Andone C. Lavery

Impulsive airgun transmissions collected during the New England Seamount Acoustics pilot experiments are used to investigate deep sediment structure in the Sohm abyssal plain south of the Atlantis II Seamount. Airgun signals from a moving source were recorded on a fixed vertical line array, deployed at a depth of 5050 m, which was equipped with six receivers spanning water depths from 304 to 1434 m and environmental sensors. Distinct arrivals consistent with sub-bottom eigenray paths are identified at ranges up to 15 km. Geoacoustic inversion is performed using the relative arrival time (ΔT) between single-bottom-interacting and corresponding single-sub-bottom-interacting eigenray arrivals. Modeled ΔT is computed using BELLHOP eigenrays in the water column combined with analytic formulas for layered sediment models in the sub-bottom. Constant sound speed and n2 linear parameterizations are evaluated for geoacoustic inversion. Layer parameters, including thicknesses, sound speeds, and gradients, are estimated using adaptive simplex simulated annealing, and model selection is performed using the Bayesian information criterion. Results favor a model with a constant sound speed upper layer over a deeper n2 gradient layer, indicating the presence of two resolvable sub-bottom layers with a combined thickness of approximately 500 m and sound speeds consistent with consolidated sediments.

The turbulent energy cascade of ocean mixing remains poorly understood at geophysically large Reynolds numbers (). Here we propose a new conceptual model for the structure of small‐scale mixing grounded in high‐resolution multibeam echosounding observations from the mouth of the Connecticut River, a shallow salt‐wedge estuary. Tidal forcing and bottom topography slope the pycnocline, sustaining interfacial shear and Kelvin–Helmholtz instabilities well below the marginal instability threshold on a vertical scale of order 1 m. Acoustic backscatter, used here as a proxy for salinity microstructure dissipation, provides time‐resolved two‐dimensional imagery of the structure and evolution of turbulent mixing. At , we find that mixing is dominated not by the collapse of slowly evolving billow cores as at , but instead by fast turbulence within the m thin, shallow‐sloping braids that connect them, energized by baroclinic shear. We interpret these observations using two‐dimensional direct numerical simulation at field‐matched parameters, which predicts the emergence of secondary Kelvin–Helmholtz instabilities and turbulence within the braids, pre‐empting further steepening and primary overturn. We then estimate turbulent dissipation and scalar variance dissipation at the pycnocline and braid scales using physics‐based scalings involving only the local slope, shear and layer thickness. Laboratory experiments in an inclined duct, an analog of the turbulent braid regime, visualize mixing down to dissipative scales, where it remains organized in myriad thin, sheared, intense filaments rather than overturns. We conclude that high‐ mixing hotspots with sustained shear forcing follow fundamentally different dynamics than previously thought.

A method is presented to distinguish single finite targets from acoustic clutter, that is, unresolvable echoes associated with multiple targets. It exploits frequency-dependent interference of multiple targets and the resultant frequency-dependent variation of cross-spectral phase to differentiate between echoes from single targets and coincident echoes from multiple targets. The method is demonstrated, in the context of split-beam echosounders, through mathematical modeling of the cross-spectral phase for an arbitrary number of targets as well as through laboratory and at-sea experiments on elastic spheres. Fresnel zone and resonance effects are used to interpret the findings, providing insights into the connection between the underlying scattering physics and the effects of coincident echoes on cross-spectral phase. False acceptance rates of the method are then calculated and compared with those of existing approaches in the literature. Finally, the method is applied to infer the in situ target strengths of individual gas-bearing organisms.

An acoustic autonomous underwater vehicle (AUV) deployed during the New England Shelf Break Acoustics experiment in May 2021 investigated the effect of environmental variability on acoustic propagation and scattering. Shipboard 38 kHz echosounder data revealed the presence of a biological deep scattering layer (DSL) approximately 100 m below the AUV mission depth of 300 m. A prolate spheroid gas-filled swimbladder scattering model was used to estimate the DSL acoustic attenuation resulting from resonant scattering by fish. These range-dependent attenuation coefficients were applied to a comparison of 2.5–4.5 kHz AUV source signals received on mooring hydrophones following propagation paths through and above the DSL. A parabolic equation model was used to simulate transmission loss through the scattering layer along the selected propagation paths, as well as account for physical oceanographic variability. The DSL in this study was estimated to account for roughly 1–1.5 dB/km of attenuation through the layer, which improves the final propagation path comparison results and emphasizes the importance of accounting for both biological scattering and physical oceanographic variability in attenuation studies.

Split-beam echosounders are widely used in acoustical oceanography to measure the abundance and distribution of marine organisms and to study their movement and behavior in situ. Abundance estimates rely on target strength measurements, which depend on accurate angle of arrival (AoA) estimates to compensate for transducer directivity, and analyses of movement and behavior also depend on AoA to determine spatial positions. Recent years have seen the transition from narrowband to broadband systems, which offer improved acoustic detection and characterization of targets, yet the method for AoA estimation with split-beam echosounders has remained unchanged. The split-aperture correlator (SAC), originally developed for narrowband systems, remains the only available option despite inadequate assessment for broadband systems and fundamental restrictions to single-echo scenarios. In this work, we assess SAC's applicability to broadband systems, demonstrating its inaccuracy in estimating the AoAs of broadband single echoes from targets with highly frequency-dependent scattering responses, such as gas-bearing fish. An extended SAC is introduced to address this limitation. Furthermore, we introduce a broadband maximum likelihood estimator for estimating the AoAs and spectra of overlapping echoes. The estimator's efficacy and applicability to a broadband split-beam echosounder are demonstrated through controlled laboratory experiments.

A multi-channel back-propagation approach was used to acoustically localize an autonomous underwater vehicle (AUV), which was out of range for bottom-lock navigation and had resorted to unreliable dead-reckoning. This opportunistic study was performed as part of the New England Shelf Break Acoustics experiment in spring 2021, when an AUV was deployed within a network of acoustic transceiver moorings and ship-towed sources. The AUV was equipped with a 2.5–4.5 kHz transducer and towed a 16-channel hydrophone array. This localization method involves ray back-propagation of direct and surface reflected arrivals originating from the AUV source and received on nearby mooring hydrophones. Additional back-propagation of ship-towed source signals received on the AUV array is used to determine vehicle azimuth. The effects of mooring tilt are analyzed and incorporated into the localization result to minimize error. Localization uncertainty related to source-receiver geometry and physical oceanographic variability is investigated, and a geometric model for characterizing uncertainty is presented. The final localization result is then compared to that of other localization methods, including AUV dead-reckoning and multi-channel back-propagation without a depth constraint.

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.

Bistatic laboratory measurements are presented for acoustic scattering from both smooth and rough elastic cylinders insonified by directional spherical waves. A scattering model, accounting for incident directional spherical waves while assuming negligible end effects, was derived in a previous article [Mursaline, Stanton, Lavery, and Fischell, J. Acoust. Soc. Am. 154, 307–322 (2023)] but only evaluated for monostatic scattering by smooth cylinders. The evaluation is extended here to bistatic geometries for both smooth and rough cylinders. The effect of axi-symmetric Gaussian roughness (axi-symmetric random variations in cylinder radius) on the cylinder on overall scattering levels and resonances is investigated. Particular emphasis is given to the influence of roughness on the excitation of axially propagating guided wave resonances associated with oblique incident angles. Bistatic laboratory observations presented herein further substantiate the effects on scattering due to the properties of the incident field from practical sonars, such as spherical spreading, as observed in the above-mentioned article. For smooth cylinders, axially propagating guided wave resonances are seen to become more prominent during bistatic in-plane scattering compared to bistatic orthogonal-plane scattering and previously published monostatic data. For rough cylinders, both overall scattering levels and resonances are found to be diminished compared to the smooth case.

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Our ocean plays a major role in climate regulation, hosts vast biodiversity, sustains economies, ensures food security, and supports international shipping, tourism, recreation, and human health and security. Yet, the ocean is experiencing unprecedented changes: sea level rise, coral bleaching, ocean acidification, anoxia, accelerated melting of the major ice sheets, shifts in ocean circulation, pollution, overfishing, and increased industrialization. Sound plays a vital role in exploring, understanding, and monitoring these changes over a range of spatial and temporal scales. In this talk, I will focus on the multitude of approaches taken to using sound to understand our changing ocean, the advances in platforms used to enable these studies, the expansion of global ocean observing systems that include measurements of sound as an essential ocean variable, and the enormous complexities in interpreting acoustic signals in a dynamic and evolving ocean landscape. I will end by discussing the opportunities and obstacles associated with globalizing approaches for using sound to improve ocean stewardship.