SUDATUMK2DR - Kirchhoff datuming of receivers for 2D prestack data	
		(shot gathers are the input)				

    sudatumk2dr  infile=  outfile=  [parameters] 			

 Required parameters:							
 infile=stdin		file for input seismic traces			
 outfile=stdout	file for common offset migration output  	
 ttfile=		file for input traveltime tables		
   The following 9 parameters describe traveltime tables:		
 fzt= 			first depth sample in traveltime table		
 nzt= 			number of depth samples in traveltime table	
 dzt=			depth interval in traveltime table		
 fxt=			first lateral sample in traveltime table	
 nxt=			number of lateral samples in traveltime table	
 dxt=			lateral interval in traveltime table		
 fs= 			x-coordinate of first source			
 ns= 			number of sources				
 ds= 			x-coordinate increment of sources		

 fxi=                  x-coordinate of the first surface location      
 dxi=                  horizontal spacing on surface                   
 nxi=                  number of input surface locations               
 sgn=			Sign of the datuming process (up=-1 or down=1)  

 Optional Parameters:							
 dt= or from header (dt) 	time sampling interval of input data	
 ft= or from header (ft) 	first time sample of input data		
 surf="0,0;99999,0"  The first surface defined the recording surface 
 surf="0,0;99999,0"  and the second one, the new datum.              
                       "x1,z1;x2,z2;x3,z3;...
 fzo=fzt		z-coordinate of first point in output trace 	
 dzo=0.2*dzt		vertical spacing of output trace 		
 nzo=5*(nzt-1)+1 	number of points in output trace		",	
 fxso=fxt		x-coordinate of first shot	 		
 dxso=0.5*dxt		shot horizontal spacing		 		
 nxso=2*(nxt-1)+1  	number of shots 				
 fxgo=fxt		x-coordinate of first receiver			
 dxgo=0.5*dxt		receiver horizontal spacing			
 nxgo=nxso		number of receivers per shot			
 fmax=0.25/dt		frequency-highcut for input traces		
 offmax=99999		maximum absolute offset allowed in migration 	
 aperx=nxt*dxt/2  	migration lateral aperature 			
 angmax=60		migration angle aperature from vertical 	
 v0=1500(m/s)		reference velocity value at surface		
 dvz=0.0  		reference velocity vertical gradient		
 antiali=1             Antialiase filter (no-filter = 0)               
 jpfile=stderr		job print file name 				
 mtr=100  		print verbal information at every mtr traces	
 ntr=100000		maximum number of input traces to be migrated	

 verbose=0		silent, =1 chatty				

 Notes:								
 1. Traveltime tables were generated by program rayt2d (or other ones)	
    on relatively coarse grids, with dimension ns*nxt*nzt. In the	
    datuming process, traveltimes are interpolated into shot/gephone 	
    positions and output grids.					
 2. Input traces must be SU format and organized in common rec. gathers
 3. If the offset value of an input trace is not in the offset array 	
    of output, the nearest one in the array is chosen. 		
 4. Amplitudes are computed using the reference velocity profile, v(z),
    specified by the parameters v0= and dvz=.				
 5. Input traces must specify source and receiver positions via the header
    fields tr.sx and tr.gx. Offset is computed automatically.		


 Author:  Trino Salinas, 05/01/96,  Colorado School of Mines

 This code is based on sukzmig2d.c written by Zhenyue Liu, 03/01/95.
 Subroutines from Dave Hale's modeling library were adapted in
 this code to define topography using cubic splines.

 This code implements a Kirchhoff extraplolation operator that allows to
 transfer data from one reference surface to another.  The formula used in
 this application is a far field approximation of the Berryhill's original
 formula (Berryhill, 1979).  This equation is the result of a stationary
 phase analysis to get an analog asymptotic expansion for the two-and-one
 half dimensional extrapolation formula (Bleistein, 1984).

 The extrapolation formula permits the downward continuation of upgoing
 waves  and  upward  continuation  of  downgoing waves.  For upward conti-
 nuation of upgoing waves and downward continuation of downgoing waves,
 the conjugate transpose of the equation is used (Bevc, 1993).

 References :

 Berryhill, J.R., 1979, Wave equation datuming: Geophysics,
   44, 1329--1344.

 _______________, 1984, Wave equation datuming before stack
   (short note) : Geophysics, 49, 2064--2067.

 Bevc, D., 1993, Data parallel wave equation datuming with
   irregular acquisition topography :  63rd Ann. Internat.
   Mtg., SEG, Expanded Abstracts, 197--200.

 Bleistein, N., 1984, Mathematical methods for wave phenomena,
   Academic Press Inc. (Harcourt Brace Jovanovich Publishers),
   New York.