Lavery Andone C.

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Lavery

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Andone C.

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0000-0002-0713-1623

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

(Acoustical Society of America, 2020-02-06) Bassett, Christopher ; Lavery, Andone C. ; Lyons, Anthony P. ; Wilkinson, Jeremy P. ; Maksym, Ted

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.
Laboratory observations of double-diffusive convection using high-frequency broadband acoustics

( 2008-09) Ross, Tetjana ; Lavery, Andone C.

High-frequency broadband (200-300 kHz) acoustic scattering techniques have been used to observe the diffusive regime of double-diffusive convection in the laboratory. Pulse compression signal processing techniques allow 1) centimetre-scale interface thickness to be rapidly, remotely, and continuously measured, 2) the evolution, and ultimate merging, of multiple interfaces to be observed at high-resolution, and 3) convection cells within the surrounding mixed layers to be observed. The acoustically measured interface thickness, combined with knowledge of the slowly-varying temperatures within the surrounding layers, in turn allows the direct estimation of double-diffusive heat and buoyancy fluxes. The acoustically derived interface thickness, interfacial fluxes and migration rates are shown to support established theory. Acoustic techniques complement traditional laboratory sampling methods and provide enhanced capabilities for observing the diffusive regime of double-diffusion in the ocean.
Broadband acoustic quantification of stratified turbulence

(Acoustical Society of America, 2013-07) Lavery, Andone C. ; Geyer, W. Rockwell ; Scully, Malcolm E.

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

(Acoustical Society of America, 2020-08-18) Bassett, Christopher ; Lavery, Andone C. ; Stanton, Timothy K. ; DeWitt Cotter, Emma

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.
Acoustic detection of oceanic double-diffusive convection : a feasibility study

(American Meteorological Society, 2010-03) Ross, Tetjana ; Lavery, Andone C.

The feasibility of using high-frequency acoustic scattering techniques to map the extent and evolution of the diffusive regime of double-diffusive convection in the ocean is explored. A scattering model developed to describe acoustic scattering from double-diffusive interfaces in the laboratory, which accounted for much of the measured scattering in the frequency range from 200 to 600 kHz, is used in conjunction with published in situ observations of diffusive-convection interfaces to make predictions of acoustic scattering from oceanic double-diffusive interfaces. Detectable levels of acoustic scattering are predicted for a range of different locations in the world’s oceans. To corroborate these results, thin acoustic layers detected near the western Antarctic Peninsula using a multifrequency acoustic backscattering system are shown to be consistent with scattering from diffusive-convection interfaces.
Measurements of acoustic scattering from zooplankton and oceanic microstructure using a broadband echosounder

(Oxford University Press, 2009-10-29) Lavery, Andone C. ; Chu, Dezhang ; Moum, James N.

In principle, measurements of high-frequency acoustic scattering from oceanic microstructure and zooplankton across a broad range of frequencies can reduce the ambiguities typically associated with the interpretation of acoustic scattering at a single frequency or a limited number of discrete narrowband frequencies. With this motivation, a high-frequency broadband scattering system has been developed for investigating zooplankton and microstructure, involving custom modifications of a commercially available system, with almost complete acoustic coverage spanning the frequency range 150–600 kHz. This frequency range spans the Rayleigh-to-geometric scattering transition for some zooplankton, as well as the diffusive roll-off in the spectrum for scattering from turbulent temperature microstructure. The system has been used to measure scattering from zooplankton and microstructure in regions of non-linear internal waves. The broadband capabilities of the system provide a continuous frequency response of the scattering over a wide frequency band, and improved range resolution and signal-to-noise ratios through pulse-compression signal-processing techniques. System specifications and calibration procedures are outlined and the system performance is assessed. The results point to the utility of high-frequency broadband scattering techniques in the detection, classification, and under certain circumstances, quantification of zooplankton and microstructure.
Acoustic scattering from density and sound speed gradients : modeling of oceanic pycnoclines

(Acoustical Society of America, 2011-12-20) Ross, Tetjana ; Lavery, Andone C.

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.
Biogeographic variations in diel vertical migration determined from acoustic backscattering in the northwest Atlantic Ocean

(Elsevier, 2023-03-02) Wiebe, Peter H. ; Lavery, Andone C. ; Lawson, Gareth L.

Active acoustic scattering techniques, which capitalize on the fact that different kinds of organisms scatter sound differently as the frequency changes, are uniquely suited to synoptic studies of zooplankton and fish distributions. Acoustic systems are particularly useful for documenting diel vertical migrations (DVM). Measurements with a hull mounted echosounder operating at frequencies of 43, 120, 200, and 420 kHz were made near-continuously on a 26-day cruise in the Northwest Atlantic Ocean in August 2011. The distribution of backscattering in relation to changing environmental quantities along the cruise track enabled characterization of the rates and amplitudes of diel vertical migrations and acoustic indices of pelagic animal abundance in relation to hydrographic regimes. The most pervasive phenomenon observed acoustically was a regular DVM evident along a series of six study transects. In the mode waters (∼18 °C) of the Sargasso Sea, a non-migrating layer was observed around 200 m, which was not present in the other surveyed regions (Gulf Stream, GS/Labrador Sea loop, Flemish Cap, Slope Water). A deeper (>450 m) non-migrating layer existed throughout the survey region. Migration speeds varied between 361 m/hour in the Sargasso Sea and 72.5 m/hour in the GS/Labrador Sea Loop. Acoustic backscattering at 43 kHz in the upper 500 m was highest in the Sargasso Sea (day overall median = −79.5 dB; night median = −76.5 dB) and lower in the more northern stations (all the other regions day = −86.8 dB and night = −81.1 dB). Variations in the depth of the DSL at the sampled locations are not limited by subsurface oxygen levels in the Northwest Atlantic and Northern North Atlantic.•Acoustic backscattering layers differ between hydrographic regions in the NW Atlantic.•Diel Vertical Migration (DVM) is a dominant feature of the Northwestern Atlantic Ocean.•DVM varies in speed and distance between hydrographic regions in the NW Atlantic Ocean.•The Sargasso Sea has deeper and faster DVM than other regions in the NW Atlantic Ocean.
Broadband acoustic quantification of mixed biological aggregations at the New England shelf break

(Acoustical Society of America, 2022-10-25) Loranger, Scott ; Jech, Michael J. ; Lavery, Andone C.

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

(Acoustical Society of America, 2016-10-04) Bassett, Christopher ; Lavery, Andone C. ; Maksym, Ted

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

(Acoustical Society of America, 2015-12-21) Jech, J. Michael ; Horne, John K. ; Chu, Dezhang ; Demer, David A. ; Francis, David T. I. ; Gorska, Natalia ; Jones, Benjamin A. ; Lavery, Andone C. ; Stanton, Timothy K. ; Macaulay, Gavin J. ; Reeder, D. Benjamin ; Sawada, Kouichi

Analytical and numerical scatteringmodels 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 m2) 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
Classification of broadband target spectra in the mesopelagic using physics-informed machine learning

(Acoustical Society of America, 2021-06-04) DeWitt Cotter, Emma ; Bassett, Christopher ; Lavery, Andone C.

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. I. INTRODUCTI
High-frequency acoustic scattering from turbulent oceanic microstructure : the importance of density fluctuations

(Acoustical Society of America, 2003-11) Lavery, Andone C. ; Schmitt, Raymond W. ; Stanton, Timothy K.

Acoustic scattering techniques provide a unique and powerful tool to remotely investigate the physical properties of the ocean interior over large spatial and temporal scales. With high-frequency acoustic scattering it is possible to probe physical processes that occur at the microstructure scale, spanning submillimeter to centimeter scale processes. An acoustic scattering model for turbulent oceanic microstructure is presented in which the current theory, which only accounts for fluctuations in the sound speed, has been extended to include fluctuations in the density as well. The inclusion of density fluctuations results in an expression for the scattering cross section per unit volume, σv, that is explicitly dependent on the scattering angle. By relating the variability in the density and sound speed to random fluctuations in oceanic temperature and salinity, σv has been expressed in terms of the temperature and salinity wave number spectra, and the temperature-salinity co-spectrum. A Batchelor spectrum for temperature and salinity, which depends on parameters such as the dissipation rates of turbulent kinetic energy and temperature variance, has been used to evaluate σv. Two models for the temperature-salinity co-spectrum have also been used. The predictions indicate that fluctuations in the density could be as important in determining backscattering as fluctuations in the sound speed. Using data obtained in the ocean with a high resolution vertical microstructure profiler, it is predicted that scattering from oceanic microstructure can be as strong as scattering from zooplankton.
Use of the distorted wave Born approximation to predict scattering by inhomogeneous objects : application to squid

(Acoustical Society of America, 2009-01) Jones, Benjamin A. ; Lavery, Andone C. ; Stanton, Timothy K.

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

(Acoustical Society of America, 2008-03) Jones, Benjamin A. ; Stanton, Timothy K. ; Lavery, Andone C. ; Johnson, Mark P. ; Madsen, Peter T. ; Tyack, Peter L.

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.
Exploiting signal processing approaches for broadband echosounders

(International Council for the Exploration of the Sea, 2017-08-28) Lavery, Andone C. ; Bassett, Christopher ; Lawson, Gareth L. ; Jech, J. Michael

Broadband echosounders, which transmit frequency-modulated pulses, increase the spectral characterization of targets relative to narrowband echosounders, which typically transmit single-frequency pulses, and also increase the range resolution through broadband matched-filter signal processing approaches. However, the increased range resolution does not necessarily lead to improved detection and characterization of targets close to boundaries due to the presence of undesirable signal processing side lobes. The standard approach to mitigating the impact of processing side lobes is to transmit tapered signals, which has the consequence of also reducing spectral information. To address this, different broadband signal processing approaches are explored using data collected in a large tank with both a Kongsberg–Simrad EK80 scientific echosounder with a combination of single- and split-beam transducers with nominal centre frequencies of 18, 38, 70, 120, 200, and 333 kHz, and with a single-beam custom-built echosounder spanning the frequency band from 130 to 195 kHz. It is shown that improved detection and characterization of targets close to boundaries can be achieved by using modified replica signals in the matched filter processing. An additional benefit to using broadband echosounders involves exploiting the frequency dependence of the beam pattern to calibrate single-beam broadband echosounders using an off-axis calibration sphere.
Twilight zone observation network: a distributed observation network for sustained, real-time interrogation of the ocean’s twilight zone

(Marine Technology Society, 2021-05-01) Thorrold, Simon R. ; Adams, Allan ; Bucklin, Ann ; Buesseler, Ken O. ; Fischer, Godi ; Govindarajan, Annette F. ; Hoagland, Porter ; Di, Jin ; Lavery, Andone C. ; Llopez, Joel ; Madin, Laurence P. ; Omand, Melissa M. ; Renaud, Philip ; Sosik, Heidi M. ; Wiebe, Peter ; Yoerger, Dana R. ; Zhang, Weifeng G.

The ocean's twilight zone (TZ) is a vast, globe-spanning region of the ocean. Home to myriad fishes and invertebrates, mid-water fishes alone may constitute 10 times more biomass than all current ocean wild-caught fisheries combined. Life in the TZ supports ocean food webs and plays a critical role in carbon capture and sequestration. Yet the ecological roles that mesopelagic animals play in the ocean remain enigmatic. This knowledge gap has stymied efforts to determine the effects that extraction of mesopelagic biomass by industrial fisheries, or alterations due to climate shifts, may have on ecosystem services provided by the open ocean. We propose to develop a scalable, distributed observation network to provide sustained interrogation of the TZ in the northwest Atlantic. The network will leverage a “tool-chest” of emerging and enabling technologies including autonomous, unmanned surface and underwater vehicles and swarms of low-cost “smart” floats. Connectivity among in-water assets will allow rapid assimilation of data streams to inform adaptive sampling efforts. The TZ observation network will demonstrate a bold new step towards the goal of continuously observing vast regions of the deep ocean, significantly improving TZ biomass estimates and understanding of the TZ's role in supporting ocean food webs and sequestering carbon.
Modeling characterization of the vertical and temporal variability of environmental DNA in the mesopelagic ocean

(Nature Research, 2021-10-28) Andruszkiewicz Allan, Elizabeth ; DiBenedetto, Michelle H. ; Lavery, Andone C. ; Govindarajan, Annette F. ; Zhang, Weifeng G.

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

(Acoustical Society of America, 2014-12-18) Bassett, Christopher ; Lavery, Andone C. ; Maksym, Ted ; Wilkinson, Jeremy P.

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.
Wideband (15–260 kHz) acoustic volume backscattering spectra of Northern krill (Meganyctiphanes norvegica) and butterfish (Peprilus triacanthus)

(International Council for the Exploration of the Sea, 2017-08-28) Jech, J. Michael ; Lawson, Gareth L. ; Lavery, Andone C.

Measurements of acoustic backscatter made over a wide frequency band have the potential for improved classification relative to traditional narrowband methods, by characterizing more fully the frequency response of scatterers. In January 2014, five wideband transceivers [Simrad EK80 Wideband Transceivers (WBTs)] and split-beam transducers with nominal centre frequencies of 18, 38, 70, 120, and 200 kHz were used to collect acoustic data spanning a nearly continuous 15–260 kHz bandwidth. The acoustic samples were from ca. 2 m below the surface to the seabed in an area along the US continental shelf break. Bottom trawls and zooplankton nets were also used to sample scatterers contributing to selected features of the acoustic backscatter. Measurements of frequency-dependent volume backscattering strength (i.e. volume backscattering spectra) from aggregations of euphausiids (mostly Northern krill, Meganyctiphanes norvegica) clearly resolved the transition from Rayleigh to geometric scattering, consistent with modelled backscatter from the type and length of animals sampled with bongo nets. Volume backscattering spectra from aggregations dominated by butterfish (Peprilus triacanthus) revealed a frequency response that was suggestive of superimposed scattering by soft tissue and bone. Backscatter predicted by Kirchhoff ray mode models of butterfish corresponded to trends in the measured spectra, supporting the assumption that acoustic scattering by butterfish is dominated by soft tissue and vertebrae.

Lavery Andone C.

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