Program sffd2bs | 2-D Fourth-order Finite-difference wave extrapolation 
[SYNOPSIS]
sffd2bs > out.rsf vel=vel.rsf < source.rsf opt=y dt= nt= isx= isz= nb=30 c=0.01
[PARAMETERS]
float   | c | =0.01 |   | 	decaying parameter
float   | dt | = |   | 	
int     | isx | = |   | 	
int     | isz | = |   | 	
int     | nb | =30 |   | 	
int     | nt | = |   | 	
bool    | opt | =y |  [y/n] | 	if y, determine optimal size for efficiency 
file    | vel | = |   | 	auxiliary input file name
[DIRECTORY]
user/songxl
