Program sffagrad | Calculating frequency attenuation gradient. 
[SYNOPSIS]
sffagrad < in.rsf > out.rsf grad=y lperc=65. hperc=85. freq= type=
[PARAMETERS]
float   | freq | = |   | 	Frequency corresponding to energy ratio, valid when type=ratio 
bool    | grad | =y |  [y/n] | 	If y, output attenuation gradient; if n, output absorption factor 
float   | hperc | =85. |   | 	High percentage of total energy 
float   | lperc | =65. |   | 	Low percentage of total energy 
string  | type | = |   | 	[low,full,ratio,attenuation] attribute type, the default is attenuation  
[DIRECTORY]
user/yliu
