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Vienna, Austria

Drob D.P.,U.S. Navy | Garces M.,University of Hawaii at Manoa | Hedlin M.,University of California at San Diego | Brachet N.,International Data Center
Pure and Applied Geophysics | Year: 2010

Expert knowledge suggests that the performance of automated infrasound event association and source location algorithms could be greatly improved by the ability to continually update station travel-time curves to properly account for the hourly, daily, and seasonal changes of the atmospheric state. With the goal of reducing false alarm rates and improving network detection capability we endeavor to develop, validate, and integrate this capability into infrasound processing operations at the International Data Centre of the Comprehensive Nuclear Test-Ban Treaty Organization. Numerous studies have demonstrated that incorporation of hybrid ground-to-space (G2S) enviromental specifications in numerical calculations of infrasound signal travel time and azimuth deviation yields significantly improved results over that of climatological atmospheric specifications, specifically for tropospheric and stratospheric modes. A robust infrastructure currently exists to generate hybrid G2S vector spherical harmonic coefficients, based on existing operational and emperical models on a real-time basis (every 3- to 6-hours) (Drobet al.,2003). Thus the next requirement in this endeavor is to refine numerical procedures to calculate infrasound propagation characteristics for robust automatic infrasound arrival identification and network detection, location, and characterization algorithms. We present results from a new code that integrates the local (range-independent) τp ray equations to provide travel time, range, turning point, and azimuth deviation for any location on the globe given a G2S vector spherical harmonic coefficient set. The code employs an accurate numerical technique capable of handling square-root singularities. We investigate the seasonal variability of propagation characteristics over a five-year time series for two different stations within the International Monitoring System with the aim of understanding the capabilities of current working knowledge of the atmosphere and infrasound propagation models. The statistical behaviors or occurrence frequency of various propagation configurations are discussed. Representative examples of some of these propagation configuration states are also shown. © 2010 US Government.

Wilmut M.J.,University of Victoria | Chapman N.R.,University of Victoria | Prior M.,International Data Center
IEEE Journal of Oceanic Engineering | Year: 2010

The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) operates a network of underwater hydrophones as part of the International Monitoring System (IMS). Data from this network are processed at the International Data Centre (IDC), Vienna, Austria. One of the objectives is to identify the signals that are due to an underwater explosion, the so-called H-phase signals. Data provided by IDC were examined to investigate new automated processing schemes that could significantly reduce the number of signals needed to be analyzed by human experts, while still detecting with high probability the rare H-phase signals. A variant of quadratic classification (QC) using four signal characteristics from the time, frequency, and cepstrum domains was applied to the problem. It was found that 97.5% of the received H-phase signals are detected by the automated QC process. These H-phase signals are among only about 1% of the signals which are allowed to form event solutions for further analysis by experienced experts. © 2005 IEEE.

Evers L.G.,Royal Netherlands Meteorological Institute | Evers L.G.,Technical University of Delft | Brown D.,International Data Center | Heaney K.D.,Ocean Acoustical Services and Instrumentation Systems | And 4 more authors.
Geophysical Research Letters | Year: 2014

Atmospheric low-frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here we report for the first time on atmospheric infrasound from a large underwater earthquake (M w 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves. Key Points Evanescent wave coupling links the solid Earth, oceans, and atmosphere Acoustic waves use anomalous transparency of the water-air interface Underwater geophysical processes and events can be heard in the atmosphere ©2014. American Geophysical Union. All Rights Reserved.

Le Bras R.J.,University of Vienna | Le Bras R.J.,International Data Center | Kuzma H.,Chatelet Resources | Sucic V.,University of Rijeka | Bokelmann G.,University of Vienna
Journal of the Acoustical Society of America | Year: 2016

A notable sequence of calls was encountered, spanning several days in January 2003, in the central part of the Indian Ocean on a hydrophone triplet recording acoustic data at a 250 Hz sampling rate. This paper presents signal processing methods applied to the waveform data to detect, group, extract amplitude and bearing estimates for the recorded signals. An approximate location for the source of the sequence of calls is inferred from extracting the features from the waveform. As the source approaches the hydrophone triplet, the source level (SL) of the calls is estimated at 187 ± 6 dB re: 1 μPa-1 m in the 15-60 Hz frequency range. The calls are attributed to a subgroup of blue whales, Balaenoptera musculus, with a characteristic acoustic signature. A Bayesian location method using probabilistic models for bearing and amplitude is demonstrated on the calls sequence. The method is applied to the case of detection at a single triad of hydrophones and results in a probability distribution map for the origin of the calls. It can be extended to detections at multiple triads and because of the Bayesian formulation, additional modeling complexity can be built-in as needed. © 2016 Acoustical Society of America.

Fee D.,University of Alaska Fairbanks | Waxler R.,University of Mississippi | Assink J.,University of Mississippi | Gitterman Y.,Geophysical Institute of Israel | And 8 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

Three large-scale infrasound calibration experiments were conducted in 2009 and 2011 to test the International Monitoring System (IMS) infrasound network and provide ground truth data for infrasound propagation studies. Here we provide an overview of the deployment, detonation, atmospheric specifications, infrasound array observations, and propagation modeling for the experiments. The experiments at the Sayarim Military Range, Israel, had equivalent TNT yields of 96.0, 7.4, and 76.8 t of explosives on 26 August 2009, 24 January 2011, and 26 January 2011, respectively. Successful international collaboration resulted in the deployment of numerous portable infrasound arrays in the region to supplement the IMS network and increase station density. Infrasound from the detonations is detected out to ~3500 km to the northwest in 2009 and ~6300 km to the northeast in 2011, reflecting the highly anisotropic nature of long-range infrasound propagation. For 2009, the moderately strong stratospheric wind jet results in a well-predicted set of arrivals at numerous arrays to the west-northwest. A second set of arrivals is also apparent, with low celerities and high frequencies. These arrivals are not predicted by the propagation modeling and result from unresolved atmospheric features. Strong eastward tropospheric winds (up to ~70 m/s) in 2011 produce high-amplitude tropospheric arrivals recorded out to >1000 km to the east. Significant eastward stratospheric winds (up to ~80 m/s) in 2011 generate numerous stratospheric arrivals and permit the long-range detection (i.e., >1000 km). No detections are made in directions opposite the tropospheric and stratospheric wind jets for any of the explosions. Comparison of predicted transmission loss and observed infrasound arrivals gives qualitative agreement. Propagation modeling for the 2011 experiments predicts lower transmission loss in the direction of the downwind propagation compared to the 2009 experiment, consistent with the greater detection distance. Observations also suggest a more northerly component to the stratospheric winds for the 2009 experiment and less upper atmosphere attenuation. The Sayarim infrasound calibration experiments clearly demonstrate the complexity and variability of the atmosphere, and underscore the utility of large-scale calibration experiments with dense networks for better understanding infrasound propagation and detection. Additionally, they provide a rich data set for future scientific research. Key Points Three large ground-truth infrasound experiments were conducted in 2009 and 2011 Strong wind jets permitted long range detection Atmospheric specifications sufficient for qualitative propagation modeling © 2013. American Geophysical Union. All Rights Reserved.

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