Program sfwave2dss | 1-D finite-difference wave extrapolation 
[SYNOPSIS]
sfwave2dss < inp.rsf > out.rsf vel=vel.rsf grad1=grad1.rsf grad2=grad2.rsf sl=nx/2 what=2 nb1=20 nb2=20
[PARAMETERS]
file    | grad1 | = |   | 	auxiliary input file name
file    | grad2 | = |   | 	auxiliary input file name
int     | nb1 | =20 |   | 	x boundary nodes 
int     | nb2 | =20 |   | 	z boundary nodes
int     | sl | =nx/2 |   | 	source location
file    | vel | = |   | 	auxiliary input file name
int     | what | =2 |   | 	2nd or 4th order for FD
[DIRECTORY]
user/songxl
