Infrasound Recordings Research Articles

A Preliminary Study of Development of Standard Background Noise Model for the KMA Infrasound Stations

The infrasound is transmitted through the air, so the characteristics are changed by temperatures, humidity, and wind of the air. It is difficult to obtain high quality infrasound signals due to the background noise. The main sources of the background noise are human activities and wind. The development of a background noise model that quantifies the frequency band is important in the design of filters to improve the signal-to-noise ratio.In order to secure the quality of the infrasound signals. In this paper, we obtain the power spectral densities for the year of the 2015 infrasound recordings of Yanggu and Cheolwon station, and develop the model that obtained by maximum density per frequency in the upper 95% and lower 5% regions. Power spectral density analysis showed that the Yanggu station had relatively higher noise levels than the Cheolwon station and exceeded the international standard background noise model in certain frequency bands. The analysis of the meteorological data for the year of 2015 shows that the wind interfered the signal due to the high altitude of the station. The results of this paper are expected to be used as a background noise model to obtain high quality infrasound data. Keywords: Power Spectral Density, Infrasound Station of KMA, Background Noise

The influence of periodic wind turbine noise on infrasound array measurements

Aerodynamic noise emissions from the continuously growing number of wind turbines in Germany are creating increasing problems for infrasound recording systems. These systems are equipped with highly sensitive micro pressure sensors accurately measuring acoustic signals in a frequency range inaudible to the human ear. Ten years of data (2006–2015) from the infrasound array IGADE in Northern Germany are analysed to quantify the influence of wind turbine noise on infrasound recordings. Furthermore, a theoretical model is derived and validated by a field experiment with mobile micro-barometer stations. Fieldwork was carried out 2004 to measure the infrasonic pressure level of a single horizontal-axis wind turbine and to extrapolate the sound effect for a larger number of nearby wind turbines. The model estimates the generated sound pressure level of wind turbines and thus enables for specifying the minimum allowable distance between wind turbines and infrasound stations for undisturbed recording.This aspect is particularly important to guarantee the monitoring performance of the German infrasound stations I26DE in the Bavarian Forest and I27DE in Antarctica. These stations are part of the International Monitoring System (IMS) verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT), and thus have to meet stringent specifications with respect to infrasonic background noise.

Homing in for New Year: impact parameters and pre-impact orbital evolution of meteoroid 2014 AA

On 2008 October 7, small asteroid 2008 TC3 turned itself into the parent body of the first meteor ever to be predicted before entering the Earth's atmosphere. Over five years later, the 2014 AA event became the second instance of such an occurrence. The uncertainties associated with the pre-impact orbit of 2008 TC3 are relatively small because thousands of observations were made during the hours preceding the actual meteor airburst. In sharp contrast, 2014 AA was only observed seven times before impact and consequently its trajectory is somewhat uncertain. Here, we present a recalculation of the impact parameters -location and timing- of this meteor based on infrasound recordings. The new values and their uncertainties together with Monte Carlo and N-body techniques, are applied to obtain an independent determination of the pre-impact orbit of 2014 AA. Our orbital solution is used to investigate the possible presence of known near-Earth objects (NEOs) moving in similar orbits. Prior to impact, 2014 AA was subjected to a web of overlapping secular resonances and it followed a path similar to those of 2011 GJ3, 2011 JV10, 2012 DJ54, and 2013 NJ4. NEOs in this transient group have their orbits controlled by close encounters with the Earth-Moon system at perihelion and Mars at aphelion, perhaps constituting a dynamical family. Extensive comparison with other studies is also presented.

Evidence of Large Infrasonic Radiation Induced by Earthquake Interaction with Alluvial Sediments

The M w 5.9 Ferrara earthquake that struck northern Italy on 20 May 2012 was recorded with an infrasound array at a source‐to‐receiver distance of 300 km. The infrasound record revealed early and late detections characterized by large back‐azimuth variations, suggesting the existence of an extended area of infrasound radiation. Unlike most previous studies, the modeled area of maximum infrasound radiation appears to mimic an extended flat area (plain of the Po River) with no significant contributions from nearby mountain ranges. The ShakeMap of the earthquake and the map of the reported acoustic boom are in good agreement with the modeled area of infrasound radiation, suggesting how the transition of seismic waves into acoustic atmospheric waves is efficiently exciting infrasound recorded at far distances from the source. Such a result is in agreement with the significant seismic amplification within the Po plain alluvial sediments.

On experience in recording the voice of the sea in the water area of the Black Sea

Some data on a high-frequency infrasound recorded within a range of 2–16 Hz (voice of the sea) in the water area of the Black Sea are given. Different parameters of the recorded infrasonic signal—the direction and phase velocity of arriving infrasonic waves, spectral composition, and coherence—have been studied. In the course of measurements, both wind and wave conditions in the water area of the Black Sea have been studied in detail. The collision of two atmospheric vortices was observed a few hours before the first arrivals of infrasonic waves, and the collision of differently directed sea waves was observed during infrasound recording. The direction of the arrivals of infrasonic waves coincides with the direction between the zone of collision of sea waves and the point of infrasound recording. The assumption was made that, in order to explain the observed infrasonic waves, it is necessary to use the mechanism responsible for the emission of infrasound into the atmosphere by standing surface waves formed due to the nonlinear interaction of surface waves propagating in opposite directions and to take into account the frequency-filtering properties of both wind-velocity and temperature stratifications of the atmosphere itself along the path of infrasound propagation. This assumption calls for additional verification.

Strombolian surface activity regimes at Yasur volcano, Vanuatu, as observed by Doppler radar, infrared camera and infrasound

In late 2008 we recorded a continuous multi-parameter data set including Doppler radar, infrared and infrasound data at Yasur volcano, Vanuatu. Our recordings cover a transition in explosive style from ash-rich to ash-free explosions followed again by a phase of high ash discharge. To assess the present paradigm of Strombolian behavior in this study we investigate the geophysical signature of these different explosive episodes and compare our results to observations at Stromboli volcano, Italy. To this end we characterize Yasur's surface activity in terms of material movement, temperature and excess pressure. The joint temporal trend in these data reveals smooth variations of surface activity and regime-like persistence of individual explosion forms over days. Analysis of all data types shows ash-free and ash-rich explosive styles similar to those found at Stromboli volcano. During ash-free activity low echo powers, high explosion velocities and high temperatures result from the movement of isolated hot ballistic clasts. In contrast, ash-rich episodes exhibit high echo powers, low explosion velocities and low temperatures linked to the presence of colder ash-rich plumes. Furthermore ash-free explosions cause high excess pressure signals exhibiting high frequencies opposed to low-amplitude, low-frequency signals accompanying ash-rich activity. To corroborate these findings we compare fifteen representative explosions of each explosive episode. Explosion onset velocities derived from Doppler radar and infrared camera data are in excellent agreement and consistent with overall observations in each regime. Examination of infrasound recordings likewise confirms our observations, although a weak coupling between explosion velocity and excess pressure indicates changes in wave propagation. The overall trend in explosion velocity and excess pressure however demonstrates a general correlation between explosive style and explosion intensity, and points to stability of the uppermost conduit on timescales shorter than at Stromboli volcano.

Infrasonic analysis of the October 28, 2014 Antares rocket failure at Wallops Island, Virginia, using video recordings as ground truth

We used close-in video recordings of the October 28, 2014 Antares rocket failure at Wallops Island, VA to establish an event timeline to aid in the analysis of infrasound recordings made at nearby stations of the IRIS Transportable Array. Our timeline is ignition at 22:22:38 UTC, liftoff at + 4 s, bright plume and first explosion at an altitude of 300 m at + 15 s, second large explosion as the rocket hits the ground at + 25 s, followed by the excess fuel burn lasting some 400 s. Both explosions and the fuel burning events are seen in the infrasound data recorded at IRIS station S61A at a distance of 23 km, and the two explosions can be seen out to distances of 130 km. High resolution time frequency analyses of the infrasound signals at the distant stations show a dispersed signal from 0.5–8 Hz with a peak at 1.7 Hz and corresponding group velocity of 360 m/s. This dispersion curve corresponds to a low velocity duct at the surface with a thickness of approximately 1.2 km. The relatively fast group velocity...

Seismo-acoustic evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system: An example from the 2012 Tongariro eruption

The 6 August 2012 Te Maari eruption comprises a complex eruption sequence including multiple eruption pulses, a debris avalanche that propagated ~2km from the vent, and the formation of a 500m long, arcuate chasm, located ~300m from the main eruption vent.The eruption included 6 distinct impulses that were coherent across a local infrasound network marking the eruption onset at 11:52:18 (all times UTC). An eruption energy release of ~3×1012J was calculated using a body wave equation for radiated seismic energy. A similar calculation based on the infrasound record, shows that ~90% of the acoustic energy was released from three impulses at onset times 11:52:20 (~20% of total eruption energy), 11:52:27 (~50%), and 11:52:31 (~20%). These energy impulses may coincide with eyewitness accounts describing an initial eastward directed blast, followed by a westward directed blast, and a final vertical blast.Pre-eruption seismic activity includes numerous small unlocatable micro-earthquakes that began at 11:46:50. Two larger high frequency earthquakes were recorded at 11:49:06 and 11:49:21 followed directly by a third earthquake at 11:50:17. The first event was located within the scarp based on an arrival time location from good first P arrival times and probably represents the onset of the debris avalanche. The third event was a tornillo, characterised by a 0.8Hz single frequency resonance, and has a resonator attenuation factor of Q~40, consistent with a bubbly fluid filled resonator. This contrasts with a similar tornillo event occurring 2.5weeks earlier having Q~250–1000, consistent with a dusty gas charged resonator. We surmise from pre-eruption seismicity, and the observed attenuation change, that the debris avalanche resulted from the influx of fluids into the hydrothermal system, causing destabilisation and failure. The beheaded hydrothermal system may have then caused depressurisation frothing of the remaining gas charged system leading to the onset of explosive activity.

Infrasonic Signals Associated with the Aftershocks of LuShan Earthquake of April 20th, 2013

On April 20th, 2013, a strong earthquake (Ms 7.0, depth of 13 km) occurred in LuShan County, Southwestern Sichuan province, China, followed by a series of aftershocks (Ms = 3.0∼5.4). Infrasonic waves associated with the earthquake were detected and recorded by a digital infrasound recording system in our lab located in Chengdu city. Fourier analysis was used to determine the amplitude and frequency characteristics of each infrasonic recording. A certain frequency distribution range was clearly identified in the spectrograms of all analyzed shocks. Furthermore, a logarithm regression relation is also found between aftershock magnitude and peak value of corresponding infrasound events in amplitude spectra based on analysis results of more than 30 observed events. This study presents a data set and an empirical relationship. This relationship requires additional testing with observations from other earthquake aftershock sequences. These results improve our understanding of local infrasound produced by the coupling of earth surface motion to the atmosphere.

Aeroacoustics of volcanic jets: An overview

Explosive volcanic eruptions can inject large volumes of ash into heavily traveled air corridors; they pose a significant societal and economic hazard. They also generate large amplitude atmospheric infrasound waves (~0.01-20 Hz), which can be recorded at thousands of kilometers from the eruption and can provide detailed information on the timing, duration, and relative vigor of the volcanic explosions. In order to provide more detail about the eruption process based on acoustic signals, a quantitative model for the acoustic source process within the volcanic eruption column is needed. Volcanic eruption columns are modeled by a momentum-driven jet flow, transitioning with altitude into a thermally buoyant plume. Infrasound recordings from such activity resemble the large-scale turbulence similarity spectrum, indicating that large-scale volcanic jet flows generate an infrasonic form of jet noise. However, volcanic jet noise deviates from pure-air laboratory jet noise because of complexities such as multiphase flow (especially loading with ash particles); nozzle/crater geometry and roughness; buoyancy effects; and high temperature and density effects. We propose a new framework for understanding acoustic sources at volcanoes based on aeroacoustics research, which is being developed through multi-disciplinary integration of field, numerical, and laboratory studies.

Passive acoustic remote sensing and anomalous infrasound propagation studies

In this talk, we will present recent work on various infrasound remote sensing studies. We will focus on bi-directional stratospheric ducting during a Sudden Stratospheric Warming (SSW) event and the associated infrasonic signature. We present infrasound data in which the described effect is captured with microbarom signals in the Mediterranean region. Microbarom source locations are modeled using operational ocean wave models. The modeling reveals a previously unidentified microbarom source region in the Eastern Mediterranean besides the more typical microbarom source region in the Atlantic Ocean. This work illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a SSW event. Furthermore, we will present a case study in which the influence of atmospheric dynamics on infrasound propagation is studied. We make use of over 6 years of nearly continuous volcanic infrasound recordings from Mount Etna, Italy (37 N) that are available through the Atmospheric dynamics Research InfraStructure in Europe (ARISE) network. The infrasound observables are compared to theoretical estimates obtained from propagation modeling using existing European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric databases. While a good agreement is often found, we also report on significant discrepancies around the equinox period and during intervals during which anomalous detections occur during the winter.

The first second of volcanic eruptions from the Erebus volcano lava lake, Antarctica—Energies, pressures, seismology, and infrasound

We describe a multiparameter experiment at Erebus volcano, Antarctica, employing Doppler radar, video, acoustic, and seismic observations to estimate the detailed energy budget of large (up to 40 m‐diameter) bubble bursts from a persistent phonolite lava lake. These explosions are readily studied from the crater rim at ranges of less than 500 m and present an ideal opportunity to constrain the dynamics and mechanism of magmatic bubble bursts that can drive Strombolian and Hawaiian eruptions. We estimate the energy budget of the first second of a typical Erebus explosion as a function of time and energy type. We constrain gas pressures and forces using an analytic model for the expansion of a gas bubble above a conduit that incorporates conduit geometry and magma and gas parameters. The model, consistent with video and radar observations, invokes a spherical bulging surface with a base diameter equal to that of the lava lake. The model has no ad hoc free parameters, and geometrical calculations predict zenith height, velocity, and acceleration during shell expansion. During explosions, the energy contained in hot overpressured gas bubbles is freed and partitioned into other energy types, where by far the greatest nonthermal energy component is the kinetic and gravitational potential energy of the accelerated magma shell (>109 J). Seismic source energy created by explosions is estimated from radar measurements and is consistent with source energy determined from seismic observations. For the generation of the infrasonic signal, a dual mechanism incorporating a terminally disrupted slug is proposed, which clarifies previous models and provides good fits to observed infrasonic pressures. A new and straightforward method is presented for determining gas volumes from slug explosions at volcanoes from remote infrasound recordings.

Characteristic atmosphere–ocean–solid earth interactions in the Antarctic coastal and marine environment inferred from seismic and infrasound recording at Syowa Station, East Antarctica

Abstract Several characteristic waves detected by seismographs in Antarctic stations have been recognized as originating from the physical interaction between the solid earth and the atmosphere–ocean–cryosphere system surrounding the Antarctic and may be used as a proxy for characterizing ocean wave climate. A Chaparral-type infrasound sensor was installed at Syowa Station (SYO; 39.6E, 69.0S), East Antarctica, in April 2008 during the International Polar Year (IPY2007–2008). Matching data are also available for this time period from the existing broadband seismic recorder located close by. Continuous infrasound data for 2008–2009 include background signals (microbaroms) with a broad peak in the wave period between the values of 4 and 10 s. Signals with the same period are recorded by the broadband seismograph at SYO (microseisms). This period band is identified as double-frequency microseisms/baroms (DFM). The DFM have relatively lower amplitudes during winter. We suggest that this is due to the sea-ice extent around the coast causing a decreased ocean loading effect. In contrast, the single frequency microseisms/baroms with a peak in period between 12 and 30 s are observed under storm conditions, particularly in winter. On the infrasound data, stationary signals are identified with harmonic overtones at a few Hertz to lowermost human audible band, which we suggest is due to local effects such as sea-ice cracking and vibration. Microseism measurements are a useful proxy for characterizing ocean wave climate, complementing other oceanographic and geophysical data. At SYO, continuous monitoring by both broadband seismograph and infrasound contributes to the Federation of Digital Seismographic Networks, the Comprehensive Nuclear-Test-Ban Treaty in the high southern latitudes and the Pan-Antarctic Observations System under the Scientific Committee on Antarctic Research.

Combining local and remote infrasound recordings from the 2009 Redoubt Volcano eruption

The explosive phase of the 2009 Redoubt Volcano eruption produced predominantly short duration, high-amplitude infrasound signals recorded up to 4500km away. All 19 numbered explosive events were recorded at a local microphone (DFR, 12km), as well as at an infrasound array in Fairbanks, Alaska (I53US, 547km), most with high signal to noise ratios. The local microphone provides an estimate of the source parameters, and comparison between the two datasets allows the unique opportunity to evaluate acoustic source term estimation at a remote array. High waveform similarity between DFR and I53US occurs during much of the explosive phase due to strong stratospheric ducting, permitting accurate source constraints inferred from I53US data. Cross-correlation analysis after applying a Hilbert transform to the I53US data shows how the acoustic energy has passed through a single caustic, as predicted by ray theory. Similar to previous studies, significant low-frequency infrasound from Redoubt recorded at I53US is coincident with high-altitude ash emissions. The largest events also produced considerable energy at greater than 50s periods, likely related to the initial oscillations of the volcanic plume or jet. Many of the explosive events have emergent onsets, somewhat unusual for explosive, short-duration eruptions. Comparison of the satellite-derived SO2 emissions with the relative amount of acoustic energy at I53US shows a very high, statistically significant correlation. This study reiterates the utility of using remote infrasound arrays for detection of hazardous emissions and characterization of large volcanic eruptions, and demonstrates how, under typical meteorological conditions, remote infrasound arrays can provide an accurate representation of the acoustic source.

Insights into explosive activity at closely-spaced eruptive vents using infrasound signals: Example of Mt. Etna 2008 eruption

On 13 May 2008 an eruptive fissure opened on Mount Etna's eastern flank feeding both explosive activity and lava effusion from multiple vents for about 14 months. During the investigated May-September 2008 eruptive period, infrasound recordings from a 4 station-sparse network allowed tracking of the explosive activity in terms of location and dynamics. In order to focus on activity from the eruptive fissure, the infrasonic events generated by the summit craters were selected by using both spectral features and time delays between pairs of stations and excluded from our analysis. Then, to accurately locate events from the fissure, we used a composite method, based on the semblance and brightness functions. This enabled the study of the co-existence of more than one infrasound source and/or its migration along the eruptive fissure. Hence, results permitted us to discriminate the number of active vents and their location along the fissure even when, due to poor weather conditions, it was not possible to access the vents or carry out direct observations. The eruptive activity was characterised by variations in the number of active vents according to the overall intensity of the eruptive event. Variability of the infrasound waveforms highlighted either that distinct vents produced signals with different waveforms, or that single vents generated different events during distinct periods of time, or finally both the previous phenomena. We applied the strombolian bubble vibration model to model waveform differences and attributed the signal variations to bubble radius changes.

New insights into banded tremor from the 2008–2009 Mount Etna eruption

We investigated the banded tremor activity occurring at Mt. Etna volcano between August–October 2008 during the 2008–2009 eruption. The banded tremor occurred in episodes lasting 25–30 min with intervals in between the episodes of about 25 min. Seismic signal analyses showed that the banded tremor was characterized by spectral contents, wavefields, and source locations that differed from the “ordinary” volcanic tremor. The infrasound recordings exhibited an intermittent infrasonic tremor alternating with the banded tremor episodes. Finally, nonlinear analyses suggested that a banded tremor system can be considered chaotic, implying (1) sensitive dependence on initial conditions, suggesting not only that a banded tremor system requires particular conditions to generate but also that slight variations of these conditions are able to greatly change the features of the banded tremor or even to stop it; and (2) long‐term unpredictability, that is, the impossibility to forecast the long‐term evolution of the banded tremor. On the basis of all these results and analogies with geyser models, we suggest a model of banded tremor that invokes alternating recharge‐discharge phases. Banded tremor is due to “perturbations” in shallow aquifers, such as fluid movement and bubble growth or collapse due to hydrothermal boiling, triggered by the heat and hot fluid transfer from the underlying magma bodies. This heat‐fluid transfer also causes an increasing pressure in the aquifer, leading to fluid discharge. During this process, the seismic radiation decreases, and, if the fluid discharge is well coupled with the atmosphere, acoustic signals are generated.

Multiparameter quantification of gas release during weak Strombolian eruptions at Pacaya Volcano, Guatemala

Activity at Pacaya volcano, Guatemala is characterized by persistent degassing, coupled at times with lava effusion and/or coincident Strombolian‐style explosions. We used 0.25 Hz sample rate sulfur dioxide emission data from a UV camera and coincident infrasound recordings to investigate the link between varied emission rates and infrasound events. Degassing rates varied on two scales during this study; smaller, more rapid variations on the scale of 1 to 3 minutes are superimposed on larger, slower changes over 30 minutes to 1 hour. The acoustic record was dominated by repeated N‐shaped events characteristic of Strombolian explosions. Because of the high‐resolution of the gas measurements, we were able to calculate gas masses for individual explosions from both datasets and find the two estimates agree within an order of magnitude. The infrasound explosion records correlate with small pulses in degassing, but the longer‐term degassing trends may represent a deeper process.

Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano, Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m3 from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.

Characteristics of porous‐hose infrasonic wind‐noise‐reducing filters.

Spatial wind‐noise‐reducing filter technologies are widely used in infrasound recording because wind is the primary source of noise in this frequency band. While permanent infrasound arrays often make use of large solid‐pipe rosette spatial filters, portable or temporary arrays must use other materials, the most popular of which is porous garden hose. While undeniably effective at reducing wind noise, porous hoses have distinct frequency‐dependent effects on signal amplitudes and by changing the phase response of the microphone can have deleterious effects on bearing accuracy. In this presentation the effects of the use of porous‐hose wind‐noise‐reducing filters are discussed, both in terms of the effectiveness of noise reduction and the effects on signal characteristics.

Characterization of infrasound from lightning

During thunderstorm activity in the Netherlands, electromagnetic and infrasonic signals are emitted due to the process of lightning and thunder. It is shown that correlating infrasound detections with results from a electromagnetic lightning detection network is successful up to distances of 50 km from the infrasound array. Infrasound recordings clearly show blastwave characteristics which can be related to cloud‐ground discharges, with a dominant frequency between 1–5 Hz. Amplitude measurements of CG discharges can partly be explained by the beam pattern of a line source with a dominant frequency of 3.9 Hz, up to a distance of 20 km. The ability to measure lightning activity with infrasound arrays has both positive and negative implications for CTBT verification purposes. As a scientific application, lightning studies can benefit from the worldwide infrasound verification system.