The European Infrasound Bulletin highlights infrasound activity produced mostly by anthropogenic sources, recorded all over Europe and collected in the course of the ARISE and ARISE2 projects (Atmospheric dynamics Research InfraStructure in Europe). Data includes high-frequency (> 0.7 Hz) infrasound detections at 24 European infrasound arrays from nine different national institutions complemented with infrasound stations of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Data were acquired during 16 years of operation (from 2000 to 2015) and processed to identify and locate ∼ 48,000 infrasound events within Europe. The source locations of these events were derived by combining at least two corresponding station detections per event. Comparisons with ground-truth sources, e.g., Scandinavian mining activity, are provided as well as comparisons with the CTBT Late Event Bulletin (LEB). Relocation is performed using ray-tracing methods to estimate celerity and back-azimuth corrections for source location based on meteorological wind and temperature values for each event derived from European Centre for Medium-range Weather Forecast (ECMWF) data. This study focuses on the analysis of repeating, man-made infrasound events (e.g., mining blasts and supersonic flights) and on the seasonal, weekly and diurnal variation of the infrasonic activity of sources in Europe. Drawing comparisons to previous studies shows that improvements in terms of detection, association and location are made within this study due to increasing the station density and thus the number of events and determined source regions. This improves the capability of the infrasound station network in Europe to more comprehensively estimate the activity of anthropogenic infrasound sources in Europe.
Large earthquakes that occurred in the Sumatra region in 2004 and 2005 generated acoustic waves recorded by the Diego Garcia infrasound array. The Progressive Multi‐Channel Correlation (PMCC) analysis is performed to detect the seismic and infrasound signals associated with these events. The study is completed by an inverse location procedure that permitted reconstruction of the source location of the infrasonic waves. The results show that ground motion near the epicenter and vibrations of nearby land masses efficiently produced infrasound. The analysis also reveals unique evidence of long period pressure waves from the tsunami earthquake (M9.0) of December 26, 2004.
Volcanic eruptions are valuable calibrating sources of infrasonic waves worldwide detected by the International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO) and other experimental stations. In this study, we assess the detection capability of the European infrasound network to remotely detect the eruptive activity of Mount Etna. This well-instrumented volcano offers a unique opportunity to validate attenuation models using multi-year near-and far-field recordings. The seasonal trend in the number of detections of Etna at the IS48 IMS station (Tunisia) is correlated to fine temporal fluctuations of the stratospheric waveguide structure. This observed trend correlates well with the variation of the effective sound speed ratio which is a proxy for the combined effects of refraction due to sound speed gradients and advection due to along-path wind on infrasound propagation. Modeling results are consistent with the observed detection capability of the existing regional network. In summer, during the downwind season, a minimum detectable amplitude of ~10 Pa at a reference distance of 1 km from the source is predicted. In winter, when upwind propagation prevails, detection thresholds increase up to ~100 Pa. However, when adding four experimental arrays to the IMS network, the corresponding thresholds decrease down to ~20 Pa in winter. The simulation results provide here a realistic description of long- to mid-range infrasound propagation and allow predicting fine temporal fluctuations in the European infrasound network performance with potential application for civil aviation safety.
The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is exploring methods to enhance the current operational infrasound processing system at the International Data Centre (IDC) for infrasound data recorded by the International Monitoring System (IMS). Several enhancements are under development and are currently being tested. The first enhancement is the incorporation of methods for determining signal amplitude. The following signal amplitudes are being determined for each infrasound detection: peak-to-peak amplitude, root mean squared (RMS) amplitude, and instantaneous amplitude as revealed by the analytic trace via the Hilbert Transform. Initial efforts consider a variety of frequency bands, with the utility in network processing being of primary importance. A second enhancement is the incorporation of station noise characterization in terms of the power spectral density (PSD). This determines the noise field at each station for various times of day and as a function of season. This information is useful in determining network detection capability.
Atmospheric infrasound signals are observed across a wide frequency range (~0.01-20 Hz, Campus and Christie, 2010). We may consider three general frequency bands of interest: • Above 0.5 Hz: impulse signals of natural or man-made origin, which may propagate over distances of several hundred kilometers. • Between 0.1 and 0.5 Hz: microbaroms and isolated large remote events such as explosions, meteorites and volcanoes. • Below 0.1 Hz: large-scale atmospheric disturbances such as mountain associated waves and auroral infrasound.
A global‐scale analysis of detections made at all 36 currently operating International Monitoring System (IMS) infrasound arrays confirms that the primary factor controlling signal detectability is the seasonal variability of the stratospheric zonal wind. At most arrays, ∼80% of the detections in the 0.2‐ to 2‐Hz bandpass are associated with propagation downwind of the dominant stratospheric wind direction. Previous IMS infrasound network performance models neglect the time‐ and site‐dependent effects of both stratospheric meteorological variability and ambient noise models. In this study both effects are incorporated; we compare empirical and improved specifications of the stratospheric wind and include station‐dependent wind noise models. Using a deterministic approach, the influence of individual model parameters on the network performance is systematically assessed. At frequencies of interest for detecting atmospheric explosions (0.2–2 Hz), the simulations predict that explosions equivalent to ∼500 t of TNT would be detected by at least two stations at any time of the year. The detection capability is best around January and July when stratospheric winds are strongest, compared to the equinox periods when zonal winds reduce and reverse. The model predicts that temporal fluctuations in the ground‐to‐stratosphere meteorological variables generate detection threshold variations on daily and seasonal timescales of ∼50 and ∼500 t, respectively. While the strong zonal winds lead to an improvement in detection capability, their highly directional nature leads to an increase in the location uncertainty owing to the decreased azimuthal separation of the detecting stations.
