SUMIGFFD - Fourier finite difference migration for		    
	    zero-offset data. This method is a hybrid migration which 
	    combines the advantages of phase shift and finite difference", 
	    migrations.						

 sumigffd <infile >outfile vfile= [optional parameters]		

 Required Parameters:						  
 nz=		   number of depth sapmles			 ", 
 dz=		   depth sampling interval			 
 vfile=		name of file containing velocities	      

 Optional Parameters:						  
 dt=from header(dt) or .004    time sampling interval		  
 dx=from header(d2) or 1.0     midpoint sampling interval	      
 ft=0.0			first time sample			
 fz=0.0			first depth sample		      

 tmpdir=	if non-empty, use the value as a directory path	
		prefix for storing temporary files; else if the	
		the CWP_TMPDIR environment variable is set use	 
		its value for the path; else use tmpfile()	     
 
 The input velocity file \'vfile\' consists of C-style binary floats.  ",  
 The structure of this file is vfile[iz][ix]. Note that this means that
 the x-direction is the fastest direction instead of z-direction! Such a
 structure is more convenient for the downward continuation type	
 migration algorithm than using z as fastest dimension as in other SU  ", 
 programs.						     

 Because most of the tools in the SU package (such as  unif2, unisam2, ", 
 and makevel) produce output with the structure vfile[ix][iz], you will
 need to transpose the velocity files created by these programs. You may
 use the SU program \'transp\' in SU to transpose such files into the  
 required vfile[iz][ix] structure.				     




 Credits: CWP Baoniu Han, July 21th, 1997


 Trace header fields accessed: ns, dt, delrt, d2
 Trace header fields modified: ns, dt, delrt