Monitoring the Earth’s Atmosphere with the Global IMS Infrasound Network

Abstract

The comprehensive nuclear-test-ban treaty organization (CTBTO) is tasked with monitoring compliance with the comprehensive nuclear-test-ban treaty (CTBT), which bans nuclear weapon explosions underground, in the oceans, and in the atmosphere. The verification regime includes a globally distributed network of seismic, hydroacoustic, infrasound, and radionuclide stations, which collect and transmit data to the International Data Centre (IDC) in Vienna, Austria, shortly after the data are recorded at each station. The infrasound network defined in the protocol of the CTBT comprises 60 infrasound array stations. Each array is built according to the same technical specifications; it is typically composed of 4–9 sensors, with 1–3 km aperture geometry. This constitutes the first global infrasound network ever built with such a large and uniform distribution of stations.

Infrasound data at the IDC are processed at the station level using the progressive multichannel correlation (PMCC) algorithm, which calculates the signal correlation between sensors at an infrasound array. If the signal is sufficiently correlated and consistent over an extended period of time and frequency range, detection is created. Groups of detections are then categorized according to their propagation and waveform features, and a phase name is assigned for infrasound, seismic, or noise detections.

Nonnoise detections are then used in network processing at the IDC along with seismic and hydroacoustic technologies. The arrival phases detected on the three waveform technologies may be combined and used for locating events in an automatically generated bulletin of events. This automatic event bulletin is routinely reviewed by analysts during the interactive review process. The contribution of infrasound data to the final IDC event bulletin is currently at an early stage of testing and development. The IDC is exploring new software and procedures for fusing infrasound data with data from seismic and hydroacoustic stations. The number of false events generated by the large number of infrasound detections is a factor that complicates the introduction of infrasound data into routine operations. The IDC is currently working on reducing the number of false alarm infrasound events in the automatic bulletin to a manageable number before the analysts routinely review this bulletin.

A large collection of infrasound reference events (IRED) has been built by the IDC during the last few years, originating from natural or man-made sources. Most of the detected signals are associated to local or regional phenomena recorded by a single IMS infrasound station: man-made cultural activity, wind farms, aircraft, artillery exercises, ocean surf, thunderstorms, rumbling volcanoes, iceberg calving, aurora, and avalanches. Other signals may be recorded by several IMS infrasound stations at larger distances: ocean swell, sonic booms, and mountain-associated waves. However, only a small fraction of events meet the event definition criteria considering the Treaty verification mission of the Organization. Candidate event types for the IDC reviewed event bulletin (REB) include atmospheric or surface explosions, meteor explosions, rocket launches, signals from large earthquakes, and explosive volcanic eruptions.