<s>
In	O
2001	O
,	O
ARL	O
fielded	O
infrasonic	O
sensors	O
(	O
<	O
20Hz	O
)	O
in	O
the	O
Republic	O
of	O
Korea	O
to	O
meet	O
a	O
U.S.	O
Army	O
requirement	O
for	O
passive	O
surveillance	O
systems	O
to	O
detect	O
artillery	O
fire	O
(	O
8	O
–	O
10Hz	O
)	O
in	O
the	O
vicinity	O
of	O
the	O
Demilitarized	B-General_Concept
Zone	I-General_Concept
.	O
</s>
