England Shelf Break Research Articles

High-resolution acoustically informed maps of sound speed.

As oceanographic models advance in complexity, accuracy, and resolution, in situ measurements must provide spatiotemporal information with sufficient resolution to inform and validate those models. In this study, water masses at the New England shelf break were mapped using scientific echosounders combined with water column property measurements from a single conductivity, temperature, and depth (CTD) profile. The acoustically-inferred map of sound speed was compared with a sound speed cross section based on two-dimensional interpolation of multiple CTD profiles. Long-range acoustic propagation models were then parameterized by the sound speed profiles estimated by the two methods and differences were compared.

Exploiting environmental asymmetry for vessel localization from the vertical coherence of radiated noise.

A method to determine the range and bearing of a moving broadband acoustic source, such as a surface vessel, using the coherence measured on two omni-directional, vertically separated hydrophones is demonstrated using acoustic data recorded near Alvin Canyon on the New England shelf break. To estimate the vessel's range, two theoretical approaches, a half-space model and a Pekeris waveguide model based on normal modes, establish simple relationships between the broadband signal coherence and frequency, source range, and the vertical separation of the receiver hydrophones. A brute force inversion produces a passive acoustic estimate of vessel range. Rapidly changing bathymetry with large features, such as that near Alvin Canyon, produces azimuthal asymmetry in the plan-view coherence pattern about the receivers due to horizontal refraction, focussing, and the up- (down-) slope compression (extension) of modal interference patterns. For vessels with a constant speed and heading, this generates an asymmetry in the received power and vertical coherence fringing pattern. This effect is first demonstrated using reciprocal three-dimensional parabolic equation and raytracing models in an idealized Gaussian canyon, then observed in Alvin Canyon measurements. By comparing the experimental observations to the modeled coherence, the vessel's bearing and range relative to the receivers are obtained.

Acoustic propagation through a biological deep scattering layer at the New England shelf break

The New England Shelf Break is a dynamic ocean environment with strong spatial variability due to complex physical processes and interactions with warm core rings originating from the Gulf Stream. An acoustic autonomous underwater vehicle (AUV) was deployed in the New England Shelf Break Acoustics (NESBA) experiment in May 2021 to investigate environmental variability and its effect on sound propagation. The AUV system was comprised of a modified REMUS 600 vehicle with a hull-mounted 3.5 kHz transducer, a downward-facing EK80 echosounder, and a towed linear hydrophone array. Shipboard EK80 data measured during the AUV mission showed the presence of a biological deep scattering layer. Preliminary analysis of AUV source signals passing through the layer suggested significant attenuation compared to signals not passing through the layer. To investigate this further, a parabolic equation (PE) model with range-dependent attenuation patches in the water column was constructed to replicate the biological attenuation through the scattering layer. The model uses estimates of absorption and scattering derived from shipboard and AUV EK80 data. Through this study we aim to better understand the scattering effects of this deep biological layer on acoustic propagation. [Work supported by the Office of Naval Research.]

Effects of seabed corrugation and stratification on 3D acoustic propagation along the New England Shelf Break

Acoustic signals transmitted and recorded during the New England Shelf Break Acoustics Experiment in 2021 (NESBA) display evidence of complex scattering effects which were not captured in in situ modeling efforts. In particular, although the acoustic parabolic equation models used can predict later arrivals of a single transmission, initial arrivals show mismatch between model predictions and observation. In this presentation, estimates of sub-bottom sediment structure are supplemented by a high-resolution model of the bathymetry to simulate 3D sound propagation across a realistic seabed. The influence of the seabed corrugation and stratification on acoustic arrival times is examined in the context of the NESBA environment. [This research is supported by the Office of Naval Research].

Seabed Analysis on the New England shelf break using ambient sound data and trans-dimensional geoacoustic inversion

The New England Shelf Break Acoustics (NESBA) Signals and Noise experiment was conducted in April-May 2021. Ambient sound data were collected over several days in the mud-patch area as well as in deeper water closer to the shelf break. A 16-hydrophone vertical array was used to measure the natural sound of breaking waves on the sea-surface. Using beamforming in the 500–700 Hz band, these data were used to obtain an estimate of the bottom reflection coefficient as well as the seabed layering. The reflection coefficient data were subsequently used with trans-dimensional inversion techniques to produce a geoacoustic model for the seabed (e.g., sound speed, density, and layering). Results show the ambient sound data can be used to produce well resolved geo-acoustic parameters, especially in the upper part of the seabed (e.g., <10 m). These results are compared between several locations on the New England Shelf Break area and are also compared with other published results using different estimation techniques. In addition, some of the issues related to the impacts of data errors and preferred measurement geometries will also be presented. [Work supported by the Office of Naval Research.]

Geoacoustic inversion using 3D modal rays on the New England Shelf Break

During the Seabed Characterization Experiment in 2021 (SBCEX21), hydrophones deployed on the seabed of the New England Shelf Break recorded acoustic signals from SUS charge explosions. Acoustic data from one of these TOSSIT hydrophones displays evident modal structures which cannot be fully captured with 2D propagation models. In this presentation, 3D acoustic modal ray tracing is shown to provide a stronger match for recorded modal travel times, and is used to invert for geoacoustic parameters of the seabed. Performance of 2D and 3D modal propagation models are presented, and limitations of the methods are shown in the context of large parameter space inversion efforts. [This research is supported by The Office of Naval Research.]

Probabilistic acoustic predictions leveraging NESBA and ARGO measurements

Ocean acoustic propagation models used for sonar performance prediction often rely on global or regional ocean modeling and data assimilation, or monthly climatologic averages, for estimating the 4D ocean temperature and salinity (T/S) field. Ocean models rely heavily on satellite-based ocean surface measurements, e.g., sea surface temperature (SST) and height (SSH), plus typically vastly less resolved in space and time vertical water column measurements derived using ship-based and/or ARGO float instruments. Systems also exist that create 3D synthetic T/S fields from satellite SST/SSH measurements, e.g., the Modular Ocean Data Assimilation System (MODAS) and the more recent Improved Synthetic Ocean Profile (ISOP) system. It is unclear, however, how well these synthetic T/S fields capture significant ocean ducting conditions. Similarly, climatologic averages combine historical vertical T/S profile data in spatial cells by month; but the averaging process tends to blur out important features. This talk will address the use of 2021 New England Shelf Break Acoustics (NESBA) experiment and ARGO vertical profile data for: (1) testing the degree to which ocean models or climatology capture important acoustic features under a range of conditions; and (2) providing more realistic sonar performance prediction means and uncertainties. [Work supported by the Office of Naval Research].

Optimally Distributed Receiver Placements Versus an Environmentally Aware Source: New England Shelf Break Acoustics Signals and Noise Experiment

Optimally Distributed Receiver Placements Versus an Environmentally Aware Source: New England Shelf Break Acoustics Signals and Noise Experiment

In situ measurements of sediment shear wave speed from the New England Mud Patch and shelf break areas using the acoustic coring system

The acoustic coring system (ACS) is a probe-equipped gravity corer that provides in situ measurements of compressional and shear wave speed and attenuation. During an April 2022 coring survey, the ACS was deployed at 36 locations within the New England Mud Patch and New England Shelf Break areas. Data from these measurements will be presented to characterize the depth-dependent structure of the geoacoustic seabed properties as well as their spatial variability. The in situ measurements will be interpreted in the context of stratigraphic layering measured by a seismic survey. Historically, in situ shear speed measurements of the seabed have proved difficult to obtain. From the 2022 survey, depth-dependent (up to 4 meters) shear speed profiles at 800 and 1200 Hz were produced from a subset of the 36 deployments. These results, along with methods for analysis, challenges in the measurements, and supplemental laboratory experiments will be discussed. The results will also be compared to measurements from previous experiments. [Work sponsored by ONR.]

Acoustic ducting by shelf water streamers at the New England shelfbreak.

Greater sound speed variability has been observed at the New England shelfbreak due to a greater influence from the Gulf Stream with increased meander amplitudes and frequency of Warm Core Ring (WCR) generation. Consequently, underwater sound propagation in the area also becomes more variable. This paper presents field observations of an acoustic near-surface ducting condition induced by shelf water streamers that are related to WCRs. The field observations also reveal the subsequent disappearance of the streamer duct due to the passage of a WCR filament. These two water column conditions are investigated with sound propagation measurements and numerical simulations.

Localization of an acoustic autonomous underwater vehicle using multi-channel back-propagation methods

Autonomous underwater vehicles (AUVs) are extremely useful tools for studying the acoustics of complex ocean environments due to their ability to detect environmental changes with greater spatial resolution than fixed moorings. During the New England Shelf Break Acoustics (NESBA) experiments in May 2021, an AUV system was deployed to collect acoustic data for investigating the local biological, physical, and geological oceanography. This acoustic AUV system was comprised of a modified REMUS 600 vehicle, a hull-mounted 3.5 kHz transducer, and a towed multi-channel hydrophone array. Along mission profiles where the AUV is fully submerged but too shallow for bottom-lock navigation, one challenge is accurate localization of the AUV. Localization was performed in post-processing using multi-channel back-propagation methods applied to AUV source signals received at mooring hydrophones in the NESBA network as well as ship-towed sound source signals received at the AUV-towed array. Uncertainty in the localization estimates due to spatiotemporal sound speed changes was investigated, and hydrophone mooring tilt angle was determined by minimizing the localization uncertainty. Following localization, this AUV acoustic data was used to investigate local seafloor sub-bottom properties and the acoustic effects of biological scattering layers and varying physical oceanography. [Work supported by the Office of Naval Research.]

In situ measurements of compressional and shear wave speed from the New England Mud Patch and Shelf Break using the Acoustic Coring System

In situ measurements of geoacoustic properties provide direct characterization of the seabed at near ambient conditions. The Acoustic Coring System (ACS) is a gravity corer equipped with acoustic probes that obtain in-situ compressional wave (30–200 kHz) and shear wave (400–1200 Hz) measurements as the corer penetrates the seabed. During the April 2022 R/V Endeavor coring survey, the ACS was deployed at 36 locations within the New England Mud Patch and New England Shelf Break areas. Data from these measurements will be presented to characterize the depth-dependent structure of the geoacoustic seabed properties as well as their spatial variability. The in-situ measurements will be interpreted in the context of stratigraphic layering measured by a seismic survey. Depth-dependentprofiles of compressional speed from a subset of these deployments in the NEMP will be compared to profiles previously collected at nearby locations in 2016. In situ compressional wave records from both areas will be compared with ex-situ sediment core measurements, including data collected from core loggers and laboratory analyses. Finally, preliminary shear speed results will be discussed. [Work sponsored by ONR.]

Identifying the presence and importance of 3D acoustic effects in New England Shelf Break Acoustics experiment data

During the 2021 New England Shelf Break Acoustics (NESBA) experiments, a real-time physical oceanographic and ocean acoustic modeling effort sought to predict planned acoustic transmissions between a network of moorings in situ. While positive comparisons between simulated acoustics and hydrophone observation were made during field experiments, updates to ocean environment models post-cruise have improved the model and data agreement. Of importance to this real-time modeling effort is to use the disagreement between acoustic simulation and acoustic data to estimate model environment errors. By identifying spatial locations of likely error in the modeled sound speed, in situ acoustic simulations can identify acoustically significant locations to perform measurements of ocean water state properties such as temperature and salinity, as well as marine geological features. This presentation will focus on comparison efforts, extracting model environment error estimates, and explore the influence of 3D effects on this process. [This research is supported by the Office of Naval Research.]

Study of acoustic propagation across an oceanic front at the edge of the New England shelf.

Propagation of low-frequency sound across a warm core ring-enhanced oceanic front at lower horizontal grazing angles is presented. The data were collected from an experimental source tow conducted during the New England Shelf Break Acoustics (NESBA) experiments in spring 2021. The 3 h tow track provides spatiotemporal measurements of acoustic propagation through the front across varying geometries. Coincident oceanographic measurements are used to estimate the strong temperature gradient of the water column and three-dimensional (3D) sound speed field. Two-dimensional (2D) adiabatic mode and full-field sound propagation models are utilized to investigate the acoustic sensitivity to the frontal structure. Then, the joint effects of acoustic ducting and bathymetric slope refraction are examined using 3D sound propagation models. Key components of the measured acoustic impulse response are captured in the 3D numerical model, and the sensitivity of low-frequency propagation to the front geometry is demonstrated.

Sound waves characterize ocean front and its inhabitants

Shipboard broadband sonars identify and measure species in the New England shelf break, a commercial-fishing zone.

Real-time joint ocean acoustics and circulation modeling in the 2021 New England Shelf Break Acoustics experiment (L).

During the spring of 2021, a coordinated multi-vessel effort was organized to study physical oceanography, marine geology and biology, and acoustics on the northeast United States continental shelf, as part of the New England Shelf Break Acoustics (NESBA) experiment. One scientific goal was to establish a real-time numerical model aboard the research vessel with high spatial and temporal resolution to predict the oceanography and sound propagation within the NESBA study area. The real-time forecast model performance and challenges are reported in this letter without adjustment or re-simulation after the cruise. Future research directions for post-experiment studies are also suggested.

Variations of acoustic ducting in the presence of gulf stream warm core rings at the New England shelfbreak

The physical oceanographic (PO) conditions at the New England shelfbreak have become more dynamic with increased warming and meandering of the gulf stream. Consequently, underwater sound propagation in the area has also become more variable. The most profound PO feature that breaks off from Gulf Stream instabilities is warm core rings, which have complex interactions with bathymetry and seasonal shelf water masses. Warm core rings can cause shelf water streamers to extend offshore, resulting in surface and/or subsurface acoustic ducting conditions. This study observed acoustic surface ducting and breakdown conditions induced by a shelf water streamer at the New England shelfbreak. Ducting conditions were variable when a subsequent warm core ring intruded the acoustic source and receiver network, mixing with the shelf water streamer and influencing the sound propagation during and after the acoustic duct breakdown period. The field observations were also compared with a broadband numerical model. With increasing gulf stream warm core ring formations, streamer induced surface ducts are becoming more relevant in shelf break acoustics. [Work supported by the Office of Naval Research.]

Broadband acoustic quantification of mixed biological aggregations at the New England shelf break.

At the New England shelf break, cold, less saline shelf water collides with warmer saltier slope water to form a distinct oceanographic front. During the Office of Naval Research Sediment Characterization Experiment in 2017, the front was mapped by narrowband (18 and 38 kHz) and broadband (70-280 kHz) shipboard echo sounders. The acoustically determined cross-shelf velocity of the front ranged in amplitude from 0.02 to 0.33 m/s. Acoustic surveys revealed aggregations of scatterers near the foot of the front. Acoustic backscatter in conjunction with Northeast Fisheries Science Center bottom trawl surveys identified longfin squid (Doryteuthis pealeii) and mackerel (Scomber scombrus) as the most likely scatterers in the aggregations. A mixed species scattering model was developed and further refined by the use of a matching method used for distribution of the lengths of each species. The mean length of squid and mackerel, respectively, using the matching method was 4.45 ± 1.00 and 20.25 ± 1.25 cm compared with 6.17 ± 2.58 and 22.76 ± 1.50 cm from the trawl data. The estimated total biomass of the aggregation was a factor of 1.64 times larger when using the matching method estimated length distribution compared to the trawl length distribution.

High resolution mapping of sound speed and density with broadband echo sounders

Advances in acoustic propagation and physical oceanographic modeling necessitate improved spatiotemporal resolution of water column properties (temperature, salinity, sound speed, and density) to inform and validate models. Shipboard profiling provides adequate vertical resolution, but high-resolution horizontal sampling is expensive and time consuming. Stationary arrays generate critical long-term data sets and high vertical resolution, however, there is a trade-off between dense horizontal sampling and total spatial coverage. Towed systems provide dense coverage horizontally but lack vertical resolution in a single location. During the ONR funded New England Shelf Break Acoustics experiments in 2021, sparse shipboard CTD and expendable bathythermograph (XBT) casts were combined with backscatter measurements by shipboard broadband echo sounders to map water column physical properties of the New England Shelf Break front with high resolution vertically and horizontally. The shipboard echo sounders detected backscatter from the strong gradients of the shelf break front, physical microstructure, and internal waves present at the frontal boundary. Acoustically tracking the front between CTD casts resulted in high resolution profiles of water column properties. Acoustic propagation modeling at 500 Hz and 4 kHz was compared when using the acoustically determined water column profile and when using profiles from CTD and XBT casts alone.

Influence of stratified sub-bottom sediment layers on acoustic simulations in the New England shelf break environment

During the 2021 New England Shelf Break Acoustics (NESBA) Experiment, a large number of acoustic signal transmissions along and across the continental shelf were compared with combined physical oceanographic (PO) and ocean acoustic (OA) models. This experiment aimed to examine the influence of environmental changes in the water column on acoustic propagation in the shelf break environment. Sub-bottom surveys carried out by a combination of autonomous underwater vehicles (AUVs) and ship-board instruments revealed distinctly stratified sediment layers. Accounting for the properties of sub-bottom sediment layers is an integral component of diagnosing mismatch between experimental acoustic data and simulations, as these properties can influence the attenuation and scattering of received signals. A comparison of homogeneous half-space bottom models calculated at sea in May of 2021 and improved models which incorporate the results of sub-bottom surveys is presented, and the importance of this modeling consideration is examined for the purposes of data and model comparisons. [This work was supported by the Office of Naval Research.]