[Users] Start time in SMT SEG-Y files ...?

[Bert Bril]

Dear users,


As you know, SEG-Y import can be a difficult job. We try to provide a 
flexible loading tool that can handle many variants, even some that are 
clearly violating the standard.

Most 'large' vendors nowadays deliver Revision 1 SEG-Y files (as 
OpendTect does), which makes life a lot easier. This includes Landmark, 
Schlumberger, H&R and SMT software.

One of the problems still remaining in the SEG-Y standard is that there 
is no clear statement on how to handle the time of the first sample. The 
two common ways are: via the 'delrt' or the 'laga' trace header field. 
The 'laga' method is used - as far as I know - by H&R and I believe also 
Petrel and SMT. The sign of this value is described in a rather cryptic 
way in the SEG-Y standard, but I agree with H&R (and I think Petrel) 
that it should be opposite to the 'delrt' method. So data starting at 
100 ms should get '-100' in the 'laga' field, and/or '100' in delrt 
(OpendTect reads/writes both).

A few days ago I got a support call about a problem with start time with 
an SMT-generated SEG-Y file. It said that the sign in the SMT file was 
opposite the above stated (i.e. laga was filled in the same way as 
delrt). I think that is not right, and all SEG-Y files I have seen so 
far had the opposite sign.

Is there maybe a setting in the software to prevent this?

Otherwise I can only hope that SMT will change this. If you are 
confronted with this problem, you can work around it by using an 
overrule for the start time.


Bert

-- 
-- Bert Bril / OpendTect developer at dGB
-- mailto:Bert.Bril at opendtect.org , http://opendtect.org