Distributed acoustic sensing (DAS) is a novel tool in underwater acoustics that measures strain in an optical fiber from the phase change of backscattered coherent light to estimate acoustic pressure. This technology transforms fiber-optic cables into dense, synchronized arrays of thousands of acoustic sensors, offering unique opportunities for marine research. However, the directivity and frequency response of DAS measurements remain poorly understood. An experiment was conducted in 2025 to characterize the DAS acoustic pressure sensitivity of the GeoLab cable off the southern coast of Madeira Island. Sound source was deployed from R/V Discovery at 14 m depth, with a reference hydrophone suspended 1 m below the source. Data acquisition covered three short towed transects and six stationary stations at 100–3000 m ocean depths. The source repeatedly transmitted a signal comprising chirps in the 105–160 Hz band and 11 tonals between 55–195 Hz. Ambient and DAS self-noise were evaluated during periods between the transmissions. Gaussian beam and wide-angle parabolic equation-based propagation modeling was performed to account for sloping bathymetry and surface reflections. Results of the measurements of the frequency and angular dependence of DAS pressure sensitivity and signal-to-noise ratio will be presented. [Work supported by NSF and NERC.]

During the 2023 and 2024 New England Sea Mounts Acoustics (NESMA) experiments, the Naval Postgraduate School deployed a network of near-bottom Moored Autonomous Noise Recorders (MANRs) near the Atlantis II Seamounts. MANRs recorded ambient sound continuously for 2 months in 2023 and 6 months in 2024. Sub-millisecond timing accuracy was maintained with on-board chip-scale atomic clocks. This paper investigates the retrieval of acoustic empirical Green’s functions (EGFs) from cross-correlations of ambient sound simultaneously recorded by MANR#2 and MANR#3 located on steep seamount flanks and MANR#4 located on an abyssal plain. MANRs were located 4 m above the seafloor at depths of 2994, 4067, and 4443 m, respectively. Stacking cross-correlation functions from approximately 100 h of noise recordings is found to produce EGFs with stable causal and acausal direct arrivals in the 15–50 Hz frequency band. We then track the temporal evolution of the reciprocal and non-reciprocal components of the passively measured acoustic travel time, which contain information about the evolution of sound speed and flow velocity along the propagation path. Acoustic observations of the environmental variability in deep water in the vicinity of Atlantis II Seamounts are compared to tide-resolving MITgcm hydrodynamic simulations. [Work supported by ONR.]

A network of four Moored Autonomous Noise Recorders (MANRs) was deployed in the vicinity of the Atlantis II Seamounts for 52 days during the 2023 New England Seamounts Acoustics (NESMA) pilot experiment and for 5 months during the 2024 NESMA field experiment. In each experiment, two MANRs were placed on the steep flanks of the Atlantis II Seamounts and two were placed in the abyssal plain. Receiver depths ranged from 2500 to 4475 m. The moored receivers provided stable observation platforms in a region with highly variable and occasionally strong currents. This network of receivers was designed to investigate the feasibility of utilizing passive acoustic remote sensing methods to study a highly dynamic ocean region with complex bathymetry. Strong ocean variability at the experimental site was induced by its proximity to the Gulf Stream and, in 2024, the passage of Hurricane Ernesto. The analyses presented in this paper are based on ambient sound spectra and spectrograms acquired concurrently by multiple near-bottom receivers. The relation between spectral features of ambient sound and ocean dynamics is explored along with the statistical distribution of ambient sound, spatial and temporal variability of ambient sound spectra, and the dependence of ambient sound on environmental conditions.

The Naval Postgraduate School deployed a network of Moored Autonomous Noise Recorders (MANRs) in the Northwest Atlantic near the Atlantis II Seamounts during the 2023 NESMA Pilot Experiment. This study examines the retrieval of ocean current speeds from flow noise recorded over a 52-day deployment. A strong correlation was observed between acoustic noise intensity at infrasonic frequencies (<20 Hz) and current speeds. Distinct spectral properties enabled differentiation between flow noise and ambient sound, including shipping noise. A regression tree machine-learning model trained using data from MANR #1, equipped with both a hydrophone and a current meter, facilitated the inference of current speeds with 1-min resolution at MANR #2, which was equipped solely with a hydrophone. MANRs #1 and #2 were located on steep seamount flanks at depths of 2573 and 2994 m, respectively. The regression tree model estimated current speeds of up to 107.7 cm/s at MANR #2, which was validated by spectral comparisons of flow noise at the two moorings. These findings underscore the potential for hydrophones to function as effective tools for long-term current monitoring, offering critical insights into deep-sea currents and their impact on seafloor dynamics and sediment transport. [Work supported by ONR.]

Naval Postgraduate School operated a network of Moored Autonomous Noise Recorders (MANRs) in the vicinity of the Atlantis II Seamounts during the 2023 New England Seamounts Acoustics (NESMA) Pilot experiment. Each MANR had a single hydrophone located a few meters above the seafloor. Acoustic pressure was recorded continuously by three MANRs for about 2 months. Two of the MANRs were located on steep flanks of the Atlantis II Seamounts and the other MANR was deployed in a deep trench. In addition to ambient sound, MANRs recorded signals from various compact sources of opportunity, including passing ships, chirp signals from tomographic moorings SIO-E and SIO-N of the Scripps Institution of Oceanography, and the powerful impulsive sound generated by the catastrophic demise of the SIO-E mooring. In this work, arrival patterns from SIO-E chirps and the impulsive sound are analyzed to determine time-dependent source depth. Frequency and angular dependence of the amplitudes of identified ray arrivals are employed to estimate roughness and constrain geoacoustic parameters of the seafloor on SIO-E—MANR propagation paths. Additional constraints on seafloor roughness and reflectivity are derived from the observed Lloyd’s mirror-type interference pattern of broadband noise of the R/V Neil Armstrong. [Work supported by ONR.]

Episodes of exceptionally strong near-bottom currents were encountered at depths of over 2500 m in a 52-day deployment of moored autonomous acoustic noise recorders (MANRs) over the Atlantis II Seamounts in the Northwest Atlantic. A strong correlation is found between the current speed and acoustic noise intensity, especially at infrasonic frequencies below 20 Hz. Flow noise and ambient sound, including shipping noise, made comparable contributions to the measured acoustic intensity but had distinct spectral properties. This paper explores a way to identify and quantify the differences between flow noise and ambient sound in the pressure fluctuations measured by a hydrophone and find statistical characteristics of the fluctuations which contain robust information about the flow speed. A regression tree machine learning model was developed to relate the acoustic features of flow noise to directly measured current speeds. By training the model using data from a MANR equipped with a hydrophone and current meter, the time series of current speed was obtained with 1-min resolution at another MANR, where only acoustic data were available. Accuracy of the inferred current speeds was confirmed by comparing the dependence of flow noise spectra on the current speed at the two MANRs.

Naval Postgraduate School deployed a network of Moored Autonomous Noise Recorders (MANRs) in the vicinity of the Atlantis II Seamounts for 2 months during the 2023 New England Seamounts Acoustics (NESMA) pilot experiment and for 5 months during the 2024 NESMA field experiment. In each experiment, two MANRs were placed on the steep flanks of the Atlantis II Seamounts and two were placed along a deep trench. Strong, episodic near-bottom flows were observed a few meters above the seafloor during both experiments at water depths between 2550 and 4450 m. An assemblage of measurements from tilt current meters, flow noise over the MANR hydrophones and variations in in situ temperature were used to identify the unexpected flows. In this work, we examine the characteristics of the irregular flows, their spatial and temporal relationship with the Gulf Stream, and the implications of the flows for sound propagation and acoustic measurements in this region of complex bathymetry and oceanography. [Work supported by ONR.]

Knowledge of near-bottom ocean current velocities and especially their extreme values is necessary to understand geomorphology of the seafloor and composition of benthic biological communities and quantify mechanical energy dissipation by bottom drag. Direct measurements of near-bottom currents in deep ocean remain scarce because of logistical challenges. Here, we report the results of flow velocity and pressure fluctuation measurements at three sites with depths of 2573–4443 m in the area where the Gulf Stream interacts with the New England Seamounts. Repeated episodes of unexpectedly strong near-bottom currents were observed, with the current speed at 4443 m of more than 0.40 m/s. At 2573 m, current speeds exceeded 0.20 m/s approximately 5% of the time throughout the entire eight-week measurement period. The maximum flow speeds of over 1.10 m/s recorded at this site significantly surpass the fastest previously reported directly measured current speeds at comparable or larger depths. A strong correlation is found between the noise intensity in the infrasonic band and the measured current speed. The noise intensity and the characteristic frequency increase with the increasing current speed. Machine-learning tools are employed to infer current speeds from flow-noise measurements at the site not equipped with a current meter.

Ambient sound was continuously recorded for 52 days by three synchronized, single-hydrophone, near-bottom receivers. The receivers were moored at depths of 2573, 2994, and 4443 m on flanks and in a trough between the edifices of the Atlantis II seamounts. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.5 to 4000 Hz. Statistical distribution of sound intensity exhibits much heavier tails than in the expected exponential intensity distribution throughout the frequency band of observations. It is established with high statistical significance that the data are incompatible with the common assumption of normally distributed ambient noise in deep water. Spatial variability of the observed ambient sound appears to be controlled by the seafloor properties, bathymetric shadowing, and nonuniform distribution of the noise sources on the sea surface. Temporal variability of ambient sound is dominated by changes in the wind speed and the position of the Gulf Stream relative to the experiment site. Ambient sound intensity increases by 4-10 dB when the Gulf Stream axis is within 25 km from the receivers. The sound intensification is attributed to the effect of the Gulf Stream current on surface wave breaking.

Ambient sound was recorded for about two months by three synchronized, single-hydrophone receivers that were moored at depths of 2573, 2994, and 4443 m on seamount flanks. Hydrophones were located within 5 m from the seafloor. The data reveal the power spectra and intermittency of the ambient sound intensity in a 13-octave frequency band from 0.5 to 4000 Hz. Statistical distribution of pressure amplitude exhibits much heavier tails than the expected Rayleigh distribution throughout the frequency band of observations. Spatial variability of the observed ambient sound is controlled by the seafloor properties, bathymetric shadowing, and nonuniform distribution of the noise sources on the sea surface due to the Gulf Stream and its meanders. Interferometry of the ambient sound recorded by the receivers with the horizontal separations of 7.0 km shows strong variations of the acoustic travel time between the receivers along a surface-reflected path due to the evolution of the Gulf Stream position. The magnitude of the variations of the passively measured acoustic travel time is consistent with the available contact measurements of the sound speed profiles. Environmental inferences derived from the ambient, shipping, and flow noise data will be discussed. [Work supported by the ONR TFO DRI.]

Motivated by the 2012 Florida Straits Noise Interferometry Experiment, this paper investigates a timereversal-based approach to acoustic remote sensing of the ocean. The signal processing mimics operation of a physical time-reversal mirror. The input data for the simulated time-reversal mirror can be obtained using either a compact, broadband sound source or cross-correlation of the diffuse noise recorded by spatially separated receivers. Low-frequency sound propagation is considered over ranges that are large compared to the water depth. The approach exploits the notion that the "best" focusing of the backpropagated time-reversed signal occurs at the "right point," when the backpropagation takes place in the same propagation medium as the one, where the data have been acquired. Various metrics of the focusing quality are considered. A combination of spatial and temporal characteristics of the focus is proposed that leads to a robust and unique solution of the geoacoustic inverse problems considered for a single-element passive time-reversal mirror. Inputs with rather low signal-to-noise ratio prove acceptable, which is particularly important in the passive remote sensing context.

Acoustic noise interferometry in the ocean relies on synchronized measurements of ambient sound at spatially separated points to passively measure the acoustic travel times between receiver locations. A network of four autonomous, moored, near-bottom acoustic receivers were deployed in the vicinity of the New England Seamounts for 52 days as part of the larger New England Seamount Acoustics (NESMA) Pilot experiment. Receiver depths ranged from 2500–4475 m. The moored receivers provided stable observation platforms in a region with highly variable and occasionally strong currents. The noise interferometry network aimed to investigate the feasibility of utilizing passive acoustic remote sensing methods to study a highly dynamic ocean region with complex bathymetry. Strong ocean variability at the experimental site was induced by its proximity to the Gulf Stream. This paper presents the initial analysis of water-depth dependence and intermittency of ambient sound spectra and spectrograms on near-bottom receivers. The relation between the spectral features of ambient sound and ocean dynamics is explored. Additionally, the feasibility of using the onboard Chip Scale Atomic Clocks to synchronize the experimental data and calculate broadband noise cross-correlation functions for each of the receiver pairs is discussed.

Applications of acoustic noise interferometry to passive remote sensing of the ocean rely on retrieval of empirical Green's functions (EGFs) from cross-correlations of ambient sound at spatially separated points. At ranges of tens of ocean depths, obtaining stable and accurate EGF estimates usually requires noise averaging periods of hours or days. Using data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey, it is found that at ranges of 40–70 ocean depths, the EGFs can be retrieved with noise averaging times as short as 64 s. The phenomenon is observed for various receiver pairs but does not occur simultaneously in all azimuthal directions. The rapidly emerging EGFs have a wider frequency band and a richer normal mode content than the EGFs obtained in previous studies using long averaging times and are better suited for monitoring physical processes in the water column. Available acoustic and environmental data is examined to understand the conditions leading to rapid EGF emergence from diffuse noise. Strong intermittency is observed in the horizontal directionality of ambient sound. Rapid emergence of EGF in shallow-water waveguide is found to occur when the directionality of diffuse ambient noise is favorable.

This paper presents some initial findings from the New England Seamount Acoustics (NESMA) Pilot experiment, focusing on the data obtained in April—June 2023 with three moored autonomous noise recorders (MANRs). Two MANRs were each equipped with a tilt current meter (TCM). Unexpectedly strong near-bottom currents were observed at depths of 2500m and 4443m, with the flow speeds reaching 110 cm/s and 80 cm/s, respectively. At 2500 m, recorded current speeds exceeded 20 cm/s approximately 5% of the time throughout the entire eight-week measurement period. Collocated ambient sound measurements revealed episodic large increases in the low-frequency noise intensity. A strong correlation is found between the noise intensity in the 2–20 Hz band and the measured current speed. As expected for flow noise, the intensity and the characteristic frequency of the power spectrum peaks increase with the increasing current speed. Observations of the flow noise by the MANR not equipped with a TCM indicate the presence and timing of strong current events and allow for an estimate of the current speeds at that location. The possible origins of the strong near-bottom currents and the flow noise effect on operation of deep-water acoustic systems will be discussed.

Noise interferometry offers an opportunity for passive ocean remote sensing through retrieval of empirical Green’s functions (EGFs) from cross-correlations of ambient sound. At ranges of tens of ocean depths, obtaining stable and accurate EGF estimates usually requires noise averaging periods of hours or days. Decreasing the necessary averaging times without compromising the EGF accuracy is critically important for operational applications. Using data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey, it has been found that occasionally averaging periods as short as a few minutes are sufficient. The phenomenon is observed for various receiver pairs but does not occur simultaneously in all azimuthal directions. Short averaging periods give EGFs in a broader frequency band and with a richer mode content. These are better suited to monitor physical processes in the water column. The conditions conducive to rapid EGF emergence are studied using in situ temperature measurements, meteorological information, and the acoustic data acquired with a horizontal line array. Strong intermittency is observed in the horizontal directionality of ambient sound. For various receiver pairs, EGFs are found to emerge rapidly when ambient sound directionality favors propagation in the direction from one receiver to the other.

Acoustic noise interferometry is applied to retrieve empirical Green's functions (EGFs) from the ambient and shipping noise data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Despite strong internal wave-induced perturbations of the sound speed in water, EGFs are found on 31 acoustic paths by cross-correlating the noise recorded on a single hydrophone with noise on the hydrophones of a horizontal linear array about 3.6 km away. Datasets from two non-overlapping 15-day observation periods are considered. Dispersion curves of three low-order normal modes at frequencies below 110 Hz are extracted from the EGFs with the time-warping technique. The dispersion curves from the first dataset were previously employed to estimate the seabed properties. Here, using this seabed model, we invert the differences between the dispersion curves obtained from the two datasets for the variation of the time-averaged sound speed profile (SSP) in water between the two observation periods. Results of the passive SSP inversion of the second dataset are compared with the ground truth derived from in situ temperature measurements. The effect of temporal variability of the water column during noise-averaging time on EGF retrieval is discussed and quantified.

Applications of acoustic noise interferometry to passive remote sensing of the ocean and improvement of sonar performance predictions rely on retrieval of empirical Green’s functions (EGFs) from synchronized measurements of ambient sound at spatially separated points. Usually, long noise averaging times of many hours and even days are employed to achieve precise passive measurements of the ray or normal mode travel times that are necessary for evaluation of the environmental parameters with oceanographically relevant accuracy. However, long averaging times have limited utility when nonlinear internal waves or other oceanographic processes cause significant, time-dependent variations in sound propagation conditions. Analysis of the noise records acquired during the Shallow Water 2006 experiment reveals that EGFs can be reliably retrieved from cross-correlations of noise recorded over periods as short as 10 minutes by moored receivers horizontally separated by 40–50 ocean depths. The measured EGFs evolve as the sound speed field changes. Suitability of the EGFs for acoustic remote sensing is demonstrated by geoacoustic inversion of the passively measured normal mode dispersion curves and comparison to previously reported inversion results. Rapid EGF retrieval has far-reaching implications for operational use of the underwater acoustic noise interferometry. [Work supported by NSF.]

This paper investigates application of a signal processing technique, which is motivated by time reversal, to geoacoustic inversions in a shallow-water waveguide. The signal processing mimics operation of a physical time-reversal mirror. The input data for the simulated time-reversal mirror can be obtained using either a compact, broadband sound source or cross-correlation of the diffuse noise recorded by spatially separated receivers. Low-frequency sound propagation is considered over ranges that are large compared to the water depth. The approach exploits the notion that the “best” focusing of the backpropagated time-reversed signal occurs at the “right point,” when the backpropagation takes place in the same propagation medium as the one, where the data have been acquired. Various metrics of the focusing quality are considered. A combination of spatial and temporal characteristics of the focus is proposed that leads to a unique solution of the geoacoustic inverse problem for a single-element passive time-reversal mirror. [Work supported by NSF.]

Empirical Green's functions are obtained for 31 paths in a highly dynamic coastal ocean by cross-correlation of ambient and shipping noise recorded in the Shallow Water 2006 experiment on a horizontal line array and a single hydrophone about 3600 m from the array. Using time warping, group speeds of three low-order normal modes are passively measured in the 10–110 Hz frequency band and inverted for geoacoustic parameters of the seabed. It is demonstrated that, despite very strong sound speed variations caused by nonlinear internal waves, noise interferometry can be successfully used to acoustically characterize the seafloor on a continental shelf.

Acoustic noise interferometry is applied to retrieve empirical Green's functions (EGFs) from the ambient and shipping noise data acquired in the Shallow Water 2006 experiment on the continental shelf off New Jersey. Despite strong internal wave-induced perturbations of the sound speed in water, EGFs are found on 31 acoustic paths by cross-correlating the noise recorded on a single hydrophone with noise on the hydrophones of a linear array about 3.6 km away. Two fifteen day-long datasets are considered. Dispersion curves of three low-order normal modes at frequencies below 110 Hz are extracted from the EGFs with time-warping technique. The dispersion curves from the first dataset were previously employed to estimate the seabed properties [T. Tan et al., J. Acoust. Soc. Am. 147, EL453–EL459 (2020)]. Here, using this geoacoustic model, we invert the differences between the higher-frequency part of the dispersion curves obtained from the two datasets for the variation of the time-averaged sound speed profile in water between the two observation periods. Results of the passive inversion are compared to the ground truth derived from in situ temperature measurements. The effect of temporal variability of the water column during noise averaging time on EGF retrieval is discussed and quantified.