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DZDV - determine depth derivative with respect to the velocity

  parameter, dz/dv,  by ratios of migrated data with the primary
  amplitude and those with the extra amplitude                   

 dzdv <infile afile=afile dfile=dfile>outfile [parameters]       

 Required Parameters:                                      
 infile=    input of common image gathers with primary amplitude 
 afile=     input of common image gathers with extra amplitude   
 dfile=     output of imaged depths in common image gathers      
 outfile=   output of dz/dv at the imaged points                 
 nx=         number of migrated traces                    
 nz=          number of points in migrated traces                
 dx=        horizontal spacing of migrated trace                 
 dz=          vertical spacing of output trace             
 fx=          x-coordinate of first migrated trace               
 fz=          z-coordinate of first point in migrated trace            
 off0=         first offset in common image gathers              
 noff=              number of offsets in common image gathers          
 doff=              offset increment in common image gathers           
 cip=x1,z1,r1,..., cip=xn,zn,rn         description of input CIGS
      x     x-value of a common image point                      
      z     z-value of a common image point     at zero offset         
      r     r-parameter in a common image gather                 

 Optional Parameters:                                      
 nxw, nzw=0       window widths along x- and z-directions in     
                  which points are contributed in solving dz/dv.

 This program is used as part of the velocity analysis technique developed
 by Zhenyue Liu, CWP:1995.                                 

 Author: CWP: Zhenyue Liu,  1995
 Liu, Z. 1995, "Migration Velocity Analysis", Ph.D. Thesis, Colorado
      School of Mines, CWP report #168.