Sound Fixing Research Articles

Localisation for underwater acoustic networks with a piece-wise linear sound speed profile

ABSTRACT Underwater agent localisation is a precondition for plenty of ocean applications. The variation of the underwater sound speed (USS) is one of the main factors that constrain the performance of underwater localisation. In this paper, a scheme is designed to reduce the agent localisation error arising from the USS variation in deep water and an agent localisation algorithm named ‘localisation with a piecewise linear USS' (LPL) is proposed. According to the characteristics of a real USS in deep water, a piecewise linear USS model is introduced to approximate the real USS. The agent above under the ‘sound fixing and ranging’ (SOFAR) channel are both considered. The LPL algorithm consists of three parts: initial value estimation, horizontal distance estimation and localisation. Considering the piecewise linear USS profile, the proposed LPL algorithm improves the performance for localisation with 60% more precision than the state-of-the-art methods.

Hydroacoustic Monitoring of Mayotte Submarine Volcano during Its Eruptive Phase

Submarine volcanoes are more challenging to monitor than subaerial volcanoes. Yet, the large eruption of the Hunga Tonga-Hunga Ha’apai volcano in the Tonga archipelago in 2022 was a reminder of their hazardous nature and hence demonstrated the need to study them. In October 2020, four autonomous hydrophones were moored in the sound fixing and ranging channel 50 km offshore Mayotte Island, in the North Mozambique Channel, to monitor the Fani Maoré 2018–2020 submarine eruption. Between their deployment and July 2022, this network of hydrophones, named MAHY, recorded sounds generated by the recent volcanic activity, along with earthquakes, submarine landslides, marine mammals calls, and marine traffic. Among the sounds generated by the volcanic activity, impulsive signals have been evidenced and interpreted as proxy for lava flow emplacements. The characteristics and the spatio-temporal evolution of these hydroacoustic signals allowed the estimation of effusion and flow rates, key parameters for volcano monitoring. These sounds are related to the non-explosive quenching of pillow lavas due to the rapid heat transfer between hot lava and cold seawater, with this process releasing an energy equivalent to an airgun source as used for active seismic exploration. Volcano observatories could hence use autonomous hydrophones in the water column to detect and monitor active submarine eruptions in the absence of regular on-site seafloor survey.

Analysis of Nuclear Explosion Detection Capability of IMS Hydroacoustic Network

The Comprehensive Nuclear Test Ban Treaty (CTBT) organization has established a hydroacoustic network to ensure the effective detection of nuclear explosions in the vast ocean and in the atmosphere over it in order to effectively guarantee the implementation of the treaty. To assess its effectiveness, according to the reciprocity of underwater acoustic field, this study employs the hydrophone position as the central sound source and utilizes the parabolic equation, along with seabed topography and sound velocity profile data, to estimate the transmission loss. Background noise levels of station evaluation are conducted by using historical data so as to estimate the underwater nuclear explosion sound source level via a passive sonar equation. Utilizing the full ship shock trial (FSST) results of the “Ford” aircraft carrier on June 18, 2021, as a case study, the detection capability of the HA10 station for nuclear explosions is investigated. Subsequently, the network’s overall detection capability was evaluated using the source level at 20 Hz corresponding to a 1 kiloton nuclear explosion yield as a reference. Findings indicate the network’s proficiency in identifying long‐distance propagation signals, particularly those with high explosion source levels within the sound fixing and ranging (SOFAR) channel. The sensitivity analysis demonstrates the network’s ability to detect explosions smaller than 1 kg equivalent in close proximity to a station, and larger explosions exceeding 1 kg or 1 ton at greater distances. This evaluation underscores the network’s significance in detecting and monitoring underwater nuclear explosions across extensive oceanic regions.

Improved Acoustic Tracking of RAFOS-Enabled Profiling Floats through the New Software Package artoa4argo

Abstract In sea ice–covered polar oceans, profiling Argo floats are often unable to surface for 9 months or longer, rendering acoustic RAFOS tracking the only method to obtain unambiguous under-ice positions. Tracking RAFOS-enabled floats has historically relied on the ARTOA3 software, which had originally been tailored toward nonprofiling floats in regions featuring the sound fixing and ranging (SOFAR) channel with acoustic ranges of approximately 1000 km. However, in sea ice–covered regions, RAFOS tracking is challenged due to (i) reduced acoustic ranges of RAFOS signals, and (ii) enhanced uncertainties in float and sound source clock offsets. A new software, built on methodologies of previous ARTOA versions, called artoa4argo, has been created to overcome these issues by exploiting additional float satellite fixes, resolving ambiguous float positions when tracking with only two sources and systematically resolving float and sound source clock offsets. To gauge the performance of artoa4argo, 21 RAFOS-enabled profiling floats deployed in the Weddell Sea during 2008–12 were tracked. These have previously been tracked in independent studies with a Kalman smoother and a multiconstraint method. The artoa4argo improves tracking by automating and streamlining methods. Although artoa4argo does not necessarily produce positions for every time step, which the Kalman smoother and multiconstraint methods do, whenever a track location is available, it outperforms both methods. Significance Statement Argo is an international program that collects oceanic data using floats that drift with ocean currents and sample the water column from 2000-m depth to the surface every 7–10 days. Upon surfacing, the float acquires a satellite position and transmits its data via satellite. In polar regions, with extensive seasonal sea ice coverage, floats are unable to surface for many months. Thus, any under-ice samples collected are missing positions, hampering their use in scientific endeavors. Since monitoring of polar regions is imperative to better understand and predict the effects of climate change, hydroacoustic tracking is employed there. Here a new acoustic tracking software, artoa4argo, is introduced, which improves tracking of these floats.

Seismic T Phases in the Western-Central Mediterranean: Source of Seismic Hazard?

Abstract The Algerian offshore earthquake of 18 March 2021, Mw 6.0, was felt by people in various Italian regions, also at large epicentral distance. This unusual human perception far from the source prompted us to analyze the waveforms recorded by land seismic stations installed along the Iberian, French, and Italian coasts. On some seismograms of the selected network, prominent T phases are detected. T waves can travel in the SOund Fixing And Ranging (SOFAR) channel over great distances (thousands of kilometers) with little loss in signal strength and be recorded by near-coastal seismometers after the P (primary) and S (secondary) phases (hence T or tertiary phases). To explain the subjective perception of ground shaking with quantities that are measured on the seismogram, we estimated the empirical macroseismic intensities for both body and T phases and we calculated the body-wave seismic attenuation. The P-wave anelastic attenuation analysis shows two main wave propagation patterns that reflect lithosphere heterogeneity of the Algerian, Liguro-Provençal, and Tyrrhenian basins. We find that in some cases, in particular along the Italian and French coasts, the largest ground shaking is caused by the T phase. Our observations confirm that the central-western Mediterranean Sea is a favorable site for T-wave propagation and suggest that the T phases should be taken into account in ground-shaking hazard assessment for the central-western Mediterranean.

The AtmoSOFAR Channel: First Direct Observations of an Elevated Acoustic Duct

AbstractThe Sound Fixing and Ranging (SOFAR) channel in the ocean allows for low frequency sound to travel thousands of kilometers, making it particularly useful for detecting underwater nuclear explosions. Suggestions that an elevated SOFAR‐like channel should exist in the stratosphere date back over half a century and imply that sources within this region can be reliably sensed at vast distances. However, this theory has not been supported with evidence of direct observations from sound within this channel. Here we show that an infrasound sensor on a solar hot air balloon recorded the first infrasound detection of a ground truth airborne source while within this acoustic channel, which we refer to as the AtmoSOFAR channel. Our results support the existence of the AtmoSOFAR channel, demonstrate that acoustic signals can be recorded within it, and provide insight into the characteristics of recorded signals. Results also show a lack of detections on ground‐based stations, highlighting the advantages of using balloon‐borne infrasound sensors to detect impulsive sources at altitude.

Three-Hop Underwater Wireless Communications: A Novel Relay Deployment Technique

Underwater long-distance wireless communication (ULWC) is a critical challenge in many applications, such as marine environmental monitoring, underwater remote control, and underwater navigation, to name a few. However, very little literature focuses on ULWC, especially where the communication distance ups to thousands of kilometers, which is urgently required in the underwater Internet of Things (UIoT) in the large-scale and deep sea. In this paper, to improve the underwater communication capacity at a level of thousands of kilometers, we propose a three-hop underwater wireless acoustic communication (3H-UWAC) structure based on the Sound Fixing and Ranging (SOFAR) channel. The proposed 3H-UWAC consists of transmitters, relay stations (RSs), and receivers. Different from the existing ULWC, 3H-ULWC can improve energy efficiency with a small vertical directivity angle (VDA). Due to the characteristics of UWAC, the straight-line communication link can be realized in the proposed three hops, and the communication distance can be increased to thousands of kilometers. Respecting the randomness of underwater devices, tools from stochastic geometry are used to model the spatial distributions of transmitters’, receivers’, and RSs’ locations. RSs are set on the SOFAR channel at a known depth. In the first hop, the transmitter sends information to the nearest first relay station (NFRS) on the SOFAR plane. In the second hop, the NFRS sends information to the nearest relay station, which is called the nearest second relay station (NSRS), to the receiver on the SOFAR plane. In the third hop, SNFS sends information to the receiver. All three communication hops can be achieved with a narrow beam width, where the energy efficiency is improved critically. With given densities of transmitters, RSs, and receivers, the coverage probabilities (CPs) of the three hops (transmitter to FRS, FRS to SRS, and SRS to receiver) are analyzed, and the final CP from a transmitter to a receiver through the 3H link is derived. Insights about the effects of VDAs at the transmitters, FNRS, and SNRS, as well as the depths of transmitters and receivers, are revealed. A rapid optimization method is proposed based on the analytical results. The accuracy of the analysis is verified by Monte-Carlo simulations.

Contrasting T-Phase and P-Wave Patterns from the 2015 and 2017 Eruptions of the Submarine Volcano Kick-‘em-Jenny: Influence of Cardinal Direction on Recorded First Phase Arrival

ABSTRACT Kick-‘em-Jenny (KeJ)—the most active volcano in the Lesser Antilles and located about 8 km north of Grenada—is the only known, live submarine volcano in the region. Unrest and eruption episodes are approximately decadal. Typically, a KeJ episode, which may consist of several eruptions, is short-lived and rapidly returns to quiescence, with virtually no background seismicity. Although two previous eruptions broke the surface of the sea, and some have had seismic effects felt in northern Grenada and sometimes Martinique, instrumental recordings of T phases were considered to provide firm evidence of eruptions. T phases are hydroacoustic waves that propagate efficiently in the Sound Fixing and Ranging channel allowing long-range recording. The most recent episodes occurred on 29 April 2017 and 23, 24 July 2015, with no reported surface evidence. The 2015 hr-long seismic signals, among the longest durations ever recorded, manifested differences in the first phase arrivals at some of the same recording stations suggesting a fundamental difference in T-phase wave trains’ generation at KeJ. The single eruption in the 2017 volcanic episode had a duration similar to those prior to the 2015 eruptions; however, except for the four stations in the immediate vicinity of KeJ, the detected signals propagated only as T waves. Real-time and posteruption deductions that the episodes occurred from different cardinal directions on the volcano were found to be supported by the findings from bathymetric analyses done shortly after the episodes. This characteristic enhances the KeJ profile, possibly improving real-time activity assessments in future KeJ episodes, with a view to strengthening hazard and risk communication.

Seismic detection with distributed acoustic sensors using a convolutional neural network in the frequency wavenumber spectrum.

With the development of optical fiber distributed acoustic sensors (DAS), their application to seismic observation has become popular. We conducted DAS measurements from November 19 to December 2, 2019, using dark fiber of an ocean bottom cable seismic and tsunami observation system off the Sanriku coast in northeastern Japan and investigated seismic detection methods from the obtained strain rate data. We examined a new seismic detection method using a convolutional neural network, to the best of our knowledge, treating a frequency wavenumber spectrum of strain rate as an image. This method effectively captured a characteristic wave described as the T-phase in a sound fixing and ranging channel even with low signal-to-noise ratio data.

Temporally Varying Mesoscale Eddy Characteristics in the California Current System Identified from RAFOS Floats

The SOund Fixing And Ranging (RAFOS) floats were deployed by the Naval Postgraduate School (NPS) near California coast from 1992 to 2004 at depth between 150 and 600 m (http://www.oc.nps.edu/npsRAFOS/). Each drifter trajectory is adaptively decomposed using the empirical mode decomposition (EMD) into low-frequency (non-diffusive, i.e., current) and high-frequency (diffusive, i.e., eddies) components. The identified eddies are mostly anticyclonic with total 203 anticyclonic and 36 cyclonic spirals. Eddy characteristics are determined from the time series of individual RAFOS float trajectory. They are the current velocity scale, eddy radial scale, eddy velocity scale, eddy Rossby number, and eddy-current kinetic energy ratio. The California Current System is found an eddy-rich system with the overall eddy-current kinetic energy ratio of 1.78. It contains submesoscale and mesoscale eddies. The horizontal length scale of 10 km is a good threshold of the eddy radial scale (Leddy) to separate the two kinds of eddies. The mean eddy Rossby number is 0.72 for the submesoscale eddies and 0.06 for the mesoscale eddies. The current-eddy kinetic energy ratio is similar between submesoscale and mesoscale eddies. This may imply similar current-eddy kinetic energy transfer for submesoscale and mesoscale eddies. Statistical characteristics and interannual variability of current velocity scale and eddy characteristic parameters are also presented.

Reduction of Low-Frequency Vessel Noise in Monterey Bay National Marine Sanctuary During the COVID-19 Pandemic

Low-frequency sound from large vessels is a major, global source of ocean noise that can interfere with acoustic communication for a variety of marine animals. Changes in vessel activity provide opportunities to quantify relationships between vessel traffic levels and soundscape conditions in biologically important habitats. Using continuous deep-sea (890 m) recordings acquired ∼20 km (closest point of approach) from offshore shipping lanes, we observed reduction of low-frequency noise within Monterey Bay National Marine Sanctuary (California, United States) associated with changes in vessel traffic during the onset of the COVID-19 pandemic. Acoustic modeling shows that the recording site receives low-frequency vessel noise primarily from the regional shipping lanes rather than via the Sound Fixing and Ranging (SOFAR) channel. Monthly geometric means and percentiles of spectrum levels in the one-third octave band centered at 63 Hz during 2020 were compared with those from the same months of 2018–2019. Spectrum levels were persistently and significantly lower during February through July 2020, although a partial rebound in ambient noise levels was indicated by July. Mean spectrum levels during 2020 were more than 1 dB re 1 μPa2 Hz–1 below those of a previous year during 4 months. The lowest spectrum levels, in June 2020, were as much as 1.9 (mean) and 2.4 (25% exceedance level) dB re 1 μPa2 Hz–1 below levels of previous years. Spectrum levels during 2020 were significantly correlated with large-vessel total gross tonnage derived from economic data, summed across all California ports (r = 0.81, p < 0.05; adjusted r2 = 0.58). They were more highly correlated with regional presence of large vessels, quantified from Automatic Identification System (AIS) vessel tracking data weighted according to vessel speed and modeled acoustic transmission loss (r = 0.92, p < 0.01; adjusted r2 = 0.81). Within the 3-year study period, February–June 2020 exhibited persistently quiet low-frequency noise and anomalously low statewide port activity and regional large-vessel presence. The results illustrate the ephemeral nature of noise pollution by documenting how it responds rapidly to changes in offshore large-vessel traffic, and how this anthropogenic imprint reaches habitat remote from major ports and shipping lanes.

Temperature-driven seasonal and longer term changes in spatially averaged deep ocean ambient sound at frequencies 63-125 Hz.

The soundscape of the Northeast Pacific Ocean is studied with emphasis on frequencies in the range 63-125 Hz. A 34-year (1964-1998) increase and seasonal fluctuations (1994-2006) are investigated. This is achieved by developing a simple relationship between the total radiated power of all ocean sound sources and the spatially averaged mean-square sound pressure in terms of the average source factor, source depth, and sea surface temperature (SST). The formula so derived is used to predict fluctuations in the sound level in the range 63-125 Hz with an amplitude of 1.2 dB and a period of 1 year associated with seasonal variations in the SST, which controls the amount of sound energy trapped in the sound fixing and ranging (SOFAR) channel. Also investigated is an observed 5 dB increase in the same frequency range in the Northeast Pacific Ocean during the late 20th century [Andrew, Howe, Mercer, and Dzieciuch (2002). ARLO 3, 65-70]. The increase is explained by the increase in the total number of ocean-going ships and their average gross tonnage.

T-Phases Observed at the Ionian Seafloor: Seismic Source and Bathymetric Effects

AbstractT-waves are acoustic waves generated by earthquakes at the land–water interface. They can propagate efficiently for thousands of kilometers within the ocean’s low-velocity waveguide—the SOund Fixing And Ranging (SOFAR) channel. In the present work, we studied T-waves that propagate in the Ionian basin and are generated by regional earthquakes (epicentral distance <1000 km) located in the Hellenic Arc (Greece). The Ionian Sea is a small basin that has strong bathymetric variations and is limited at its western edge by a steep continental slope—the Malta escarpment. T-waves from Greece were recorded by a broadband seismometer onboard one of seafloor-observing units of the Western Ionian Regional Facility of EMSO—European Multidisciplinary Seafloor and water column Observatory Research Infrastructure (see Data and Resources) deployed in the western Ionian Sea (Italy) at about 2100 m water depth. By studying the particle motion and T-phase energy flux (TPEF) of the T-waves recorded at the observatory, we find that the western Ionian Sea bathymetry is an efficient reflector for T-waves within the SOFAR channel. To investigate whether factors other than T-wave path effects drive TPEF levels, we also study the source part of the T-wave generation process.

Lagrangian Drifter to Identify Ocean Eddy Characteristics

Deterministic–stochastic empirical mode decomposition (EMD) is used to obtain low-frequency (non-diffusive; i.e., background velocity) and high-frequency (diffusive; i.e., eddies) components from a Lagrangian drifter‘s trajectory. Eddy characteristics are determined from the time series of eddy trajectories from individual Lagrangian drifters such as eddy radius, eddy velocity, eddy Rossby number, and the eddy–current kinetic energy ratio. A long-term dataset of the Sound Fixing and Ranging (RAFOS) float time series obtained near the California coast by the Naval Postgraduate School from 1992 to 2004 at depth between 150 and 600 m is used as an example to demonstrate the capability of the deterministic–stochastic EMD.

All-terminal reliability of multi-AUV cooperative systems in horizontally stratified SOFAR channel

ABSTRACT This work develops a calculation method of all-terminal reliability for multi-AUV cooperative systems in horizontally stratified sound fixing and raging channel (SOFAR channel), considering the sound speed profile in SOFAR channel. The working environment of multi-AUV (autonomous underwater vehicle) cooperative systems is complex and the reliability under different environmental conditions is different. This work adopts the Kraken normal mode method to get the transmission loss, and calculates the reliability of the communication link based on the relationship between the transmission loss and signal-to-noise ratio. All-terminal reliability of multi-AUV cooperative systems can be obtained.

Controls on the T Phase Energy Fluxes Recorded on Juan Fernandez Island by Continental Seismic Wave Paths and Nazca Bathymetry

AbstractT phases from 54 South American earthquakes with Mw > 5.2 are observed at a broadband station on Juan Fernandez Island. We computed the T phase energy flux (TPEF) values of the seismograms. The TPEF values show a large dispersion that can be explained by considering the tectonic characteristics of the South American plate and the Nazca plate bathymetry. The TPEFs generated by the 2015 Illapel and 2017 Valparaíso seismic sequences were controlled by the positions of the interface events along the dip. The central and downdip interplate earthquakes were more efficient in the generation of T phases than the near‐trench interplate earthquakes (depths of <15 km). The variations in the generation efficiency with depth are explained by the continental raypaths of the body waves and the incidence angles of waves entering the sound fixing and ranging channel. Additionally, we observed differences in the TPEFs from both earthquake sequences that were controlled by seamounts atop the Nazca plate along the T phase paths.

Ultra‐long‐range hydroacoustic observations of submarine volcanic activity at Monowai, Kermadec Arc

AbstractMonowai is an active submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. During May 2011, it erupted over a period of 5 days, with explosive activity directly linked to the generation of seismoacoustic T phases. We show, using cross‐correlation and time‐difference‐of‐arrival techniques, that the eruption is detected as far as Ascension Island, equatorial South Atlantic Ocean, where a bottom moored hydrophone array is operated as part of the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty Organization. Hydroacoustic phases from the volcanic center must therefore have propagated through the Sound Fixing and Ranging channel in the South Pacific and South Atlantic Oceans, a source‐receiver distance of ~15,800 km. We believe this to be the furthest documented range of a naturally occurring underwater signal above 1 Hz. Our findings, which are consistent with observations at regional broadband stations and long‐range, acoustic parabolic equation modeling, have implications for submarine volcano monitoring.

WIGWAM Reverberation Revisited

Operation WIGWAM was a test of a 30 kt nuclear depth charge conducted in deep water 500 miles southwest of San Diego on 14 May 1955. Its primary purpose was to determine the effectiveness of that device as an antisubmarine weapon. The acoustic pulse from the test, initially an intense shockwave, radiated throughout the North and South Pacific Oceans. Acoustic reflections from topographic features were recorded for several hours after the explosion by SOund Fixing And Ranging (SOFAR) hydrophones at Point Sur, California, and Kaneohe, Hawaii. Sheehy and Halley (1957) identified peaks of the recorded coda with reflections from specific topographic features at great distances (e.g., the Hawaiian Islands, French Polynesia, or Fiji). With modern data for seafloor topography and ocean sound speed, these coda were computed with surprising accuracy using simple geodesic rays reflected from islands and seamounts. The intensity variations of the coda are mostly determined by simple ray geometry, together with modest attenuation. Coda peaks are often obtained from rays arriving simultaneously from multiple, but disparate, topographic features. Online Material: Figures of computed and measured coda and associated geodesic paths.

Elastic parabolic equation solutions for oceanic T-wave generation and propagation from deep seismic sources.

Oceanic T-waves are earthquake signals that originate when elastic waves interact with the fluid-elastic interface at the ocean bottom and are converted to acoustic waves in the ocean. These waves propagate long distances in the Sound Fixing and Ranging (SOFAR) channel and tend to be the largest observed arrivals from seismic events. Thus, an understanding of their generation is important for event detection, localization, and source-type discrimination. Recently benchmarked seismic self-starting fields are used to generate elastic parabolic equation solutions that demonstrate generation and propagation of oceanic T-waves in range-dependent underwater acoustic environments. Both downward sloping and abyssal ocean range-dependent environments are considered, and results demonstrate conversion of elastic waves into water-borne oceanic T-waves. Examples demonstrating long-range broadband T-wave propagation in range-dependent environments are shown. These results confirm that elastic parabolic equation solutions are valuable for characterization of the relationships between T-wave propagation and variations in range-dependent bathymetry or elastic material parameters, as well as for modeling T-wave receptions at hydrophone arrays or coastal receiving stations.

Passive probing of the sound fixing and ranging channel with hydro-acoustic observations from ridge earthquakes.

The International Monitoring System includes a hydro-acoustic part to verify the Comprehensive Nuclear-Test-Ban Treaty. Besides explosive signals, monitoring stations also detect acoustic waves from earthquakes that travel through the SOund Fixing And Ranging (SOFAR) channel. The travel times of such detections are listed in the Reviewed Event Bulletin, which is statistically evaluated for the stations located in the Pacific, Indian, and Atlantic Oceans. The celerities of ridge earthquakes are calculated to build up a homogeneous data-set, based on similar source mechanisms. The celerity is defined as the epicentral distance divided by the travel time. The global characteristics of these celerities can be well understood in terms of temperature variations in the SOFAR channel. A detailed velocity profile has been retrieved for the Atlantic Ocean where large differences (14 m/s) are found between the southern and northern parts of the basin. Propagation modeling with normal modes supports these findings, which shows that the celerity gives an estimate of the sound speed in the SOFAR channel. These results compare remarkably well with those from active experiments, showing the ability of passively probing the SOFAR channel with hydro-acoustic waves from earthquake sources.