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