The inaudible symphony: On the detection and source identification of atmospheric infrasound

Sound becomes inaudible if it consists of frequencies lower than 20 Hz, i.e. the human hearing threshold, and is called infrasound. Low frequency acoustic signals were first discovered after the eruption of the Krakatoa (Indonesia) in 1883. Due to its low frequency content, this infrasound traveled up to seven times around the globe while reaching altitudes over 100 kilometer. The ability to detect explosions with infrasound resulted in substantial scientific and societal interest during World War I and the era of atmospheric nuclear testing. This interest diminished as nuclear tests were confined to the underground under the Limited Test Ban Treaty in 1963. Recently, with the signature of the Comprehensive Nuclear-Test-Ban Treaty, infrasound gained renewed attention as it is being used as a verification technique. This thesis describes the complete sequence of measuring, processing and interpreting infrasound data. A microbarometer was developed and deployed in arrays to measure infrasound. Array processing and statistical detection techniques are applied to extract signals of interest from the continuous recordings. Knowledge on the atmospheric propagation appeared essential in identifying infrasonic sources, like meteors, volcanoes, sea waves and accidental chemical explosions. A large amount of coherent infrasound is continuously being detected from both natural and man-made sources, i.e. the inaudible symphony. Applications are foreseen in acoustic remote sensing where infrasound can be used as passive probe for the upper atmosphere. Non-acoustic phenomena, like gravity waves, can also be detected with the methods described in this thesis.Aerospace Engineerin

The life cycle of a sudden stratospheric warming from infrasonic ambient noise observations

A method is presented to study the life cycle of a SSW using infrasonic ambient noise observations. The potential of infrasound is shown to provide the missing observations required by numerical weather prediction to better resolve the upper atmosphere. The 2009 major SSW is reanalyzed using the Evers and Siegmund (2009) data set. Microbarom observations are evaluated to identify detections that cannot be explained by the analysis of the European Centre for Medium-Range Weather Forecasts. Identified differences can be related to either the altitude limit of the analysis, not resolving thermospheric ducts, or to an actual error in the analysis. Therefore, a first-order model is used to relate observations with the analysis, existing of the Waxler et al. (2007) microbarom source model, including bathymetry to allow column resonances, and an atmospheric propagation model using 3-D ray tracing. Daily normalized spectral powers are proposed to distinguish stratospheric from thermospheric return height, based on the different signature of solar tidal amplitude fluctuations. It is shown that a SSW is not a smooth event as following from the analysis but a series of abrupt changes with a period of 10 to 16 days, increasing in intensity and duration. This is in agreement with the wave period of Rossby waves, interacting with the stratospheric circumpolar vortex. The type of vortex disturbance, split or reversal, can be deduced from the combined effect of the change in back-azimuth direction, solar tidal signature type, and/or phase variation of the amplitude variation of the observed microbaroms.Geoscience & EngineeringCivil Engineering and Geoscience

Infrasonic signature of the 2009 major sudden stratospheric warming

The study of infrasound is experiencing a renaissance since it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty. The success of the verification technique strongly depends on knowledge of upper atmospheric processes. The ability of infrasound to probe the upper atmosphere starts to be exploited, taking the field beyond its monitoring application. Processes in the stratosphere couple to the troposphere and influence our daily weather and climate. Infrasound delivers actual observations on the state of the stratosphere with a high spatial and temporal resolution. Here we show the infrasonic signature, passively obtained, of a drastic change in the stratosphere due to the major sudden stratospheric warming (SSW) of January 2009. With this study, we infer the enormous capacity of infrasound in acoustic remote sensing of stratospheric processes on a global scale with surface based instruments.Remote SensingAerospace Engineerin

The life cycle of a Sudden Stratospheric Warming from infrasonic ambient noise observations

Geoscience & EngineeringCivil Engineering and Geoscience

Hydroacoustic Travel Time Variations as a Proxy for Passive Deep-Ocean Thermometry: A Cookbook

We report on the extraction of deep ocean travel time variations from time-lapse cross-correlations between a hydrophone station and a three-component broadband seismometer. The signals we cross-correlate in this study result from repeated activity by the Monowai seamount, one of the most active submarine volcanoes of the Tonga-Kermadec ridge. In particular, we introduce a specific workflow to exploit repetitive hydroacoustic underwater source activity, which we detail to such an extent that it serves as an example (or “cookbook”). For this reason, we have made the source code publicly available. The workflow proposed in this study (a) overcomes differences in instrument sensitivity and sample rates, (b) involves the selection of eligible cross-correlations based on a source activity criterium as well as slowness analysis, and (c) extracts the travel time variations in distinct frequency bands. In our case, the two frequency bands are 3–6 and 6–12 Hz. We find that the estimated travel time variations in both frequency bands consist of a complex periodic pattern superimposed on a robust linear trend. This linear trend is decreasing, which we attribute to increasing water temperatures along the propagation path of the hydroacoustic signals.Applied Geophysics and Petrophysic

Long-range atmospheric infrasound propagation from subsurface sources

In seismology and ocean acoustics, the interface with the atmosphere is typically represented as a free surface. Similarly, these interfaces are considered as a rigid surface for infrasound propagation. This implies that seismic or acoustic waves are not transmitted into the atmosphere from subsurface sources, and vice versa. Nevertheless, infrasound generated by subsurface sources has been observed. In this work, seismo-acoustic modeling of infrasound propagation from underwater and underground sources will be presented. The fast field program (FFP) is used to model the seismo-acoustic coupling between the solid earth, the ocean, and the atmosphere under the variation of source and media parameters. The FFP model allows for a detailed analysis of the seismo-acoustic coupling mechanisms in frequency-wavenumber space. A thorough analysis of the coupling mechanisms reveals that evanescent wave coupling and leaky surface waves are the main energy contributors to long-range infrasound propagation. Moreover, it is found that source depth affects the relative amplitude of the tropospheric and stratospheric phases, which allows for source depth estimation in the future.Applied Geophysics and Petrophysic

Infrasound from the 2009 and 2017 DPRK rocket launches

Supersonic rockets generate low-frequency acoustic waves, that is, infrasound, during the launch and re-entry. Infrasound is routinely observed at infrasound arrays from the International Monitoring System, in place for the verification of the Comprehensive Nuclear-Test-Ban Treaty. Association and source identification are key elements of the verification system. The moving nature of a rocket is a defining criterion in order to distinguish it from an isolated explosion. Here, it is shown how infrasound recordings can be associated, which leads to identification of the rocket. Propagation modelling is included to further constrain the source identification. Four rocket launches by the Democratic People's Republic of Korea in 2009 and 2017 are analysed in which multiple arrays detected the infrasound. Source identification in this region is important for verification purposes. It is concluded that with a passive monitoring technique such as infrasound, characteristics can be remotely obtained on sources of interest, that is, infrasonic intelligence, over 4500+ km.Applied Geophysics and Petrophysic

Deep ocean sound speed characteristics passively derived from the ambient acoustic noise field

The propagation of acoustic waves in the ocean strongly depends on the temperature. Lowfrequency acoustic waves can penetrate the ocean down to depths where few in situ measurements are available. It is therefore attractive to obtain a measure of the deep ocean temperature from acoustic waves. The latter is especially true if the ambient acoustic noise field can be used instead of deterministic transient signals. In this study the acoustic velocity, and hence the temperature, is derived in an interferometric approach from hydrophone array recordings. The arrays were separated by over 125 km, near Ascension Island in the Atlantic Ocean, at a depth of 850 m. Furthermore, the dispersive characteristics of the deep ocean sound channel are resolved based on the retrieved lag times for different modes. In addition, it is shown how the resolution of the interferometric approach can be increased by cross correlating array beams rather than recordings from single-sensor pairs. The observed acoustic lag times between the arrays corresponds well to modelled values, based on full-wave modelling through best-known oceanic models.Applied Geophysics and PetrophysicsAircraft Noise and Climate Effect

Probabilistic infrasound propagation using realistic atmospheric perturbations

This study demonstrates probabilistic infrasound propagation modeling using realistic perturbations. The ensembles of perturbed analyses, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), include error variances of both model and assimilated observations. Ensemble spread profiles indicate a yearly mean effective sound speed variation of up to 8 ms?1 in the stratosphere, exceeding occasionally 25 ms?1 for a single ensemble set. It is shown that errors in point estimates of effective sound speed are dominated by variations in wind strength and direction. One year of large mining explosions in the Aitik mine, northern Sweden, observed at infrasound array IS37 in northern Norway are simulated using 3-D ray tracing. Probabilistic propagation modeling using the ensembles demonstrates that small-scale fluctuations are not always necessary to improve the match between predictions and observations.Geoscience & EngineeringCivil Engineering and Geoscience

Extracting low signal-to-noise ratio events with the Hough transform from sparse array data

Low-frequency acoustic, i.e., infrasound, waves are measured by sparse arrays of microbarometers. Recorded data are processed by automatic detection algorithms based on array-processing techniques such as time-domain beam forming and f-k analysis. These algorithms use a signal-to-noise ratio (S/N) value as a detection criterion. In the case of high background noise or in the presence of multiple coinciding signals, the event's S/N decreases and can be missed by automatic processing. In seismology, detecting low-S/N events with geophone arrays is a well-known problem. Whether it is in global earthquake monitoring or reservoir microseismic activity characterization, detecting low-S/N events is needed to better understand the sources or the medium of propagation. We use an image-processing technique as a postprocessing step in the automatic detection of low S/N events. In particular, we consider the use of the Hough transform (HT) technique to detect straight lines in beam-forming results, i.e., a back azimuth (BA) time series. The presence of such lines, due to similar BA values, can be indicative of a low-S/N event. A statistical framework is developed for the HT parameterization, which includes defining a threshold value for detection as well as evaluating the false alarm rate. The method is tested on synthetic data and five years of recorded infrasound from glaciers. It is shown that the automatic detection capability is increased by detecting low-S/N events while keeping a low false-alarm rate.Applied Geophysics and Petrophysic