Topic:
How to load navigation from header when coordinate scalar is incorrect.

Description:
When navigation data is loaded from the trace header, it is automatically scaled
by the coordinate scalar found in byte 71.  Sometimes this scalar is incorrect
or has been omitted.

Solution:
Example : 

SEGY dump shows the following

  71-72   Coordinate scalar:             -10
  73-76   Source coordinate x:        523641
  77-80   Source coordinate y:       8582008
  81-84   Group coordinate x:         523641
  85-88   Group coordinate y:        8582000

Loading this navigation data in to IESX gives values of: 

       X =  52364.1 m
       Y = 858200.0 m  

The coordinate scalar in byte 71 is negative and causes the X and Y coordinates
to be divided by 10.  A positive coordinate scalar in byte 71 would, in this
case, cause the X and Y coordinates to be multiplied by 10.

Data may need to be loaded without using any coordinate scalar, or with a user
defined coordinate scalar.

1.) To load navigation data from header, without using any coordinate scalar.

    Use an expression to read the XY values directly, skipping the automatic
    scaler.

   DataManger > Load Seismic > Define Input format > Set header locations
 
   Define X and Y locations as Auxilary values A1, A2.
   Example :
   A1 =   73    IBM 32bit
   A2 =   77    IBM 32bit

  Set CDP to 'Expression', even if it is to be read from 'Trace Header'.
  This will allow you to go into the Shotpoint/CDP Expression panel.
  Set expression :

  CDP = CDP; XHDR=A1; YHDR=A2

This will not change the value of CDP that is read from the header, but allows
for creation of an expression with XHDR and YHDR to read the XY locations
directly from the trace header without using the coordinate scaler as specified
in byte 71 of the trace header.

If it is required to read the CDP as an expression this can be set in the usual
way  Eg. CDP = S1; XHDR=A1; YHDR=A2

2.) To load navigation data from headers with a user defined coordinate scalar.

   As above, but with coordinate scaling factor included in expression.
   Example :

   CDP = CDP; XHDR=A1*10; YHDR=A2*10





Last Modified on: 14-FEB-00