the checkshots in negative or positive signs?
when i upload the sonic log and the checkshots to the calibration window i get an error message that the checkshot zero time is not at the elevation of checkshot tz datum. what should i do?
I am a bit confused...
Thanks a lot!
I'm not a Petrel expert, however there's a couple of things that might help.
Checkshots are used to tie the Seismic Reference Datum to Measured Depth; both of these can have subtle wrinkles.
The seismic data is usually referenced to a given vertical datum when it is supplied to the oil company. Offshore this usually means it has been corrected to Mean Sea Level, however sometimes I have seen LAT (Lowest Astronomical Tide) used as a reference. This is usually referred to as the SRD or Seismic Reference Datum.
Onshore it might be corrected to mean sea level, however this means that when you get above ground you have "negative time"; not all software supports that concept and some might "cut off" the top of your seismic - and not all SEGY data will use the same standard in the headers to indicate this. You should have a reference datum and a velocity that was used to calculate the seismic datum onshore, since the seismic data is in time, and is recorded relative to elevation.
In a lot of interpretation software, the SRD is "fixed" to mean sea level, and can't be altered.
Wikipedia has a surprisingly useful page on measured depth:http://en.wikipedia.org/wiki/Depth_in_a_well
So - ideally, your first checkshot point should be at the Seismic reflection datum.
This is not what you are seeing - you have 12.65m for the KB, and a 12.85m measured depth for the checkshot point at the Seismic Reference Datum.
So - what might be wrong?
- someone made a mistake
Pretty easy to mistype a 6 for an 8, especially if the number had been hand-written originally. Stuff happens in the field - people are tired, under pressure, and mistakes get made. Look to see if you can find any paperwork with the KB on it- don't sweat the small stuff
You have a possible 20cm variation between the two values. The speed of sound in water is 1500 m/s. The difference in depth values corresponds to 0.266ms two-way-time - so one quarter of one millisecond. The seismic data is likely to be at a four millisecond sample interval, perhaps one millisecond at most, for most oil exploration work - or to put it another way, unless you have frequencies over 1500Hz in your seismic data, it doesn't matter. I'd take the KB measurement as being accurate, and change the first checkshot point.check the datums
Measured Depth is often relative to the kelly bushing (MDRKB), but it could be referenced to something else (drill floor : MDRDF), which might have been measured slightly differently. If you dig into things you might be able to find out there was a slightly different measurement used. However - as I comment above, its probably not worth it.