Untitled Document
Enhanced detection capability at infrasound stations
in the global CTBT verification network
Douglas R. Christie
Research School of Earth Sciences, Australian National
University, Canberra, ACT 0200, Australia
A 60-station global infrasound monitoring network is being constructed
as part of the verification regime for the Comprehensive Nuclear-Test-Ban
Treaty (CTBT). Nearly 70% of this network has been established and it
is anticipated that the network will be completed in the next few years.
The network, which is far larger and much more sensitive than any previous
infrasound monitoring network, consists of state-of-the-art infrasonic
array stations distributed as uniformly as possible over the face of
the globe. Current studies indicate that the final global network will
reliably detect signals from a 1-kiliton atmospheric nuclear explosion
at two or more monitoring stations. Research at the ANU during the past
few years has focused on the development of techniques that can lower
detection thresholds, improve location capability and enhance the overall
reliability of the network. This has resulted in the development of an
optimal infrasonic array design that eliminates problems with spatial
aliasing of high frequency signals and problems with signal coherence
between array elements. Substantial work has also been carried out on
the development of a new and effective technique for reducing wind-generated
background noise.
Wind-generated background noise is still the most significant problem
at many stations in the global infrasound monitoring network. Wind-generated
noise may seriously limit detection capability at stations located in
high wind environments with little shelter from the ambient winds. A
wind-noise-reducing pipe array is currently used at all infrasound stations
in the CTBTO verification network. While these devices provide significant
noise reduction, the level of background noise at some stations remains
unacceptably high, especially during the daytime. It is generally recognized
that further improvements in pipe array design will not resolve this
problem. Work at the ANU on infrasound background noise reduction
has therefore been concerned with a new approach to the wind-noise problem
that appears to have the potential to effectively eliminate wind noise
at most monitoring stations.
This technique is based on the use of a
turbulence-reducing enclosure constructed from a series of screens positioned
around the sensor inlet ports. A large variety of enclosures have
been constructed and tested near one of the standard International Monitoring
System (IMS) array elements at IS07, Warramunga. A larger than usual
20-m diameter enclosure (version 6) with three concentric porous walls
was tested during the year in an attempt to improve the longer period
performance of the device. Rather surprising, the performance of this
large diameter device was found to be almost identical to a smaller 14-m
diameter enclosure (version 5B) with two concentric walls. Both of these
enclosures suppress wind-generated noise by up to four orders of magnitude
at higher frequencies, even when the sensor is connected to only a single
inlet port located at the center of the enclosure.
We conclude
that version 5B shown in Figure 1 is the most effective practical design
for a turbulence-reducing enclosure.
The performance of version 5B has also been compared directly with the
performance of a standard IMS 96-port 18-m diameter pipe array at site
H1 at IS07 Warramunga. This comparison shows that the degree of noise
reduction provided by the turbulence reducing enclosure with only a single
inlet port is more than two orders of magnitude better than the standard
96-port pipe array at higher frequencies. The performance of the pipe
array is, however, slightly better at low frequencies. This suggests
that the performance of existing pipe arrays at IMS infrasound stations
can be improved very substantially by enclosing the pipe array inside
a turbulence-reducing enclosure similar to version 5B. It is recommended
that all new IMS stations should be constructed with wind-noise-reducing
pipe arrays located inside a turbulence-reducing enclosure
Version 5B of the infrasonic
wind-noise-reducing enclosure.