Sound Channel Axis Research Articles

Oceanographic Characteristics in the Three International Indonesian Archipelago Sea Lanes (IASLs) Region: Implications for Underwater Acoustics System

Graphical Abstract Highlight Research The IASL-1 entry portal in the southern and northern regions shows the emergent SOFAR channels. The shadow zone and the existence of a SOFAR deep sound channel in the IASL-2 and IASL-3 routes can be triggered by the emergent “saddle” SVP pattern. The variability seasonally and interannually due to variations in seawater properties stratification plays an important role in SOFAR channel appearances in IASLs. The diverse oceanographic characteristics of IASLs necessitate the implementation of sustainable marine geospatial data. Abstract The Indonesian Maritime Continent (IMC) provides three international sea lanes, known as the Indonesian Archipelago Sea Lanes (IASLs), allowing ships to navigate across territorial waters between the Pacific and Indian Oceans and vice versa. Gaining knowledge about the distinct oceanographic characteristics of the three IASLs can offer valuable insight into maritime safety and sustainable marine resource management. This study aims to review oceanographic characteristics in IASL regions from available past studies to provide a comprehensive insight into the processes and dynamical oceanography in the IASL region and its implications for underwater acoustic patterns. The results of research found that the IASL-1 route is characterized by a shallow shelf passage with homogeneous sound velocity profile (SVP) but a deep and narrow entry portal in the southern and northern SOFAR channels. Seasonal reversal monsoonal wind-driven current dominates the circulation. The IASL-2 and IASL-3 routes convey a deep and narrow passage with complexity of sea-air interactions; they vary on seasonal and interannual time scales. These IASLs emerge with the “saddle” SVP, which can trigger the shadow zone and the existence of a SOFAR deep sound channel that varies seasonally and interannually due to variations in seawater properties stratification. The diverse oceanographic characteristics discussed above significantly influence the underwater object detection equipment, the planning time, and the strategies for underwater defense systems. Due to these implications, it is necessary to utilize marine geospatial database. Subsequently, these data may be utilized to facilitate policy-making and provide approximations for marine activities and management along the IASLs.

Predicting underwater acoustic transmission loss in the SOFAR channel from ray trajectories via deep learning.

Predicting acoustic transmission loss in the SOFAR channel faces challenges, such as excessively complex algorithms and computationally intensive calculations in classical methods. To address these challenges, a deep learning-based underwater acoustic transmission loss prediction method is proposed. By properly training a U-net-type convolutional neural network, the method can provide an accurate mapping between ray trajectories and the transmission loss over the problem domain. Verifications are performed in a SOFAR channel with Munk's sound speed profile. The results suggest that the method has potential to be used as a fast predicting model without sacrificing accuracy.

The present and future contribution of ships to the underwater soundscape

Since the industrial revolution the ocean has become noisier. The global increase in shipping is one of the main contributors to this. In some regions, shipping contributed to an increase in ambient noise of several decibels, especially at low frequencies (10 to 100 Hz). Such an increase can have a substantial negative impact on fish, invertebrates, marine mammals and birds interfering with key life functions (e.g. foraging, mating, resting, etc.). Consequently, engineers are investigating ways to reduce the noise emitted by vessels when designing new ships. At the same time, since the industrial revolution (starting around 1760) greenhouse gas emissions have increased the atmospheric carbon dioxide fraction x(CO2) by more than 100 μmol mol-1. The ocean uptake of approximately one third of the emitted CO2 decreased the average global surface ocean pH from 8.21 to 8.10. This decrease is modifying sound propagation, especially sound absorption at the frequencies affected by shipping noise lower than 10 kHz, making the future ocean potentially noisier. There are also other climate change effects that may influence sound propagation. Sea surface warming might alter the depth of the deep sound speed channel, ice melting could locally decrease salinity and more frequent storms and higher wind speed alter the depth of the thermocline. In particular, modification of the sound speed profile can lead to the appearance of new ducts making specific depths noisier. In addition, ice melting and the increase in seawater temperature will open new shipping routes at the poles increasing anthropogenic noise in these regions. This review aims to discuss parameters that might change in the coming decades, focusing on the contribution of shipping, climate change and economic and technical developments to the future underwater soundscape in the ocean. Examples are given, contrasting the open ocean and the shallow seas. Apart from the changes in sound propagation, this review will also discuss the effects of water quality on ship-radiated noise with a focus on propeller cavitation noise.

Effect of sound speed profile on the structure of acoustic pulse and the convergence zone in deep water

Through the experimental data collected in different seasons with the propagation conditions change in the East Indian Ocean (EIO) and the South China Sea (SCS), we observe that the structure of the sound speed profiles (SSPs) in these two seas area is very different and has a significant impact on the CZs and time arrival structure of acoustic pulses. In the EIO environment, sound energy transmitting along the sound channel axis (SCA) is relatively large, and the corresponding signals arrive first, whereas signals propagating off the SCA arrive late, which is totally different with the characteristics of the waveform in the SCS. At the same time, the range of the first CZ in the EIO is 7–8 km farther than that in the SCS, and the width of the CZs in the EIO is about 2–3 km narrower when the water depths in the two experimental areas are similar. Combined with the theoretical model, the influence mechanism of SSP on the time arrival structure and CZ propagation are theoretically analyzed, which well explains the phenomenon observed in the two experiments. The results can provide a reference for the application of communication sonar in the deep-sea remote environment.

Interpreting phase shifts of basin-scale arrivals observed in the North Pacific

The 75 Hz Kauai Beacon is well situated for observing the North Pacific Ocean acoustically. Somewhat conveniently, three ocean observing networks, with acoustic receivers located within the deep sound channel: two observatories 4 mm to the East (RCO: Regional Cabled Observatory and MARS: Monterey Accelerated Research System), and the International Monitoring Station at Wake Island 3.5mm to the West. During April–June of 2023, the Beacon transmitted a 37.5 Hz bandwidth m-sequence signal several times a day every 4 days. Upon processing, a persistent arrival structure is identified, which for which each arrival exhibits a unique, yet relatively steady phase over the course of a 20-min transmission. Leveraging acoustic propagation models to better understand the effect of the fluctuating ocean environment, we address the stability of the arrivals over the 20-min transmissions, from hour to hour, and over the course of the 3-month study. This includes examination of potential causes for the slow-phase shifts observed at all of the stations, and how it relates to Doppler. This talk concludes with a discussion on the future utility and research topics of interest, including long-range geo-positioning (underwater GPS), communications, thermometry, and tomography.

Shear wave effects in hydroacoustic recordings of the Comprehensive Nuclear-Test-Ban Treaty International Monitoring System

The hydroacoustic (HA) component of the Comprehensive Nuclear-Test-Ban Treaty Organization’s International Monitoring System (IMS) comprises six stations with triplets of moored hydrophones suspended in the SOFAR channel and five near-shore seismometer stations for T-phase detection. Understanding the conversion between compressional and elastic waves at the crust-ocean boundary or at large bodies of ice can contribute to better identification and association of arrivals. In this talk, examples recorded by the IMS HA network are presented with a view to stimulating discussion and further analysis: (i) T-phases from the September 2017 announced DPRK underground nuclear test recorded by the hydrophones of HA11 (Wake Island); (ii) a “precursor” southerly arrival at HA04 (Crozet Islands), associated with the hydroacoustic anomaly related to the loss of the Argentine submarine ARA San Juan, which is inter alia compatible with a Lamb wave propagating through the Antarctic ice-sheet; (iii) T-phases from earthquakes along the Nazca subduction zone scattered horizontally by the Juan Fernandez sea-mounts and recorded by HA03 (Chile); and (iv) conversion of signals from distant underwater explosions at the crust-ocean boundary near IMS T-stations. Benchmark studies to assess the accuracy of spectral element model setups previously used for scenarios (iv) would be particularly desirable.

Experimental Testing of the RAY Computational Program for Solving Acoustic Ranging Problems on Long Trajectories Including Shelves and Deep Seas

The results of studies of the features of the formation of pulse characteristics on the axis of an underwater sound channel in waveguides with different hydrological and bathymetric conditions of the Sea of Japan and the Sea of Okhotsk are discussed. The results of model calculations and experiments characterizing the regularities of propagation of low-frequency pulse signals in complex waveguides including shelf and deep sea for hundreds of kilometers are presented. It is shown that one of the main effects that determines long-range sound propagation in complex waveguides including the shelf and deep sea is the acoustic “landslide” effect. It is also shown that numerical modeling of the process of signal propagation from the shelf to the deep sea on acoustic traces in the Sea of Japan and the Sea of Okhotsk using the RAY program provides good convergence of the calculated and experimentally obtained impulse characteristics.

Experimental Testing of Acoustic Thermometry at the Scale of the Sea of Japan with a Receiver System Placed on the Axis of an Underwater Sound Channel

We discuss the results obtained when performing a test acoustic-hydrological experiment in August 2022 at a marine test site from the coast of Sakhalin Island to Kita-Yamato Bank in the Sea of Japan. The methodology of preliminary studies in the water area intended for studying climatic variability of temperature regimes of the aquatic environment based on numerical modeling using the RAY computational program and the NEMO ocean hydrodynamic circulation model is presented. One of the main results is the average temperature of the marine environment calculated with high accuracy on the axis of the underwater sound channel in the Sea of Japan on the 1000-kilometer acoustic trace at the crossing of the vortex system. The shape of the measuring system and the technical and computational means and methods described in the article can be used as a basis for the organization of high-precision operational monitoring of thermodynamic processes in extended sea areas.

Pilot Acoustic Tomography Experiment in the Sea of Japan at 1073 km Distance

This article discusses the results obtained from performing a test acoustic-hydrological experiment in August 2022 at a marine test site from the coast of Sakhalin Island to the Kita-Yamato Bank in the Sea of Japan. A methodology for preliminary studies in the water area is presented. It is designed to study the climatic variability of the temperature regimes of the aquatic environment based on numerical modeling using the RAY computer program and the NEMO ocean hydrodynamic circulation model. One of the main results is the value of the average temperature of the marine environment calculated with high accuracy on the axis of the underwater sound channel in the Sea of Japan on a thousand-kilometer acoustic path when crossing the vortex system. The appearance of the measuring system, technical means, and methods described in the article can be used as the basis for organizing high-precision operational monitoring of thermodynamic processes in extended marine areas.

A Comparative Study of the Temperature Change in a Warm Eddy Using Multisource Data

An ocean acoustic tomography (OAT) experiment conducted in the northern South China Sea in 2021 measured a month-long record of acoustic travel times along paths of over one hundred kilometers in range. A mesoscale eddy passed through the experimental region during the deployment of four acoustic moorings, providing unique OAT data for examining the deep temperature change in the eddy and for comparison with the Hybrid Coordinate Ocean Model (HYCOM) data. The existence of the eddy is first confirmed by the merged sea level anomaly (MSLA) image and HYCOM data and it can exceed the depth of the sound channel axis. The temperature changes measured by temperature and depth (TD)/conductivity–temperature–depth (CTD) loggers and by the OAT sound speed are in accordance with those reflected on the MSLA image during the movement of the eddy. However, the eddy movement prompted by temperature changes in the HYCOM data is different from that measured by TD/CTD. The modeled eddy intensity is at least two times less than the measured eddy intensity. At the sound channel axis depth, a factor of approximately 4.17 ms−1 °C−1 can be used to scale between sound speed and temperature. The transmission/reception path-averaged temperature of the eddy derived from the OAT-computed sound speed at the depth of the sound channel axis is five times greater than those in the HYCOM data. OAT is feasible as a tool to study mesoscale eddy properties in the deep ocean, while HYCOM data are not accurate enough for this mesoscale eddy at the sound channel axis depth. It is suggested that the model be refined by the OAT path-averaged temperature as constraints when the HYCOM data capture the mesoscale eddies.

A computational model of acoustic ray propagation in the deep-sound channel axis ocean region based on the Euler-Cromer method

Research on the model of sound wave propagation in the ocean on the axis of the deep sound channel has been carried out using discrete acoustic ray propagation with varying beam lengths. This study aims to obtain parameter values in the form of maximum angle of incidence and effective beam length to simulate acoustic wave propagation around the axis of the deep sound channel. The research was conducted by simulating the acoustic beam at a depth with the minimum acoustic velocity value by varying the angle of incidence and the length of the applied beam. In this study, constant acoustic velocity gradient and gradient data were used. Calculation of the velocity gradient from discrete data using the Euler-Crommer formula. From this research, it can be seen that at a depth of 854 m the minimum acoustic velocity is 1487.53 m/s. For a constant gradient with a value of 0.054 and variations in beam length from 1 m to 4 m, the maximum angle of incidence is 0.4 radians. Analytically, the radius of curvature of the rays is 28107.12 m. Comparing the analysis and computation results, there is a very small error of 0.02%, so this model can be used to simulate acoustic wave propagation based on real data gradients. The gradient pattern based on real data and variations in beam length from 1 m to 4 m, obtained a maximum angle of incidence of 0.4 radians.

The Acoustic Nature of “Storm Nose”, “Supercell” Vortices and Tornadoes

The process of cumulonimbus cloud Cb calvus formation in the middle latitudes of real atmosphere is analyzed in this work. Its transformation from initial lifecycle stage to “maturity” undergoes due to the formation of the waveguide called “aerial acoustic channel” in the troposphere near the level of temperature minimum that is close to 2 km altitude. This “aerial acoustic channel” can be considered as analog of “deep sound channel” that corresponds to the minimal sound speed level. Tropospheric “channel” related to the thermal inversion zone is almost unlimited horizontally. Synchronous generation of two compression waves (ascending one above Cb and descending one inside Cb) is caused by Cb calvus dome ascension. The first one can provoke the aerodynamic draft previously unexplained. The second one results in the growth of its “storm nose” and in the axial and peripheral descending mechanisms in Cb. The penetration of Cb into stratosphere results in the destruction of dynamic balance around Cb top and hence in its unloading in the descending decompression wave. Here the air cools down to the “dew point” in the place of conjugation with parental cloud – due to Snellius law it results in the formation of aerosol “vortex” as condensation front; this “vortex” has calculated value of its generatrix against vertical. Due to D. Snow’s criterion, this vortex forms either “supercell” vortex or tornado vortex.

Lower turning point convegence zone in deep water with an incomplete channel

In a deep sea sound channel, rays will bend due to the sound speed profile, and convergence zone will occur when the rays are intensive. Transmission loss in the convergence zone is smaller and it is conducive to acoustic detection and communication. Therefore the study of acoustic characteristics in convergence zone is always the focus of deep-sea acoustics. A long-range sound propagation experiment is conducted in the South China Sea. An equivalent broadband explosive sound source of 1 kg is placed at a depth of 200 m, and the hydrophone receives the data at 3146 m far. The processing and analysis of the experimental data indicate that there is a convergence zone below the sound channel axis in the incomplete deep channel. Compared with the upper turning point convergence zone near the surface, this convergence zone has a high convergence gain at a long distance. The caustic lines of refracted type and refracted surface-refleted type are determined by means of ray-normal mode theory. It is found that the location of the deep convergence zone observed in the experiment is consistent with the position of the refracted caustic line. It is proved that the convergence zone is a lower turning point convergence zone formed by the superposition of a large number of normal modes in the same phase, and it has a convergence effect at a certain depth below the sound channel axis in the deep sea. The formation conditions of the convergence zone and the influence of sound source depth on the caustic structure of the convergence zone are studied. The comparisons of the transmission loss and the width between the upper and lower turning point convergence zone at a long distance aremade. The analysis shows that the convergence gain in the seventh lower turning point convergence zone is still no less than 10 dB. The influence of the vertical structure of sound velocity on the lower turning point convergence zone is studied. The theoretical analysis results are in good agreement with the experimental data.

Observations of upper ocean sound-speed structures in the North Pacific and their effects on long-range acoustic propagation at low and mid-frequencies.

Three 1000-km long, high resolution conductivity, temperature, depth sections in the North Pacific Ocean obtained by the ship towed vehicle SeaSoar are analyzed to quantify 2005 March/April upper-ocean sound-speed structure and determine the effects on low to mid-frequency transmission loss (TL) through numerical simulation. The observations reveal a variable mixed layer acoustic duct (MLAD) with a mean sonic layer depth of 91-m, and an even higher variability, 80-m-average-thickness transition layer connecting the mixed layer (ML) with the main thermocline. The sound-speed structure is hypothesized to be associated with thermohaline processes such as air-sea fluxes, eddies, submesoscale, fronts, internal waves, turbulence, and spice, but the analysis does not isolate these factors. Upper-ocean variability is quantified using observables of layer depth, ML gradient, and sound speed to compute low order moments, probability density functions, horizontal wavenumber spectra, and empirical orthogonal function decomposition. Coupled mode acoustic propagation simulations at 400 and 1000 Hz were carried out using the sound-speed observations from the upper 400-m appended to climatology, which reveal propagation physics associated with diffraction, random media effects, and deterministic feature scattering. Statistics of TL reveal important energy transfers between the MLAD and the deep sound channel.

The Influence of Intratermocline Eddies on Sound Propagation in the Deep Part of the Sea of Japan

The results of stochastic modeling of the influence of positive and negative intratermocline eddies (lenses) on the spatiotemporal structure of acoustic fields in the deep part of the Sea of Japan are presented. The lens shape was modeled by an ellipse. The lens center and the emitter were located on the axis of the underwater sound channel. The vertical distribution of the acoustic field inside the lens was studied at various ranges in front of the center, in the center and behind the center of the lens, as well as at ranges of 40 and 100 km. A negative lens focuses the ray paths on the axis of underwater sound channel at all ranges. A positive lens has a defocusing effect, yet with an increase in range it decreases significantly. The impulse response of the propagation channel was studied on the underwater acoustic channel axis at the ranges of 40 and 100 km. The maxima of impulse response during the passage of the sound emitter through the negative lens shift in time towards increasing, and when passing through the positive lens – towards decreasing with respect to the impulse response of the propagation channel in the absence of lenses.

1J3-3 Influence on sound propagation in basin by the relation between change depth of continental slope and depth of sound channel axis

1J3-3 Influence on sound propagation in basin by the relation between change depth of continental slope and depth of sound channel axis

Azimuthal, spatial, and temporal variability of acoustic intensity in along-the-shelf direction of Chukchi shelf from June to August 2017

Spatial, temporal, and azimuthal variability of sound propagation with simultaneously measured oceanography on the Chukchi shelf is reported. Broadband acoustic signals (0.7 to 1.1 kHz) were transmitted from a single sound source placed near the sound channel axis in 150 m water depth and received by two arrays about 32 km away at different angles forming two acoustic tracks. One was along the 120–150 m isobath and the other crossing 120 to 220 m isobath. The angle between the two acoustic paths was 30 deg. Sound emitted from the common source shows different behavior along each acoustic track. Concurrently detailed water column environmental parameters (i.e., salinity and temperature) were measured in the region in both “along” and “cross-shelf” directions. Emergence of a warm water masses during from late June to July caused a variable channel where transmissions showed markedly large intensity variations (∼20 dB). This presentation quantifies data analysis using correlation between acoustics and environmental signals. [Work supported by ONR 322OA.]Spatial, temporal, and azimuthal variability of sound propagation with simultaneously measured oceanography on the Chukchi shelf is reported. Broadband acoustic signals (0.7 to 1.1 kHz) were transmitted from a single sound source placed near the sound channel axis in 150 m water depth and received by two arrays about 32 km away at different angles forming two acoustic tracks. One was along the 120–150 m isobath and the other crossing 120 to 220 m isobath. The angle between the two acoustic paths was 30 deg. Sound emitted from the common source shows different behavior along each acoustic track. Concurrently detailed water column environmental parameters (i.e., salinity and temperature) were measured in the region in both “along” and “cross-shelf” directions. Emergence of a warm water masses during from late June to July caused a variable channel where transmissions showed markedly large intensity variations (∼20 dB). This presentation quantifies data analysis using correlation between acoustics and enviro...

Azimuthal, spatial, and temporal variability of acoustic intensity in cross-shelf direction during the yearlong shallow water Canada Basin Acoustic Experiment 2016–2017

A yearlong study of spatial, temporal, and azimuthal variability of sound propagation with simultaneously measured oceanography on Chukchi shelf is reported. In a shallow water region, two acoustic sources were deployed for studying the “along” and “cross-shelf” propagation. The “along-shelf” study is presented separately [J. Ascout. Sci. Am. 145 (2019)]. Here, we focus on the “cross-shelf” signal propagation in two frequencies (0.7–1.1 and 1.5–4 kHz) transmitted from a single sound source placed near the sound channel axis in 320 m water depth. Three “cross-shelf” acoustic tracks connected the source and three receiver arrays placed along 50 m isobath. The angle between east most and west most tracks was around 106 deg. Another array at 250 m isobath was deployed along the middle track. Sound emitted from the common source shows different behavior along each track. Concurrently, detailed water column salinity and temperature were measured by environmental arrays in both “along” and “cross-shelf” directions. Sea surface ice was measured by an upward looking sonar for several months. Seasonal injection of different water masses in this region and variations of sea surface conditions (full-ice, transition, and free-surface) are examined using the acoustical oceanographic data. This paper quantifies analysis using correlation between acoustics and oceanographic signals. [Work supported by ONR 322OA.]

The Comprehensive Nuclear-Test-Ban Treaty’s global International Monitoring System, its core mission and its civil and scientific applications

The Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS) comprises 337 stations distributed around the world, recording data 24/7 and sending it in near-real time to the International Data Centre (IDC) in Vienna, where the data are processed to detect signs of nuclear tests. The four monitoring technologies in the IMS are: (i) seismology, (ii) hydroacoustics, (iii) infrasound, and (iv) radionuclide. The present state of completion of the IMS is nearing 90%, with the hydroacoustic network (6 stations with triplets of moored hydrophones suspended in the SOFAR channel and 5 near-shore seismometer stations for T-phase detection) being the first technology to be completely certified as of 2017. In addition to Treaty verification, IMS raw sensor data is available for civil and scientific studies, with hydroacoustic topics encompassing ocean acoustic propagation and tomography, submarine earthquakes and volcanism, ocean soundscapes, marine mammal acoustics and disaster response. This presentation will kick off this ASA Special Session by presenting the current status of the network, with emphasis on the hydroacoustic component and on the hydrophone stations, their sustainment and pertinent technology watch, and by reviewing highlights from data analysis of events for Treaty verification and scientific and civil applications.

Seismoacoustic observations at deep ocean observatories: ACO, H2O, and OSN1

Deep seafloor (~5000 m) observations of seismoacoustic arrivals are presented from the Aloha Cabled Observatory (ACO), Hawaii-2 Observatory (H20), and Ocean Seismic Network-1 (OSN1). Acoustic observations of local earthquakes from the ACO 24 kHz hydrophone show extraordinarily high frequencies up to 165 Hz at distances to ~200 km—a unique window into earthquake source dynamics. Combined seismic and hydrophone observations show that the traditional T wave propagates as a seismoacoustic polarized interface wave “Ti” coupled to the seafloor. Seismoacoustic Ti waves propagating at the sound speed of water are routinely observed over megameter distances at H2O between Hawaii and California, even though the seafloor site is within the shadow zone for acoustic wave propagation. Internal waves or thermohaline covariation may or may not be sufficient to scatter acoustic energy from the SOFAR channel to the deep seafloor. Ti has also been observed on seismometers SSW of Oahu at the OSN1 site at the seafloor and within an ODP borehole 242 m into the basalt basement. The observation of Ti from an earthquake in Guatemala at OSN1, whose path is blocked by the Island of Hawaii, is consistent with scattering from the vicinity of the Cross Seamount.