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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.