Topic:
Problems with increments of 0.1666666 in Shot Point tables.

Description:
Seismic loading does not create accurate enough SP translation tables. Loading
long 2D lines (15000 - 25000 traces) with the SEGY headers fully populated with:
  UTM X
  UTM Y
  Floating Point Shot point number
The geometry of the line is such that there are 6 traces per shot or a shotpoint
increment of 0.16666666...When the line is loaded it creates a translation table
with 2 entries and lists the increments as 0.166 and 0.167. The first SP number
is 20.5 and the final SP is 3079.0 from the SEGY header numbers. However, if you
display the line in Seis2DV and report XY and SP numbers you find the last trace
is SP 3096 (much larger than the actual number) and all traces passed SP 3079
have 0 0 locations.

The end result is that the traces along the line are given the wrong SP numbers,
apparently due to round off errors in the increment.

This can lead to traces being located in the wrong XY location.

Solution:
1) In Renumber 2D, delete the Shot Point tables and replace it with a single
line with the correct increment. But input the increment of .1666666 exactly.
The application will not display this increment, but does correct for it
internally. If you have alot of lines with multiple tables, use IESX_UTIL,
option 39. Compression of shotpoint tables to collaspe the tables down to one
entry but you will still have to input the increment for each 2D line.

2) In Edit 2D locations, click one of the location entries and then select
   'Change' to retie the line.  

Fixed in GeoFrame 3.5

 



Last Modified on: 07-APR-00