Sound Speed Variations Research Articles

Quantifying the influence of the vertical resolution in ocean circulation modeling on the acoustic propagation through mesoscale eddies

Accurate numerical simulations of underwater acoustic propagation in a dynamic ocean—and its associated uncertainty—require using realistic environmental parameters as inputs and especially a high-fidelity representation of the expected spatio-temporal variability of the ocean sound speed in the volume of interest. In areas characterized by strong temperature and salinity variations (e.g., associated with long-living mesoscale eddies in the Gulf of Mexico), the approximate simulation of the 3D sound-speed field and its variability requires predictive oceanographic models capable of resolving such variations. This study investigates the impact of vertical resolution focusing on how it shapes the representation of the 3D sound speed variability through a suite of simulations of the northern Gulf of Mexico performed with a regional ocean model run at submesoscale permitting horizontal resolution (0.5 km) using increasing vertical resolution from 30 to 200 terrain-following layers over a one-month simulation interval (as described in the companion paper presented by Touret et al.). Geo-acoustic parameters were matched to the existing sediment database. In selected areas influenced by mesoscale eddies, ray tracing is used to determine the significance of increased resolution on acoustic propagation as a function of the sensor configurations and the expected sound speed variability.

Broadband acoustic wave propagation in temporally variable range dependent shallow water environment

Broadband acoustic wave propagation in shallow water regions involves interactions with the sea surface, sea bottom, and the water column. The topic of shallow water acoustics in the presence of variable water column has been of interest in recent years. Experimental observations have shown the importance of the effects due to the boundaries and the sound speed profile in the water column. However, more work needs to be done to establish ground truth in numerical modeling for temporally variable water column in a range dependent environment. In this paper we examine the broadband propagation on the shelf as well as the range dependent shelf break regions to depict how temporal variability of sound speed could change the dispersion characteristics of the broadband signal. Comparison between modeled and experimental data are provided. [Work supported by ONR OA program.]

Changes to mixed layer acoustic energy from spice and isopycnal tilt

Mixed layer acoustic propagation is predicted for fields that approximate two dynamically separate sources of sound speed variation: the tilting of isopycnals and “spice,” density compensated temperature and salinity changes. These fields are decomposed from a transect measurement over approximately 1000 km of the northeast Pacific Ocean. At a frequency with only one mixed layer mode, both fields contain blocking features, defined as sound speed structures less than 5 km wide that create significant acoustic energy loss. The transect shows two distinct regions where blocking features are caused by tilt or spice, indicating the acoustic importance of these dynamic processes depends on location. Statistics of mixed layer acoustic energy are investigated at two frequencies, averaging one or three mixed layer modes, both with and without blocking features. The spice field was found to produce higher loss than the tilt field. Both fields create more loss at lower frequencies when locations with blocking features were included, and more loss at higher frequencies without blocking features. Mixed layer mode amplitudes demonstrate more mixed layer modes at the higher frequency increases mode coupling, both preventing large scale acoustic energy loss and creating a more consistent loss mechanism.

Acoustic receptions at close ranges measured on autonomous underwater vehicles in the Beaufort Sea

Rapid warming of the Pacific Summer Water layer strengthens a subsurface duct in the Beaufort Sea, allowing for long-range propagation at low frequencies. An array of tomography sources was deployed within the duct as part of the Canada Basin Acoustic Propagation Experiment (CANAPE) to study acoustic propagation in this environment. The moored transceivers provide measurements of acoustic propagation at several ranges from 176 to 285 km. Additionally, two Seaglider vehicles equipped with hydrophone receivers navigated in and around the CANAPE array and recorded the transmissions from the moored sources at ranges as far as 530 km and as close as 2 km. A spatially variable sound speed environment was generated from in-situ data measured by the Seagliders and CTD casts from research vessels. Acoustic arrivals measured on the vehicles were matched to range-dependent acoustic predictions made with a broadband Parabolic Equation model to estimate source-receiver range. Acoustic receptions from multiple moored sources were used to localize the Seagliders. Here, we examine the close range (2–25 km) receptions and their impacts on acoustic localization.

Influence of the vertical resolution when simulating ocean sound speed variability in mesoscale eddies

Vertical resolution affects the representation of ocean sound speed according to a suite of regional simulations of the De Soto Canyon circulation in the Gulf of Mexico. Simulations have identical horizontal resolution of 0.5 km, partially resolving submesoscale dynamics, and increasing vertical resolution from 30 (i.e., comparable to what commonly used in mesoscale permitting or resolving hindcast and forecast products such as HYCOM) to 200 terrain-following layers. Simulations with 30- and 70-layers underestimate the ageostrophic contributions in and around the eddies below the mixed-layer and do not reproduce the sharp vorticity and density variations associated with the mesoscale circulations compared to the 140- and 200-layers runs. The ocean sound speed (based on the classical MacKenzie formula) was found to be far more variable when the submesoscale, ageostrophic circulations are captured also in their vertical structure and vertical contributions to the density field. Hence, the results of this study indicate that to better predict the influence of the submesocale oceanic circulation on ocean sound speed variability, model simulations should consider enhancing both horizontal and vertical resolution to resolve at least the first 3 baroclinic modes. To do so, more than 100 vertical layers were found to be needed in this study.

Non-intrusive mode based analyses to predict uncertainty in sound propagation in the ocean

Ocean dynamics can result in significant sound speed variability that are not well captured by deterministic numerical models and can impact differently high to low frequency acoustic propagation and bottom interactions by constraining horizontal and vertical paths. Uncertainty in the sound speed can, therefore, cause inaccuracies in acoustic based tactical decision tools and severely impact the accuracy of algorithms using sound propagation to estimate ocean volume states (e.g., ocean tomography or data assimilation). In this work, we propose to use a non-intrusive Reduced Order Modelling solution that consists of building a dictionary of static modes from historical ocean simulations with different settings and resolutions and climatology to derive an expedite uncertainty model along a central deterministic forecast. The system will then, through Orthogonal Matching Pursuit along the dictionary, use the available ocean model-data mismatches and/or acoustic innovations (e.g., arrival numbers and times, acoustic energy distribution, etc.) to define an ensemble of possible sound speed volume distributions and associated acoustic propagation outcomes. We will show results using a simplified simulation experiment that extracts “observations” from a nature reference field to document the procedure and benchmark results to reconstruct 3D sound speed fields from ocean-acoustic measurements.

A Novel FDTD–PML Scheme for Noise Propagation Generated by Biomimetic Flapping Thrusters in the Ocean Environment

Biomimetic flapping-foil thrusters can operate efficiently while offering desirable levels of thrust required for the propulsion of a small vessel or an Autonomous Underwater Vehicle (AUV). These systems have been studied both as main propulsion devices and for augmenting ship propulsion in waves. In this work, the unsteady hydrofoil loads are used to calculate the source terms of the Ffowcs Williams–Hawkings (FW-H) equation which is applied to model noise propagation in the underwater ocean acoustic environment. The solution provided by a simplified version of the Farassat formulation in free space is extended to account for a bounded domain and an inhomogeneous medium, characterizing the sea acoustic waveguide. Assuming the simplicity azimuthal symmetry of the environmental parameters, a numerical model is developed based on a Finite Difference Time Domain (FDTD) scheme, incorporating free-surface and seabed effects, in the presence of a variable sound speed profile. For the treatment of the outgoing radiating field, a Perfectly Matched Layer (PML) technique is implemented. Numerical results are presented illustrating the applicability of the method.

Experimental Study on the Impact of Seasonal Sound Speed Variability on Signal Detection Range in Arabian Sea


 
 
 Temporal variability of Signal Detection Range (SDR) with respect to measured noise level and sound speed is examined. An N x 2D acoustic model which included bathymetric variations, was used to study detection ranges for an area in Arabian Sea. Azimuthal and seasonal SDR at octave bands within 500 Hz were determined at different receiver depths. Study shows that seasonal change in sound speed profile resulted in high SDR and noise level in winter at the location. Study also confirms the significant seasonal difference in detection range corresponds to the cut off frequency at 160 Hz. Detection range for a receiver at a depth 40 m is observed to be high across the azimuth and seasons of study.
 
 

A robust method to estimate the coordinates of seafloor stations by direct-path ranging

The ranges derived from acoustic measurements between seafloor stations are relatively more accurate compared with those derived from the sea surface vessel transducer to the seafloor transponders, because measurements through mixed water layers will be affected by complex acoustic range errors. Coordinates of seafloor stations can be improved by the direct-path acoustic ranging. Systematic errors in acoustic rangings, however, will significantly deteriorate the accuracy of vertical coordinates. In order to mitigate the effects of these systematic errors (e.g., acoustic ray bending and sound speed variation errors in acoustic measurements on the seafloor station location parameters), the observation model needs to be finely constructed. First, a new observation model with acoustic ray bending and sound speed bias parameters is established. Then, using a seafloor geodetic network with four moored stations at a depth of about 3000 m in the South China Sea, the significance of the acoustic ray bending parameter is tested. The results show that (1) the acoustic ray bending parameter is significant at the 90% confidence level, which means that the acoustic ray bending error in the seafloor geodetic network is not negligible; (2) by estimating the coefficient of acoustic ray bending, the influence of the acoustic ray bending error on the vertical coordinate components can be significantly mitigated; our model improves the accuracy of the seafloor stations’ position with differences in the horizontal coordinate components less than 0.1 cm between the two-dimensional adjustment and three-dimensional adjustment, and also improves the vertical coordinate component to uncertainty less than 3.0 cm; (3) the relative movement between the moored stations is less than 50 cm, and the horizontal movement is larger than the vertical movement.

Analysis for Full-Field Photoacoustic Tomography with Variable Sound Speed.

Photoacoustic tomography (PAT) is a non-invasive imaging modality that requires recovering the initial data of the wave equation from certain measurements of the solution outside the object. In the standard PAT measurement setup, the used data consist of time-dependent signals measured on an observation surface. In contrast, the measured data from the recently invented full-field detection technique provide the solution of the wave equation on a spatial domain at a single instant in time. While reconstruction using classical PAT data has been extensively studied, not much is known for the full field PAT problem. In this paper, we build mathematical foundations of the latter problem for variable sound speed and settle its uniqueness and stability. Moreover, we introduce an exact inversion method using time-reversal and study its convergence. Our results demonstrate the suitability of both the full field approach and the proposed time-reversal technique for high resolution photoacoustic imaging.

Two-step aberration correction: application to transcranial histotripsy

Objective: Phase aberration correction is essential in transcranial histotripsy to compensate for focal distortion caused by the heterogeneity of the intact skull bone. This paper improves the 2-step aberration correction (AC) method that has been previously presented and develops an AC workflow that fits in the clinical environment, in which the computed tomography (CT)-based analytical approach was first implemented, followed by a cavitation-based approach using the shockwaves from the acoustic cavitation emission (ACE). Approach: A 700 kHz, 360-element hemispherical transducer array capable of transmit-and-receive on all channels was used to transcranially generate histotripsy-induced cavitation and acquire ACE shockwaves. For CT-AC, two ray-tracing models were investigated: a forward ray-tracing model (transducer-to-focus) in the open-source software Kranion, and an in-house backward ray-tracing model (focus-to-transducer) accounting for refraction and the sound speed variation in skulls. Co-registration was achieved by aligning the skull CT data to the skull surface map reconstructed using the acoustic pulse-echo method. For ACE-AC, the ACE signals from the collapses of generated bubbles were aligned by cross-correlation to estimate the corresponding time delays. Main results: The performance of the 2-step method was tested with 3 excised human calvariums placed at 2 different locations in the transducer array. Results showed that the 2-step AC achieved 90 ± 7% peak focal pressure compared to the gold standard hydrophone correction. It also reduced the focal shift from 0.84 to 0.30 mm and the focal volume from 10.6 to 2.0 mm3 on average compared to the no AC cases. Significance: The 2-step AC yielded better refocusing compared to either CT-AC or ACE-AC alone and can be implemented in real-time for transcranial histotripsy brain therapy.

An iterative transfer matrix approach for estimating the sound speed and attenuation constant of air in a standing wave tube.

In this work, an iterative method based on the four-microphone transfer matrix approach was developed for evaluating the sound speed and attenuation constant of air within a standing wave tube. When the air inside the standing wave tube is treated as the material under test, i.e., as if it were a sample of porous material, the transfer matrix approach can be used to identify the air's acoustic properties. The wavenumber within the tube is complex owing to the formation of a visco-thermal boundary layer on the inner circumference of the tube. Starting from an assumed knowledge of the air properties, an iterative method can be applied in the post-processing stage to estimate the complex wavenumber. Experimental results presented here show that although the results are sensitive to ambient temperature, a semi-empirical formula previously proposed by Temkin [(1981). Elements of Acoustics (John Wiley & Sons)] matches closely with the measured sound speed and attenuation constant, as does a theoretical formulation proposed by Lahiri et al. [(2014). J. Sound Vib. 333(15), 3440-3458]. Further, it is shown that the Temkin [(1981). Elements of Acoustics (John Wiley & Sons)] and Lahiri et al. [(2014). J. Sound Vib. 333(15), 3440-3458] predictions accurately represent the variation of sound speed with frequency, in contrast to the formula recommended in the ASTM E1050 standard [(2019). American Society for Testing and Materials], in which the sound speed is assumed to be independent of frequency.

Comprehensive vibro-acoustic characteristics and mathematical modeling of electric high-speed centrifugal compressor surge for fuel cell vehicles at various compressor speeds

The electric high-speed centrifugal compressor is the mainstream air compressor in present fuel cell vehicles. Surge strongly limits the stable operating range of centrifugal compressors. Therefore, it is of tremendous significance to study transient vibro-acoustic characteristics of the compressor surge evolution and early warnings of compressor surge. In this paper, comprehensive vibro-acoustic experiments of the compressor surge evolution are first conducted, and time-varying frequency characteristics are visualized through the short-time Fourier transform. The results show that the increase and fluctuation of the total sound pressure level at deep surge are mainly caused by the pressure pulsation at the centrifugal compressor outlet. Moreover, a rotating stall onset criterion based on acoustic signals is proposed, and the occurrence of severe rotating stall can be predicted by the precursor about 1.5–3 s in advance. Then, a mathematical model of the centrifugal compressor surge is established considering the variation of sound speed and introducing an additional volume, which unifies centrifugal compressor surge models at different compressor speeds. The maximum prediction error of the surge frequency is 1.95%. The model eliminates the pain point of the traditional Moore-Greitzer model which fails to predict the compressor surge characteristics at variable compressor speeds accurately. Finally, the impact of compression system dimensions on centrifugal compressor surge characteristics is quantitatively analyzed, and corresponding verification experiments are conducted. The proposed surge model can be used not only for quantifying the dynamic characteristics of compressor surge at transient speeds but also for designing active surge control strategies for variable vehicle operating conditions.

Influence of internal wave induced sound speed variability on acoustic propagation in shallow waters of North West Bay of Bengal

This study focuses on the influence of internal wave induced sound speed variability on acoustic propagation parallel to the coast in shallow waters of North West Bay of Bengal (BoB). An acoustic transmission and reception experiment had been conducted at the site during February 2018 with simultaneous acquisition of oceanographic data, to examine propagation characteristics. The 950 Hz signal from an anchored vessel was received by an array of hydrophones moored 50 km away, with a loss of ∼ 80–95 dB with respect to the source position, receiver depth and sound speed variability. Sound speed variations in the acoustic environment were simulated using a shallow water internal wave model called WAVE to characterize the acoustic environment. Derived internal wave features from oceanographic and acoustic observations from study location during the experiment period were used to simulate the internal wave acoustic environment. Sound propagation using BELLHOP model was carried out with a simulated internal wave sound speed environment for 950 Hz source frequency. Model and observed temporal variability of transmission loss with sound speed at both source depth and thermocline depth has been addressed. Transmission loss features are comparable with field observations and model limitations are reflected in the results thus showing errors between two approaches. Range dependent bathymetry and other ocean acoustic parameters influence transmission loss at the study location. Temperature and salinity are major parameters influencing sound speed fluctuation in shallow waters, and corresponding variations are reflected in transmission loss and propagation. Finally this study provides a quantitative support in transmission loss in both modeling and experimental approaches at the study location.

Viability of the mixed layer duct with observed dynamics

While the upper ocean of the north Pacific is largely mixed due to air-sea interaction, a 1000 km sea-soar transect shows the existence of stratification in the mixed layer that persists over many tens of kilometers. The effects of this stratification on mixed layer acoustic propagation depends on the sound speed differences across the different density layers, and observations show the sound speed varies rapidly both between and along the isopycnals. Acoustic models are used to separately quantify the importance of the mixed layer stratification and the density compensated sound speed variability on propagation in the mixed layer duct. The observed sound speed field is modeled as the superposition of three fields: a smooth background, the stratification field with slow isopycnal sound speed variability, and the remnant unstratified sound speed variation. The sound speed variation in the stratified mixed layer is attributed to internal waves and eddy filaments and in the unstratified field to ocean spice. Both types of observed sound speed variation significantly change, or completely block, mixed layer propagation. Examples are presented of blocking features in the tilt or spice fields separately, and where they exist only in the superposition of the two fields.

Time-domain formulation for coherent plane wave compounding in horizontally layered media

The locations of origin of ultrasonic echoes from soft tissue in conventional ultrasound beamforming are based on measured time of flight. For simplicity it is usually assumed that the region of interest is homogeneous with a constant sound speed, but in many in vivo and benchtop applications the imaging region is composed of horizontal layers, each with a distinct sound speed. In addition to time-of-flight complications, sound speed differences between the layers result in refraction through the interface of both the transmitted pulse and those reflected from the tissue, further influencing the fidelity of beamforming for target localization or other quantitative ultrasound metrics. A formulation is presented for delay-and-sum coherent Plane Wave Compounding (PWC) that accounts for variation in sound speed and refraction of ultrasound pulses in a region with two horizontal layers. The algorithm is formulated completely in the time domain, enabling simple implementation or extension of existing homogeneous PWC algorithms. Simulated and experimental images are investigated for a typical two-layer benchtop configuration, where the sound speeds in the upper and lower layers are relatively slow (water or gel) and fast (muscle), respectively. Extension of the formulation to a medium with N distinct layers is discussed.

An iterative approach for estimating the sound speed and attenuation constant of air in a standing wave tube

In the present work, an iterative method based on the four-microphone transfer matrix approach was developed for evaluating the sound speed and attenuation constant of air within a standing wave tube. In particular, when the air inside the standing wave tube is treated as the material under test, i.e., as if it were a sample of porous material, the transfer matrix approach can be used to identify the air’s acoustic properties. Note that the wavenumber within the tube is complex owing to the formation of a visco-thermal boundary layer on the inner circumference of the tube. Starting from an assumed prior knowledge of the air properties, an iterative method can be applied in the post-processing stage to estimate the complex wavenumber accurately. Experimental results presented here show that although the results are sensitive to ambient temperature, a formula previously proposed by Temkin matches closely with the measured sound speed and attenuation constant. Furthermore, it is shown that the Temkin prediction accurately represents the variation of sound speed with frequency, in contrast to the formula recommended in the ASTM E1050 standard, in which the sound speed is assumed to be independent of frequency.

Automated acoustic arrival matching using a convolutional neural network approach informed by statistics of acoustic scattering from internal waves

A machine learning model was developed to automatically align underwater acoustic measurements taken at various depths and ranges from a transmitting source in the Philippine Sea to a reference model of long range acoustic arrival structure, simultaneously determining source-receiver range and travel-time offsets associated with multipath arrivals. Ocean sound-speed variability complicates the task as the measured arrivals may exhibit scattering not present in range-independent predictions. Monte Carlo style broadband parabolic equation simulations through random internal wave fields consistent with the Garrett-Munk internal wave energy spectrum were used to generate a large data set of simulated acoustic receptions including scattered multipath arrivals with known source-receiver ranges and imposed travel time offsets. These simulated receptions were used to train and evaluate the machine learning model for arrival pattern matching to the reference model. The inclusion of various data dimensions, such as peak amplitude and width, and contextual information, such as range and depth, were also explored as input to the model. Ranging results for the machine learning model were compared to a programmatic solution engineered for the same task.

Prediction of the ocean water sound speeds via attribute-guided seismic waveform inversion

Oceanic temperature and salinity are important for studying ocean dynamics and marine ecosystems, and for developing climate prediction models. Physical oceanographers measure these properties using expendable instruments, providing accurate measurements at high vertical resolution at sparse lateral locations. Accuracy of these temperature and salinity data when laterally interpolated and used for ocean dynamics, marine biology, and weather-related studies is questionable. Marine seismic data indicate visible reflections within the water columns, which, in turn, are primarily controlled by the vertical and lateral variations of the ocean water sound speeds. Because these sound-speed variations are functions of temperature and salinity, water-column seismic reflection data have been inverted for sound speeds and used to estimate temperature and salinity. These seismically derived temperature and salinity data have lower vertical, but much finer lateral resolution than those measured by expendable instruments, and therefore are useful for physical oceanography, marine biology, and climate research. However, quality of the seismic inversion relies on a good initial model, which requires generating models at sparse locations over 2D or 3D seismic data volumes and interpolating them over the horizon, manually picked from the stacked seismic data. Because horizon picking is subject to human error and bias, replacing it with an automated process is desirable. We outline a new attribute-guided methodology for initial model generation where no horizon picking is necessary. Starting with this initial model, we run prestack waveform inversion based on genetic algorithm optimization and demonstrate that our method can predict reliable sound-speed profiles. We conclude that our method is a good way for estimating ocean water sound speeds and relating them to the temperature and salinity.

Ocean Sound Propagation in a Changing Climate: Global Sound Speed Changes and Identification of Acoustic Hotspots

AbstractClimate change is a relevant threat on a global scale, leading to impacts on ecosystems and ocean biodiversity. A considerable fraction of marine life depends on sound. Marine mammals, in particular, exploit sound in all aspects of their life, including feeding and mating. This work explores the impact of climate change in sound propagation by computing the three‐dimensional global field of underwater sound speed. The computation was performed based on present conditions (2006–2016) and a “business‐as‐usual” future climate scenario (Representative Concentration Pathway 8.5), identifying two “acoustic hotspots” where larger sound speed variations are expected. Our results indicate that the identified acoustic hotspots will present substantial climate‐change‐induced sound speed variations toward the end of the century, potentially affecting the vital activities of species in the areas. Evidence is provided of the impact of such variation on underwater sound transmission. As an example of a species impacted by underwater transmission, we considered one marine mammal endangered species, the North Atlantic right whale (Eubalaena glacialis), in the northwestern Atlantic Ocean. To the best of our knowledge, this is the first global‐scale data set of climate‐induced sound speed changes expected under a future scenario. This study provides a starting point for policies oriented research to promote the conservation of marine ecosystems and, in particular, endangered marine mammals.