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 For NMO with a velocity function of time and CDP, specify the array	     

	   cdp=cdp1,cdp2,...						     

 and, for each CDP specified, specify the vnmo and tnmo arrays as described 

 above. The first (vnmo,tnmo) pair corresponds to the first cdp, and so on. 

 Linear interpolation and constant extrapolation of 1/velocity^2 is used    

 to compute velocities at CDPs not specified.				     



 Anisotropy option:							     

 Caveat, this is an experimental option,				     



 The anisotropy coefficients anis1, anis2 permit non-hyperbolicity due	     

 to layering, mode conversion, or anisotropy. Default is isotropic NMO.     



 The same holds for the anisotropy coefficients as a function of time and   

 CDP.									     



 Moveout is defined by							     



   1		 anis1							     

  --- x^2 + ------------- x^4.						     

  v^2	     1 + anis2 x^2						     



 Note: In general, the user should set the cdp parameter.  The default is   

	to use tr.cdp from the first trace and assume only one cdp.	  

 Caveat:								     

 Nmo cannot handle negative moveout as in triplication caused by	     

 anisotropy. But negative moveout happens necessarily for negative anis1 at 

 sufficiently large offsets. Then the error-negative moveout- is printed.   

 Check anis1. An error (anis2 too small) is also printed if the	     

 denominator of the quartic term becomes negative. Check anis2. These errors

 are prompted even if they occur in traces which would not survive the	     

 NMO-stretch threshold. Chop off enough far-offset traces (e.g. with suwind)

 if anis1, anis2 are fine for near-offset traces.			     





 Credits:

	SEP: Shuki, Chuck Sword

	CWP: Shuki, Jack, Dave Hale, Bjoern Rommel

      Modified: 08/08/98 - Carlos E. Theodoro - option for lateral offset

      Modified: 07/11/02 - Sang-yong Suh -

	  added "upward" option to handle decreasing velocity function.

      CWP: Sept 2010: John Stockwell

	  replaced Carlos Theodoro's fix

	  and added the instruction in the selfdoc to use suazimuth to set 

	    offset so that it accounts for lateral offset

      note that by the segy standard "scalel" does not scale the offset

      field

 Technical Reference:

	The Common Depth Point Stack

	William A. Schneider

	Proc. IEEE, v. 72, n. 10, p. 1238-1254

	1984



 Trace header fields accessed: ns, dt, delrt, offset, cdp, scalel



=head2 User's notes (Juan Lorenzo)
untested

=cut


=head2 CHANGES and their DATES

=cut

use Moose;