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 dz=         spatial increment in vertical direction                    
 dt=         time sample interval in seconds                            
 isx=        grid point # of horizontal source positions                
 isz=        grid point # of vertical source positions                  

 Optional parameters:                                                   
 fx=0.0      first horizontal coordinate                                
 fz=0.0      first vertical coordinate                                  
 irx=        horizontal grid point # of vertical receiver lines         
 irz=        vertical grid point # of horizontal receiver lines         
 w=0.1       width of spatial source distribution (see notes)           
 sflag=2     source time function                                       
             0: user supplied source function                           
             1: impulse (spike at t=0)                                  
             2: Ricker wavelet                                          
 fmax=       maximum frequency of Ricker (default) wavelet              
 amps=1.0    amplitudes of sources                                      
 prec=0      1: precompute Bessel coefficients b_k (see notes)          
             2: use precomputed Bessel coefficients b_k                 
 fsflag=0    1: perform run with free surface b.c.                      
 vmaxu=      user-defined maximum velocity                              
 dtsnap=0.0  time interval in seconds of wave field snapshots           
 iabso=1     apply absorbing boundary conditions (0: none)              
 abso=0.1    damping parameter for absorbing boundaries                 
 nbwx=20     horizontal width of absorbing boundary                     
 nbwz=20     vertical width of absorbing boundary                       
 verbose=0   1: show parameters used                                    
             2: print maximum amplitude at every expansion term         

 velfile=vel          velocity filename                                 
 densfile=dens        density filename                                  
 sname=wavelet.su     user supplied source time function filename       
 sepxname=sectx.su    x-direction pressure sections filename            
 sepzname=sectz.su    z-direction pressure sections filename            
 snpname=snap.su      pressure snapshot filename                        
 jpfile=stderr        diagnostic output                                 

 Notes:                                                                 
  0. The combination of the Fourier method with REM time integration    
     allows the computation of synthetic seismograms which are free     
     of numerical grid dispersion. REM has no restriction on the        
     time step size dt. The Fourier method requires at least two        
     grid points per shortest wavelength.                               
  1. nx and nz must be valid numbers for pfafft transform lengths.      
     nx and nz must be odd numbers (unless opflag=1). For valid         
     numbers see e.g. numbers in structure 'nctab' in source file       
     $CWPROOT/src/cwp/lib/pfafft.c.                                     
  2. Velocities (and densities) are stored as plain C style files       
     of floats where the fast dimension is along the z-direction.       
  3. Units must be consistent, e.g. m, s and m/s.                       
  4. A 20 grid points wide border at the sides and the bottom of        
     the modeling grid is used for sponge boundary conditions           
     (default: iabso=1).                                                
     Source and receiver lines should be placed some (e.g. 10) grid     
     points away from the absorbing boundaries in order to reduce       
     reflections due to obliquely incident wavefronts.                  
  5. Dominant frequency is about fmax/2 (sflag=2), absolute maximum     
     is delayed by 3/fmax from beginning of wavelet.                    
  6. If opflag!=1 the source should be not a spike in space; the        
     parameter w determines at which distance (in grid points) from     
     the source's center the Gaussian weight decays to 10 percent       
     of its maximum. w=2 may be a reasonable choice; however, the       
     waveform will be distorted.                                        
  7. Horizontal and vertical receiver line sections are written to      
     separate files. Each file can hold more than one line.             
  8. Parameter vmaxu may be enlarged if the modeling run becomes        
     unstable. This happens if the largest eigenvalue of the modeling   
     operator L is larger than estimated from the largest velocity.     
     In particular if using the variable density acoustic wave          
     equation the eigenvalues depend also on the density and it is      
     impossible to estimated the largest eigenvalue analytically.       
  9. Bessel coefficients can be precomputed (prec=1) and stored on      
     disk to save CPU time when several shots need to be run.           
     In this case computation of Bessel coefficients can be skipped     
     and read from disk file for reuse (prec=2).                        
     For reuse of Bessel coefficients the user may need to define       
     the overall maximum velocity (vmaxu).                              
 10. If snapshots are not required, a spike source (sflag=1) may be     
     applied and the resulting impulse response seismograms can be      
     convolved later with a desired wavelet.                            
 11. The free surface (fsflag=1) does not coincide with the first       
     vertical grid index (0). It appears to be half a grid spacing      
     above that position.