Characterizing ocean ambient noise using a regional infrasound network
Alexis Le PichonThibault ArnalMarine DeCarloMarten BlixtSven Peter NäsholmJohan KeroLars CerannaFabrice Ardhuin
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Ambient noise level
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The infrasound network of the International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty has been designed for the detection and the localization of atmospheric nuclear explosions. It is composed of sixty stations, which measure micropressure changes produced in the atmosphere by infrasonic wave propagation. Most IMS infrasound stations use microbarometers MB2000 or MB2005 associated with acquisition units dedicated to geophysics. These absolute infrasound sensors measure ambient atmospheric pressure over a frequency bandwidth from DC to tens of Hz. This bandwidth not only includes the entire infrasound frequency range used for operational monitoring, but also a lower one corresponding to internal gravity waves and meteorological processes. Among gravity waves, atmospheric tides are waves with periods corresponding to integral fractions of a solar day (primarily diurnal and semidiurnal). They are produced by the atmospheric solar heating combined with upward eddy conduction of heat from the ground. Their importance is high as they regularly cause oscillations in atmospheric wind, temperature and pressure fields. However the IMS network has been designed for infrasound detection and its use for the study of such low frequency waves need a careful assessment of its ultimate capabilities.
Atmospheric wave
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Atmospheric noise
Microseism
Atmospheric wave
Swell
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Abstract Infrasound recorded in the middle stratosphere suggests that the acoustic wavefield above the Earth's surface differs dramatically from the wavefield near the ground. In contrast to nearby surface stations, the balloon‐borne infrasound array detected signals from turbulence, nonlinear ocean wave interactions, building ventilation systems, and other sources that have not been identified yet. Infrasound power spectra also bore little resemblance to spectra recorded on the ground at the same time. Thus, sensors on the Earth's surface likely capture a fraction of the true diversity of acoustic waves in the atmosphere. Future studies building upon this experiment may quantify the acoustic energy flux from the surface to the upper atmosphere, extend the capability of the International Monitoring System to detect nuclear explosions, and lay the observational groundwork for a recently proposed mission to detect earthquakes on Venus using free‐flying microphones.
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1: Introduction.- 2: The Air-Sea Boundary Interaction Zone.- 3: Fundamental Mechanisms.- 4: The Measurement of Oceanic Ambient Noise.- 5: Numerical Modeling of Ambient Noise.- 6: Research Issues.- Appendices.- Nomenclature.- Bibliography.- Index.
Ambient noise level
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Low frequency acoustic waves (infrasound) are generated by a variety of natural and anthropogenic phenomena. Although infrasound propagates throughout the atmosphere, the vast majority of acoustic studies utilize sensors on or near the Earth's surface. This paper describes results from two infrasound arrays launched into the stratosphere, one in August 2014 and the other in September 2015. The observations presented here are the first stratospheric infrasound measurements reported in scientific literature in 50 years. Acoustic signals recorded on the balloon borne sensors were different than those recorded by nearby infrasound stations on the ground. The 0.2 Hz ocean microbarom was detected in the stratosphere, but was not observed on nearby ground stations. A series of narrow band signals were also observed in the stratosphere, some of which varied in frequency over tens of minutes. The source of these signals is unclear. Wind noise decreased with altitude during the ascent, becoming negligible above 20 km. It was absent when the balloon was neutrally buoyant, although it was pervasive on ground stations operating in the same region during the day. Spectral characteristics of stratospheric infrasound were similar between the two flights and also resembled the last experiment in the early 1960s, but spatiotemporal variations in signal strength and frequency were also observed. Future efforts should focus on characterizing infrasound sensor operation in extreme environments and increasing spatial and temporal frequency of acoustic measurements in the free atmosphere. Results from this study have implications for long range detection of events such as nuclear blasts, the quantification of acoustic energy that heats the upper atmosphere, and calibration of a proposed mission to place airborne acoustic sensors on Venus.
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