SUMIGPSPI - Gazdag's phase-shift plus interpolation migration         
            for zero-offset data, which can handle the lateral         
            velocity variation.                                        

 sumigpspi <infile >outfile vfile= [optional parameters]               
 
 Required Parameters:							
 nz=		number of depth sapmles					
 dz=		depth sampling interval					
 vfile=	name of file containing velocities			
		(Please see Notes below concerning the format of vfile)	

 Optional Parameters:                                                  
 dt=from header(dt) or .004    time sampling interval                  
 dx=from header(d2) or 1.0     midpoint sampling interval              

 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()             

 Notes:								
 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, April 20th, 1998

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