SUMMARY
The detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended ...source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals that conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e. following the Bartlett or the Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a ‘clean’ spectrum. A data-driven stopping criterion for CLEAN is proposed, which relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data set and is applied to data from multiple International Monitoring System infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic that would have remained unidentified if conventional methods had been applied.
In this study we analyze infrasound signals from three earthquakes in central Italy. The Mw 6.0 Amatrice, Mw 5.9 Visso, and Mw 6.5 Norcia earthquakes generated significant epicentral ground motions ...that couple to the atmosphere and produce infrasonic waves. Epicentral seismic and infrasonic signals are detected at I26DE; however, a third type of signal, which arrives after the seismic wave train and before the epicentral infrasound signal, is also detected. This peculiar signal propagates across the array at acoustic wave speeds, but the celerity associated with it is 3 times the speed of sound. Atmosphere‐independent backprojections and full 3‐D ray tracing using atmospheric conditions of the European Centre for Medium‐Range Weather Forecasts are used to demonstrate that this apparently fast‐arriving infrasound signal originates from ground motions more than 400 km away from the epicenter. The location of the secondary infrasound patch coincides with the closest bounce point to I26DE as depicted by ray tracing backprojections.
Plain Language Summary
Underground explosions and earthquakes generate ground motions at the surface of the Earth and induce pressure fluctuations in the atmosphere. These fluctuations propagate through the atmosphere as inaudible sound called infrasound and are detected by pressure sensors hundreds of kilometers away. In this paper, we discuss how these detections can reveal information about the regions in which seismic energy radiated from underground sources couples to the atmosphere and generates infrasound. We convey a new understanding of this process and demonstrate that infrasound may originate from a much larger region and much farther away from the source than previously described in the scientific literature. Understanding the interaction of these two wavefields, seismic and infrasonic, is important for monitoring nuclear explosions, for example, but can also be used to evaluate the state of the atmosphere from naturally occurring sources like earthquakes and volcano eruptions.
Key Points
Coupling of seismic‐induced ground motions to infrasound may occur over a larger extent than previously described in scientific literature
Sources of infrasound are detected when there is an atmospheric duct connecting them to a receiver
Understanding seismoacoustic coupling is important for CTBT verification purposes
SUMMARY
The ambient infrasonic noise field is complex due to the interference of spatially distributed infrasound sources. Microbaroms are one of the most dominant omnipresent infrasonic sources ...within this wavefield. These microbaroms are generated by nonlinear ocean surface wave interactions, and have a characteristic and continuous signature within the infrasound spectrum. Under noisy conditions, microbaroms can mask infrasonic signals of interest, such as infrasound from volcanoes or explosions, which limits detection and identification of such sources. This study performs an infrasonic climatology for infrasound array I23FR, using five years of data between 2015–2020. The array is located on the Kerguelen Islands, within the Southern Ocean, and is part of the International Monitoring System for the verification of the Comprehensive Nuclear-Test-Ban Treaty. The climatology analysis addresses the expected ambient noise levels, propagation paths and potential sources within the vicinity of an infrasound sensor. Time- and frequency-domain beamforming methods have been applied to analyse the infrasonic wavefield from the I23FR observations. A recently introduced method is applied to compute so-called soundscapes, to be compared with beamform results. Although the comparison indicates a disagreement in amplitude, there is a good agreement in directionality and frequency between both.
The Royal Netherlands Meteorological Institute (KNMI) operates a three‐dimensional microbarometer array at the Cabauw Experimental Site for Atmospheric Research observatory. The array consists of ...five microbarometers on a meteorological tower up to an altitude of 200 m. Ten ground‐based microbarometers surround the tower with an array aperture of 800 m. This unique setup allows for the study of infrasound propagation in three dimensions. The added value of the vertical dimension is the sensitivity to wind and temperature in the atmospheric boundary layer over multiple altitudes. In this study, we analyze infrasound generated by an accidental chemical explosion at the Moerdijk petrochemical plant on 3 June 2014. The recordings of the tower microbarometers show two sequential arrivals, whereas the recordings on the ground show one wavefront. This arrival structure is interpreted to be the upgoing and downgoing wavefronts. The observations are compared with propagation modeling results using global‐scale and mesoscale atmospheric models. Independent temperature and wind measurements, which are available at the Cabauw Experimental Site for Atmospheric Research, are used for comparison with model output. The modeling results explain the signal arrival times; however, the tower wavefront arrivals are not explained. This study is important for understanding the influence of the atmospheric boundary layer on infrasound detections and propagation.
Key Points
A three‐dimensional infrasound array is set up for characterizing upgoing and downgoing wavefronts in the atmospheric boundary layer
The importance of atmospheric boundary layer conditions on infrasound propagation modeling is shown
The observed upgoing and downgoing wavefronts are not fully explained by ray propagation modeling
SUMMARY
Measurements of seismo-acoustic events by collocated seismic and infrasound arrays allow for studying the two wavefields that were produced by the same event. However, some of the scientific ...and technical constraints on the building of the two technologies are different and may be contradicting. For the case of a new station, an optimal design that will satisfy the constraints of the two technologies can be found. However, in the case of upgrading an existing array by adding the complementing technology, the situation is different. The site location, the array configuration and physical constraints are fixed and may not be optimal for the complementing technology, which may lead to rejection of the upgrade. The International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes 37 seismic arrays and 51 infrasound arrays. Although the CTBT verification regime is fixed in the treaty, an upgrade of the existing arrays by adding more technologies is possible.
The Mount Meron seismic array (MMAI), which is part of the IMS, is composed of 16 sites. Microbarometers were installed at five MMAI sites to form the Mount Meron infrasound array. Due to regulation and physical constraints, it was not possible to relocate the sites nor to install analogue noise reduction filters (i.e. a pipe array). In this study, it is demonstrated that the installation of the MMAI infrasound array is beneficial despite the non-optimal conditions. It is shown that the noise levels of the individual array sites are between the high and median global noise levels. However, we claim that the more indicative measures are the noise levels of the beams of interest, as demonstrated by analysing the microbaroms originated from the Mediterranean Sea. Moreover, the ability to detect events relevant to the CTBT is demonstrated by analysing man-made events during 2011 from the Libya region.
Infrasound as a Cue for Seabird Navigation Patrick, Samantha C.; Assink, Jelle D.; Basille, Mathieu ...
Frontiers in ecology and evolution,
11/2021, Volume:
9
Journal Article
Peer reviewed
Open access
Seabirds are amongst the most mobile of all animal species and spend large amounts of their lives at sea. They cross vast areas of ocean that appear superficially featureless, and our understanding ...of the mechanisms that they use for navigation remains incomplete, especially in terms of available cues. In particular, several large-scale navigational tasks, such as homing across thousands of kilometers to breeding sites, are not fully explained by visual, olfactory or magnetic stimuli. Low-frequency inaudible sound, i.e., infrasound, is ubiquitous in the marine environment. The spatio-temporal consistency of some components of the infrasonic wavefield, and the sensitivity of certain bird species to infrasonic stimuli, suggests that infrasound may provide additional cues for seabirds to navigate, but this remains untested. Here, we propose a framework to explore the importance of infrasound for navigation. We present key concepts regarding the physics of infrasound and review the physiological mechanisms through which infrasound may be detected and used. Next, we propose three hypotheses detailing how seabirds could use information provided by different infrasound sources for navigation as an acoustic beacon, landmark, or gradient. Finally, we reflect on strengths and limitations of our proposed hypotheses, and discuss several directions for future work. In particular, we suggest that hypotheses may be best tested by combining conceptual models of navigation with empirical data on seabird movements and in-situ infrasound measurements.
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