Harnessing (geoacoustic) uncertainty.

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.

Acoustic scattering by smooth elastic cylinders insonified by directional transceivers: Monostatic theory and experiments.

A theoretical model for predicting the acoustic field scattered by an elastic cylinder that is partially insonified by a directional transceiver is proposed in the form of a simple approximate one-dimensional integral. This model accounts for spherical spreading and directivity of the incident waves and extends the formulation used in a preceding article [Gurley and Stanton, J. Acoust. Soc. Am. 94, 2746-2755 (1993)] by including effects due to oblique insonification of a long cylinder assuming negligible end-contributions. The scattered field of an infinitely long cylinder for obliquely incident plane waves and point receivers is used to approximate the apparent volume flow of cylinders partially insonified by directional transceivers. The scattered pressure that is derived using the apparent volume flow, in contrast to the previous formulation, is capable of predicting axially propagating guided wave resonances; these natural modes are excited, in addition to circumferential ones, at off-normal incident angles. The model is compared with exact numerical simulations and with previously published as well as new laboratory data. The analysis illustrates the different realistic effects associated with scattering from elastic cylinders insonified by a directional transceiver both theoretically and experimentally.

Broadband backscattering from scyphozoan jellyfish.

As the ecological importance of gelatinous organisms becomes increasingly appreciated, so has the need for improved knowledge of their abundance and distribution. Acoustic backscattering measurements are routine for fisheries assessments but are not yet widely used to survey populations of gelatinous zooplankton. The use of acoustic backscattering techniques to understand the distribution and abundance of organisms requires an understanding of their target strength (TS). This study presents a framework for a sound scattering model for jellyfish based on the Distorted Wave Born Approximation that incorporates size, shape, and material properties of individual organisms. This model, with a full three-dimensional shape rendition, is applied to a common species of scyphomedusa (Chrysaora chesapeakei) and verified experimentally with broadband (52-90 and 93-161 kHz) laboratory TS measurements of live individuals. Cyclical changes in the organism's shape due to swimming kinematics were examined, as well as averages over swimming position and comparisons with scattering from simpler shapes. The model predicts overall backscattering levels and broad spectral behavior within <2 dB. Measured TS exhibits greater variability than is predicted by scaling the size of the organism in the scattering model, showing that density and sound speed vary among individuals.

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.

Modeling characterization of the vertical and temporal variability of environmental DNA in the mesopelagic ocean.

Increasingly, researchers are using innovative methods to census marine life, including identification of environmental DNA (eDNA) left behind by organisms in the water column. However, little is understood about how eDNA is distributed in the ocean, given that organisms are mobile and that physical and biological processes can transport eDNA after release from a host. Particularly in the vast mesopelagic ocean where many species vertically migrate hundreds of meters diurnally, it is important to link the location at which eDNA was shed by a host organism to the location at which eDNA was collected in a water sample. Here, we present a one-dimensional mechanistic model to simulate the eDNA vertical distribution after its release and to compare the impact of key biological and physical parameters on the eDNA vertical and temporal distribution. The modeled vertical eDNA profiles allow us to quantify spatial and temporal variability in eDNA concentration and to identify the most important parameters to consider when interpreting eDNA signals. We find that the vertical displacement by advection, dispersion, and settling has limited influence on the eDNA distribution, and the depth at which eDNA is found is generally within tens of meters of the depth at which the eDNA was originally shed from the organism. Thus, using information about representative vertical migration patterns, eDNA concentration variability can be used to answer ecological questions about migrating organisms such as what depths species can be found in the daytime and nighttime and what percentage of individuals within a species diurnally migrate. These findings are critical both to advance the understanding of the vertical distribution of eDNA in the water column and to link eDNA detection to organism presence in the mesopelagic ocean as well as other aquatic environments.

Classification of broadband target spectra in the mesopelagic using physics-informed machine learning.

Broadband echosounders measure the scattering response of an organism over a range of frequencies. When compared with acoustic scattering models, this response can provide insight into the type of organism measured. Here, we train the k-Nearest Neighbors algorithm using scattering models and use it to group target spectra (25-40 kHz) measured in the mesopelagic near the New England continental shelf break. Compared to an unsupervised approach, this creates groupings defined by their scattering physics and does not require significant tuning. The model classifies human-annotated target spectra as gas-bearing organisms (at, below, or above resonance) or fluid-like organisms with a weighted F1-score of 0.90. Class-specific F1-scores varied-the F1-score exceeded 0.89 for all gas-bearing organisms, while fluid-like organisms were classified with an F1-score of 0.73. Analysis of classified target spectra provides insight into the size and distribution of organisms in the mesopelagic and allows for the assessment of assumptions used to calculate organism abundance. Organisms with resonance peaks between 25 and 40 kHz account for 43% of detections, but a disproportionately high fraction of volume backscatter. Results suggest gas bearing organisms account for 98.9% of volume backscattering concurrently measured using a 38 kHz shipboard echosounder between 200 and 800 m depth.

Comparison of mesopelagic organism abundance estimates using in situ target strength measurements and echo-counting techniques.

Recent studies using acoustic techniques suggest that the biomass of mesopelagic fishes may be an order of magnitude higher than previously estimated from trawls. However, there is uncertainty surrounding these estimates, which are derived from shipboard echosounder measurements using necessary, but poorly constrained, assumptions. Here, an echosounder is used to measure individual target strengths at depth. These measurements are used to infer mesopelagic organism density through echo-counting. Measured target strengths are used to estimate organism density by inverting shipboard echosounder measurements. The two sampling methods agree well, but highlight the importance of accurate target strength measurements.

Frequency- and depth-dependent target strength measurements of individual mesopelagic scatterers.

Recent estimates based on shipboard echosounders suggest that 50% or more of global fish biomass may reside in the mesopelagic zone (depths of ∼200-1000 m). Nonetheless, little is known about the acoustic target strengths (TS) of mesopelagic animals because ship-based measurements cannot resolve individual targets. As a result, biomass estimates of mesopelagic organisms are poorly constrained. Using an instrumented tow-body, broadband (18-90 kHz) TS measurements were obtained at depths from 70 to 850 m. A comparison between TS measurements at-depth and values used in a recent global estimate of mesopelagic biomass suggests lower target densities at most depths.

Direct inference of first-year sea ice thickness using broadband acoustic backscattering.

Accurate measurements of sea ice thickness are critical to better understand climate change, to provide situational awareness in ice-covered waters, and to reduce risks for communities that rely on sea ice. Nonetheless, remotely measuring the thickness of sea ice is difficult. The only regularly employed technique that accurately measures the full ice thickness involves drilling a hole through the ice. Other presently used methods are either embedded in or through the ice (e.g., ice mass balance buoys) or calculate thickness from indirect measurements (e.g., ice freeboard from altimetry; ice draft using sonars; total snow and ice thickness using electromagnetic techniques). Acoustic techniques, however, may provide an alternative approach to measure the total ice thickness. Here laboratory-grown sea ice thicknesses, estimated by inverting the time delay between echoes from the water-ice and ice-air interfaces, are compared to those measured using ice cores. A time-domain model capturing the dominant scattering mechanisms is developed to explore the viability of broadband acoustic techniques for measuring sea ice thickness, to compare with experimental measurements, and to investigate optimal frequencies for in situ applications. This approach decouples ice thickness estimates from water column properties and does not preclude ice draft measurements using the same data.

Broadband acoustic backscatter from crude oil under laboratory-grown sea ice.

In ice-covered seas, traditional air-side oil spill detection methods face practical challenges. Conversely, under-ice remote sensing techniques are increasingly viable due to improving operational capabilities of autonomous and remotely operated vehicles. To investigate the potential for under-ice detection of oil spills using active acoustics, laboratory measurements of high-frequency, broadband backscatter (75-590 kHz) from crude oil layers (0.7-8.1 cm) under and encapsulated within sea ice were performed at normal and 20° incidence angles. Discrete interfaces (water-oil, oil-ice, and ice-oil) are identifiable in observations following oil injections under the ice and during the subsequent encapsulation. A one-dimensional model for the total normal incidence backscatter from oil under ice, constrained by oil sound speed measurements from -10 °C to 20 °C and improved environmental measurements compared to previous studies, agrees well with pre-encapsulation observations. At 20° incidence angles echoes from the ice and oil under ice are more complex and spatially variable than normal incidence observations, most likely due to interface roughness and volume inhomogeneities. Encapsulated oil layers are only detected at normal incidence. The results suggest that high-frequency, broadband backscatter techniques may allow under-ice remote sensing for the detection and quantification of oil spills.

Laboratory measurements of high-frequency, acoustic broadband backscattering from sea ice and crude oil.

Recent decreases in summer sea ice cover are spurring interest in hydrocarbon extraction and shipping in Arctic waters, increasing the risk of an oil spill in ice covered waters. With advances in unmanned vehicle operation, there is an interest in identifying techniques for remote, underwater detection of oil spills from below. High-frequency (200-565 kHz), broadband acoustic scattering data demonstrate that oil can be detected and quantified under laboratory grown sea ice and may be of use in natural settings. A simple scattering model based on the reflection coefficients from the interfaces agrees well with the data.

Comparisons among ten models of acoustic backscattering used in aquatic ecosystem research.

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m(2)) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge.

Broadband acoustic quantification of stratified turbulence.

High-frequency broadband acoustic scattering techniques have enabled the remote, high-resolution imaging and quantification of highly salt-stratified turbulence in an estuary. Turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors. The acoustic frequencies used span 120-600 kHz, which, for the highly stratified and dynamic estuarine environment, correspond to wavenumbers in the viscous-convective subrange (500-2500 m(-1)). The acoustically measured spectral levels are in close agreement with spectral levels measured with closely co-located micro-conductivity probes. The acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim-bladdered fish, the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure. The direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt-stratified estuary.

Deterministic forward scatter from surface gravity waves.

Deterministic structures in sound reflected by gravity waves, such as focused arrivals and Doppler shifts, have implications for underwater acoustics and sonar, and the performance of underwater acoustic communications systems. A stationary phase analysis of the Helmholtz-Kirchhoff scattering integral yields the trajectory of focused arrivals and their relationship to the curvature of the surface wave field. Deterministic effects along paths up to 70 water depths long are observed in shallow water measurements of surface-scattered sound at the Martha's Vineyard Coastal Observatory. The arrival time and amplitude of surface-scattered pulses are reconciled with model calculations using measurements of surface waves made with an upward-looking sonar mounted mid-way along the propagation path. The root mean square difference between the modeled and observed pulse arrival amplitude and delay, respectively, normalized by the maximum range of amplitudes and delays, is found to be 0.2 or less for the observation periods analyzed. Cross-correlation coefficients for modeled and observed pulse arrival delays varied from 0.83 to 0.16 depending on surface conditions. Cross-correlation coefficients for normalized pulse energy for the same conditions were small and varied from 0.16 to 0.06. In contrast, the modeled and observed pulse arrival delay and amplitude statistics were in good agreement.

Anisotropy in high-frequency broadband acoustic backscattering in the presence of turbulent microstructure and zooplankton.

High-frequency broadband (120-600 kHz) acoustic backscattering measurements have been made in the vicinity of energetic internal waves. The transducers on the backscattering system could be adjusted so as to insonify the water-column either vertically or horizontally. The broadband capabilities of the system allowed spectral classification of the backscattering. The distribution of spectral shapes is significantly different for scattering measurements made with the transducers oriented horizontally versus vertically, indicating that scattering anisotropy is present. However, the scattering anisotropy could not be unequivocally explained by either turbulent microstructure or zooplankton, the two primary sources of scattering expected in internal waves. Daytime net samples indicate a predominance of short-aspect-ratio zooplankton. Using zooplankton acoustic scattering models, a preferential orientation of the observed zooplankton cannot explain the measured anisotropy. Yet model predictions of scattering from anisotropic turbulent microstructure, with inputs from coincident microstructure measurements, were not consistent with the observations. Possible explanations include bandwidth limitations that result in many spectra that cannot be unambiguously attributed to turbulence or zooplankton based on spectral shape. Extending the acoustic bandwidth to cover the range from 50 kHz to 2 MHz could help improve identification of the dominant sources of backscattering anisotropy.

Orientation dependence of broadband acoustic backscattering from live squid.

A controlled laboratory experiment of broadband acoustic backscattering from live squid (Loligo pealeii) was conducted using linear chirp signals (60-103 kHz) with data collected over the full 360° of orientation in the lateral plane, in <1° increments. The acoustic measurements were compared with an analytical prolate spheroid model and a three-dimensional numerical model with randomized squid shape, both based on the distorted-wave Born approximation formulation. The data were consistent with the hypothesized fluid-like scattering properties of squid. The contributions from the front and back interfaces of the squid were found to dominate the scattering at normal incidence, while the arms had a significant effect at other angles. The three-dimensional numerical model predictions out-performed the prolate spheroid model over a wide range of orientations. The predictions were found to be sensitive to the shape parameters, including the arms and the fins. Accurate predictions require setting these shape parameters to best describe the most probable squid shape for different applications. The understanding developed here serves as a basis for the accurate interpretation of in situ acoustic scattering measurements of squid.

Acoustic scattering from density and sound speed gradients: modeling of oceanic pycnoclines.

A weak-scattering model that allows prediction of acoustic scattering from oceanic pycnoclines (and the accompanying sound speed gradients) based on hydrographic profiles is described. Model predictions, based on profiles from four locations, indicate that scattering from oceanic pycnoclines is measurable using standard scientific sonars operating at frequencies up to 200 kHz but generally only for pycnocline thicknesses less than 10 m. Accurate scattering models are key to assessing whether acoustic remote sensing can be used to map oceanic pycnoclines and for determining whether scattering from pycnoclines needs to be taken into account when estimating, for instance, zooplankton abundance from acoustic data.

Use of the distorted wave born approximation to predict scattering by inhomogeneous objects: application to squid.

A new method has been developed to predict acoustic scattering by weakly scattering objects with three-dimensional variability in sound speed and density. This variability can take the form of inhomogeneities within the body of the scatterer and/or geometries where the acoustic wave passes through part of the scattering body, into the surrounding medium, and back into the body. This method applies the distorted wave Born approximation (DWBA) using a numerical approach that rigorously accounts for the phase changes within a scattering volume. Ranges of validity with respect to material properties and numerical considerations are first explored through comparisons with modal-series-based predictions of scattering by fluid-filled spherical and cylindrical fluid shells. The method is then applied to squid and incorporates high resolution spiral computerized tomography (SCT) scans of the complex morphology of the organism. Target strength predictions based on the SCT scans are compared with published backscattering data from live, freely swimming and tethered squid. The new method shows significant improvement for both single-orientation and orientation-averaged scattering predictions over the DWBA-homogeneous-prolate-spheroid model.

Classification of broadband echoes from prey of a foraging Blainville's beaked whale.

Blainville's beaked whales (Mesoplodon densirostris) use broadband, ultrasonic echolocation signals with a -10 dB bandwidth from 26 to 51 kHz to search for, localize, and approach prey that generally consist of mid-water and deep-water fishes and squid. Although it is well known that the spectral characteristics of broadband echoes from marine organisms vary as a function of size, shape, orientation, and anatomical group, there is little evidence as to whether or not free-ranging toothed whales use spectral cues in discriminating between prey and nonprey. In order to study the prey-classification process, a stereo acoustic tag was deployed on a Blainville's beaked whale so that emitted clicks and the corresponding echoes from targets in the water could be recorded. A comparison of echoes from targets apparently selected by the whale and those from a sample of scatterers that were not selected suggests that spectral features of the echoes, target strengths, or both may have been used by the whale to discriminate between echoes. Specifically, the whale appears to favor targets with one or more nulls in the echo spectra and to seek prey with higher target strengths at deeper depths.

Acoustic scattering from double-diffusive microstructure.

Laboratory measurements of high-frequency broadband acoustic backscattering (200-600 kHz) from the diffusive regime of double-diffusive microstructure have been performed. This type of microstructure, which was characterized using direct microstructure and optical shadowgraph techniques, is identified by sharp density and sound speed interfaces separating well-mixed layers. Vertical acoustic backscattering measurements were performed for a range of physical parameters controlling the double-diffusive microstructure. The echoes have been analyzed in both the frequency domain, providing information on the spectral response of the scattering, and in the time domain, using pulse compression techniques. High levels of variability were observed, associated with interface oscillations and turbulent plumes, with many echoes showing significant spectral structure. Acoustic estimates of interface thickness (1-3 cm), obtained for the echoes with exactly two peaks in the compressed pulse output, were in good agreement with estimates based on direct microstructure and optical shadowgraph measurements. Predictions based on a one-dimensional weak-scattering model that includes the actual density and sound speed profiles agree reasonably with the measured scattering. A remote-sensing tool for mapping oceanic microstructure, such as high-frequency broadband acoustic scattering, could lead to a better understanding of the extent and evolution of double-diffusive layering, and to the importance of double diffusion to oceanic mixing.