Program sfwaveab | 1-D finite-difference wave extrapolation 
[SYNOPSIS]
sfwaveab < inp.rsf > out.rsf vel=vel.rsf grad=grad.rsf what=2 nb=10
[PARAMETERS]
file    | grad | = |   | 	auxiliary input file name
int     | nb | =10 |   | 	2nd or 4th order for FD
file    | vel | = |   | 	auxiliary input file name
int     | what | =2 |   | 	2nd or 4th order for FD
[DIRECTORY]
user/songxl
