Broadband Acoustics Research Articles

A new in-situ method to estimate fish target strength reveals high variability in broadband measurements

Fish Target Strength (TS) is a requirement for estimating abundance from surveys of pelagic fish using echosounders. This paper describes a novel in-situ method to estimate TS as a function of fish body length (L), according to the standard equation TS = 20 log (L) +b20, where b20 is the species-specific factor to be estimated. We made measurements of TS with a broadband scientific echosounder, which is becoming the new tool of choice in fisheries acoustics due to its enhanced signal-to-noise ratio, and higher range resolution and the ability to measure the frequency response. A split-beam 38 kHz broadband transducer was pole-mounted on a small boat in the fjords of Nuuk, Greenland. With the boat stationary, individual Atlantic cod Gadus morhua were detected as echotraces, caught with a fishing line, measured in length and then released. Video footage, from a camera attached to the line, was inspected to verify a single individual was attracted to the lure and caught. Fish echotrace tracking techniques were applied to estimate the tilt of the fish from each acoustic sample and the measured TS was adjusted accordingly with the use of a Kirchhoff-Approximation scattering model. The b20 parameter was estimated by linear regression. This method combines the benefits of associating TS from single fish of known length, which is typical of ex-situ methods, with the in-situ advantages of measuring TS of an undisturbed fish in its natural environment. Results yielded a b20 of − 65.6 dB ( ± 0.83 dB C.I.), which is within the range of previously published values for Atlantic cod measured with narrowband systems. Large variability of TS was observed within fish tracks (average s.d. of 5.55 dB). This is the first description of TS for Atlantic cod with broadband equipment. The high variability could not be attributed to variation in fish length nor tilt angle. Other physiological and behavioural aspects were also discarded. It is possible that this may be an intrinsic property of broadband acoustics, which indicates a need for re-evaluating TS to length relationships as fisheries science moves towards the wider application of broadband acoustics.

Remote acoustic sizing of tethered fish using broadband acoustics

Remote fish sizing is desirable in fisheries (e.g., pre-catch) and research (e.g., platforms without biological sampling capacity) applications. In those contexts, the high spatial resolution of pulse compressed broadband echoes combined with narrow beamwidth transducers makes it feasible to resolve the scattering from different parts of the fish body and hence can be used to measure the body size. A motorized apparatus was used to suspend individual fish in the acoustic beam of two laterally oriented transducers (45–90 kHz, 160–260 kHz, 12.2 m range) with precise control of rotation angle. Broadband scattering was measured from tethered Atlantic mackerel (Scomber scombrus), saithe (Pollachius virens), and pollack (Pollachius pollachius) ranging in standard length from 239 to 491 mm as a function of orientation angle to validate sizing based on the acoustic resolution of fish body parts. Under these controlled conditions, fish size was underestimated by 11–19 mm, varying with broadband pulse characteristics, orientation angle, species, and fish size. The best remote acoustic sizing results were obtained using 160–260 kHz pulses with a slow rise and fall of pulse amplitude (aka, taper).

Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing

Abstract Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.

Discrimination between schools and submerged trees in reservoirs: a preliminary approach using narrowband and broadband acoustics

With a growing demand for hydroelectric energy, the number of reservoirs is dramatically increasing worldwide. These new water bodies also present an opportunity for the development of fishing activities. However, these reservoirs are commonly impounded on uncut forests, resulting in many immersed trees. These trees hinder fish assessments by disrupting both gill-netting and acoustic sampling. Immersed trees can easily be confused with fish schools on echograms. To overcome this issue, we developed a method to discriminate fish schools from immersed trees. A random forest algorithm was used to classify echo-traces at 120 and 200 kHz, recorded by an EK80 (SIMRAD) in narrowband (continuous wave) and in broadband mode (frequency modulated). We obtained a good discrimination rate between trees and schools, especially in broadband (90% ratio of good classification). We demonstrate that it is possible to discriminate fish schools from immersed trees and thus facilitate the use of fisheries acoustics in reservoirs.

Remote sizing of fish-like targets using broadband acoustics

Broadband echosounder systems provide very high range resolution that could resolve different parts of fish and hence provide estimates of fish size. The potential for, and accuracy of, direct acoustic sizing of fish was tested on fish-like targets of known dimensions and orientations. Prolate spheroids, 91–477 mm long, with and without an air-filled inclusion (simulating a fish swim-bladder), made from polyvinyl-alcohol-cryogel, were suspended in an acoustic beam and rotated while the backscattered signal was recorded. The echoes from linear frequency modulated pulses of 45−90 and 160−260 kHz with a fast and a slow amplitude modulation were pulse-compressed to provide data with a range resolution of 6.6 mm at its highest. Echoes from the target boundaries were manually identified, tracked and used to directly estimate the thickness and length of the targets. An accuracy of ±11−17 mm was achieved for targets longer than about 200 mm and thicker than about 20 mm when using the 160−260 kHz pulse. The length of the air-filled inclusion could also be estimated for the larger targets. Echoes from the slow amplitude modulation were easier to interpret than the fast amplitude modulation despite the lower range resolution, due to the lower pulse compression temporal sidelobes. The results show that remote acoustic sizing of fish is feasible and of sufficient accuracy for practical use.

Diel vertical movements determine spatial interactions between cod, pelagic fish and krill on an Arctic shelf bank

Predator-prey spatial interactions are essential to understand ecosystem processes like predation rates and trophic interaction strength. In marine systems, such spatial interactions are highly dynamic and difficult to observe, as predators, prey and resources are mobile and responsive to each other, and also since shifting vertical light gradients strongly affect the space use of visual predators and their prey. We visited a bank area in the northern Barents Sea with cold bottom waters (~1°C), and combined conventional trawl and acoustic sampling with broadband hydroacoustics to obtain long-range, fine-scale observations of interactions between cod, their planktivore prey (capelin and polar cod) and krill. We caught cod in demersal trawl hauls but could not detect them with the vessel acoustics. However, broadband acoustics mounted on a submersible probe allowed us to track individual cod, revealing that they remained mostly within 10 m of the bottom throughout the diel cycle. In the morning, cod lifted slightly from the seabed indicating feeding activity, which corresponded with more fresh prey in cod stomachs in the morning. During daylight, krill pushed towards the bottom, sharing habitat with cod, while the planktivores aggregated in pelagic schools at the cost of lost feeding opportunities, overlapping with their krill prey only during twilight hours. The diel light cycle was an important driver of the spatial movements and aggregations, and krill appear to hide from capelin among the cod near bottom, while cod take advantage of descending pelagic fish after dawn to feed with a minimum of effort.

Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video.

Sound scattering layers (SSLs) are observed over a broad range of spatio-temporal scales and geographical areas. SSLs represent a large biomass, likely involved in the biological carbon pump and the structure of marine trophic webs. Yet, the taxonomic composition remains largely unknown for many SSLs. To investigate the challenges of SSL sampling, we performed a survey in a small study area in the Northern Bay of Biscay (France) by combining broadband and narrowband acoustics, net sampling, imagery and video recordings. In order to identify organisms contributing to the observed SSLs, we compared measured frequency spectra to forward predicted spectra derived from biological data. Furthermore, to assess the confidence in SSL characterization, we evaluated uncertainties in modeling, acoustical and biological samplings. Here, we demonstrate for the first time that SSL backscattering intensity in the Bay of Biscay can be dominated in springtime by resonant gas bearing organisms below 100 kHz, namely siphonophores and juvenile fishes and by pteropods at higher frequencies. Thus, we demonstrate the importance of broadband acoustics combined to nets, imagery and video to characterize resonant backscatterers and mixed mesozooplankton assemblages.

Broadband acoustic quantification of mesopelagic zooplankton

In addition to its critical role in biogeochemical cycling, the mesopelagic zone has the potential to become an important global fisheries resource. A growing number of studies have focused on estimating the global biomass of mesopelagic fishes, but that of mesopelagic zooplankton is far more uncertain. Yet, quantification of zooplankton biomass and distribution is crucial to both understanding the ecology of the mesopelagic zone and informing policy makers on sustainable exploitation. Shipboard, narrowband, volume backscattering measurements—typically performed at 18 and/or 38 kHz—are likely dominated by gas-bearing organisms, hindering accurate assessment of zooplankton biomass. A towed vehicle, Deep-See, was developed and equipped with broadband acoustics from 1–450 kHz, optical sensors, and environmental sensors to address the challenges associated with systematically acquiring holistic, quantitative data to infer mesopelagic biomass. Broadband backscattering spectra (50–160 kHz) collected by Deep-See in Summer 2018 and 2019 off the New England shelf break are used to classify scattering layers into fluid-like zooplankton (e.g., jellies, copepods, krill) or gas-bearing organisms (e.g., fishes, siphonophores), then physics-based scattering models are used to estimate abundance and biomass. Consistent with the acoustic measurements, digital holographic images collected by Deep-See reveal multiple deep scattering layers, some consisting mainly of weakly scattering krill.

Broadband acoustics: A viable tool for quantification of cod egg density and distribution?

Fish eggs are ichthyoplankton with a similar behavior in the water column as small zooplankton. Their specific density relative to sea water determines their vertical distribution and spread; horizontal transport of the layers will then determine their fate and geographical position during the hatching of the larvae. Acoustic techniques are routinely used to assess the quantities and distribution of fish populations. In comparison to fish, zooplankton and ichthyoplankton are very weak acoustic reflectors and this makes the application of acoustic techniques challenging. We assessed the potential feasibility of using broadband acoustics to detect cod eggs in a two stage process: first, we carried out small-scale tank experiments to measure the acoustic reflectivity of individual and grouped cod eggs over the 170 to 360 kHz frequency range. These experiments showed that natural densities of cod eggs could be detected above the background acoustic noise level. This lead to the second stage whereby we conducted a field test in an area with known concentrations of cod eggs, using a combined acoustic-optical towed and moored system. We postulate that the high spatial resolution, yet rapid wide area coverage possible with acoustics will lead to a better description of egg distributions and subsequent larval distributions and help to understand the extremely variable recruitment success in sub-arctic and arctic ecosystems.

Potential for broadband acoustics to improve stock assessment surveys of midwater fishes

Acoustic-trawl surveys rely on a combination of backscatter measured with echosounders and species composition data from trawls to apportion the backscatter to different species and size classes. Narrowband echosounders have been widely used in this context for decades. Multi-frequency analysis of narrowband echosounder data has been shown to be effective for discriminating between diverse taxa (e.g., euphausiids vs. swimbladdered fishes) but distinguishing morphologically similar species (e.g., swimbladdered fishes) remains a major challenge. Previous work indicates that broadband backscatter techniques have the potential to improve such acoustic target characterizations by exploiting nearly continuous frequency spectra, but these methods have not been widely applied in fisheries surveys. The recent commercial availability of broadband transceivers is accelerating the evaluation of this technology. We present operational data from two broadband acoustic scattering systems: (1) 14-160 kHz and (2) 3-10 kHz...

Broadband active acoustics for synoptic studies of marine ecosystem

Marine ecosystems host a wide variety of organisms spanning many trophic levels, ranging from the smallest planktonic organisms that comprise the base of the marine food web, to large apex predator fishes, marine mammals, such as whales, and sea birds. Active acoustical systems that span a wide range of frequencies are required to quantitatively characterize these organisms, their distribution and abundance. In this presentation, we discuss the development and use of broadband acoustic systems that are optimized for studies of fish and zooplankton through use of spectral classification methods. For swimbladder-bearing fish, a low-frequency (1–6 kHz) broadband system is described that classifies fish according to the resonance of its swimbladder. For zooplankton, a high frequency (25–600 kHz) broadband system is described that classifies zooplankton either according the resonance of their gas (gas-bearing zooplankton only) or according to the transition between the Rayleigh and geometric scattering region (non-gaseous zooplankton). Because of the resolving power of the broadband signals, the echoes also tend to be non-Rayleigh, which has additional classification information. Applications of the spectral and statistical broadband acoustics methods to ecosystem research are given. [Work supported by the Office of Naval Research and the Woods Hole Oceanographic Institution.]

A New Implementation of the Extended Helmholtz Resonator Acoustic Liner Impedance Model in Time Domain CAA

The application of wall acoustic lining is a major factor in the reduction of aircraft engine noise. The extended Helmholtz Resonator (EHR) impedance model is widely used since it is representative of the behavior of realistic liners over a wide range of frequencies. Its application in time domain CAA methods by means of [Formula: see text]-transform has been the subject of several papers. In contrast to standard liner modeling in time domain CAA, which consists in imposing a boundary condition modeling both the cavities and the perforated sheet of the liner, an alternative approach involves adding the cavities to the computational domain and imposing a condition between these cavities and the duct domain to model the resistive sheet. However, the original method may not be used for broadband acoustics since it implements an impedance condition with frequency independent resistance. This paper describes an extension of this method to implement the EHR impedance model in a time domain CAA method.

Hot topics in fish acoustics (active)

It is important to quantify the spatial distribution of fish in their natural environment (ocean, lake, and river) and how the distribution evolves in time for a variety of applications including (1) management of fish stocks to maintain a sustainable source of food and (2) to improve our understanding of the ecosystem (such as how climate change impacts fish) through quantifying predator–prey relationships and other behavior. Active fish acoustics provides an attractive complement to nets given the great distances sound travels in the water and its ability to rapidly survey a large region at high resolution. This method involves studying distributions of fish in the water through analyzing their echoes through various means. While this field has enjoyed development for decades, there remain a number of “hot topics” receiving attention from researchers today. These include: (1) broadband acoustics as an emerging tool for advanced classification of, and discrimination between, species, (2) multi-beam imaging systems used to classify fish schools by size and shape, (3) long-range (km to 10's km) detection of fish, and (4) using transmission loss to classify fish on one-way propagation paths. Recent advances in these and other topics will be presented.

On the use of high-frequency broadband sonar to classify biological scattering layers from a cabled observatory in Saanich Inlet, British Columbia

This paper explores the use of broadband acoustics to differentiate between biological scattering layers using observatory-based acoustic observations with minimal supporting biological observations. Targets and layer assemblages were classified based on 85–155 kHz acoustic data collected on the VENUS observatory in Saanich Inlet, B.C. between March 2008 and February 2010 using a clustering algorithm and different broadband acoustic data descriptors. First, a 6-h segment of data, for which there were coincident depth-resolved net-tow data, was analyzed. Clustering based on the calibrated spectrum of volume scattering strength for each target resulted in clusters that were distributed just as those resulting from clustering based on 120 kHz narrowband data because the clustering was dominated by the scattering level, rather than the spectral shape. When the target spectra were normalized, the clustering results were consistent with the different taxa found in the net samples, but often could not distinguish taxonomic groups. However, layers with distinct species assemblages had different distributions of target classifications, suggesting the assemblages could be distinguished using frequency-dependent scattering information. Ensemble-averaging the scattering observations and converting the spectral data to a 3-descriptor acoustic color representation prior to clustering was (1) more effective at distinguishing the dominant scattering layers based on their assemblages and (2) much more efficient in terms of computational cost. Clustering two years of acoustic-color data identified 4 main groups (diel migrating euphausiids and chaetognaths, fish, and a mix of pteropods and bottom-to-oxycline migrating amphipods) that were consistent with contemporaneous and historical observations of zooplankton in the inlet. A wider frequency band might be effective in better distinguishing individual zooplankton targets.

Broadband acoustics on the VENUS observatory in Saanich Inlet.

High‐frequency sonar is by far the most cost‐effective way of “profiling” the water column from an ocean observing system. From a biological oceanographic perspective, long‐term acoustic observations are rich with information on the depths and abundances of fish and zooplankton. The drawback is that it is difficult to conclusively identify which species (or even functional groups) are present at any given time. This can be done, but only with plenty of supporting data, generally acquired non‐autonomously. Broadband acoustics may be the key to making acoustic observations of fish and zooplankton less qualitative. Here we explore this idea. We present nearly two years (Apr. 2008–Feb. 2010) of broadband (85–155 kHz) echosounder data collected on the VENUS observatory in Saanich Inlet. Using historical and contemporaneous (July 30, 2009) zooplankton net‐tow data, we attempt to automate and interpret the resulting classification of scattering layers throughout the long‐term record.

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

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.

High-frequency broadband acoustic scattering for investigating double- diffusive convection

Polar regions, with their supercooled and relatively fresh surface water, are highly susceptible to the diffusive regime of double-diffusive convection (DDDC). The fluxes associated with DDDC can play a significant role in their heat/buoyancy budgets. The use of high-frequency acoustics as a tool to map the extent and evolution of DDDC in the ocean is explored through a series of laboratory 200-300 kHz broadband acoustic backscattering measurements. Pulse compression signal processing allows centimeter-scale interface thicknesses to be rapidly and remotely measured, and the evolution, and ultimate merging, of multiple centimeter-scale interfaces to be observed. Combining the acoustically measured interface thicknesses with knowledge of the relatively-constant temperatures within the surrounding layers allows the estimation of DDDC fluxes. Thus, broadband acoustics offers a rapid and remote method to infer fluxes, without the need for time-consuming microstructure measurements, suggesting that this technique could benefit field studies of DDDC. Using simple models to extrapolate to the thermohaline steps typically found associated with DDDC in polar regions, diffusive-convection interfaces are predicted to scatter at measurable levels in many areas. Narrowband (120 and 200 kHz) acoustic backscattering observations of two scattering layers in the Western Antarctic Peninsula support this prediction.

Use of broadband active acoustics to study marine organisms

Broadband signals inherently have more information than narrowband signals. In essence, they have more "channels" of information due to the wider range of frequencies spanned. It is therefore advantageous to use broadband signals in active acoustic studies of marine organisms and to apply techniques that best exploit the broadband nature of the signals. In this presentation, a brief background on the use of narrowband systems for studying marine organisms by these authors and other investigators is given. Limitations to those studies are outlined, with arguments made for the use of broadband acoustics. The past twenty years of studies centered at the Woods Hole Oceanographic Institution are subsequently reviewed in which broadband signals are used both in the laboratory and field to study a variety of organisms‐‐ swimbladder‐bearing fish and three major anatomical categories of zooplankton. The analyses are divided broadly into two major categories‐‐ time and frequency domain‐‐ for various types of analyses, including pulse compression processing and spectral analysis. Results are first shown based on studies, one organism at a time, in the laboratory. Those laboratory approaches are then applied to ocean studies of fish and large zooplankton using a new towed instrument spanning the frequency range 1.7 kHz ‐ 100 kHz.

Fluctuations of high-frequency sound field in shallow water in the presence of internal waves

The structure of space-time fluctuations of a high-frequency (several kHz) sound field in a shallow water waveguide is considered. The sound field fluctuations due to internal and surface waves show different temporal variability. The perturbation theory within the framework of ray description is applied in order to derive some analytical estimation of fluctuations of arrival time and vertical angle distribution of high frequency broadband acoustics signals. The parabolic equation is used for numerical modeling of space-time fluctuations and vertical sound field intensity distribution. The experimental measurements of ocean dynamics from recent acoustic experiments are used as input data in numerical simulation of sound field fluctuations. [Work was supported by ONR and CRDF].

Measurements of acoustic wave speeds in a fluid-saturated sandy bottom

A new laboratory has been created at NRL to study the physics associated with the scattering from buried and partially buried objects in a saturated porous media environment. The laboratory consists of a 600 000-liter deionized water tank that is instrumented with precision robotic scanners and sources and receivers for broadband (0.5 kHz–5 MHz) acoustics research. Presently, the tank is filled with a 3.0-m-deep bottom of 212-mm-mean-diam sand that has been filtered and cleaned. Preliminary measurements of the acoustical wave sound speeds propagating in this saturated porous media are presented. In addition to time-delay measurements, similar to those done by Chotiros [J. Acoust. Soc. Am. 97 (1995)], a novel synthetic array was used to map the wavefronts propagating through the bottom. From these two-dimensional synthetic array measurements, wave speeds are evaluated using analytical techniques that include wave-number frequency and ray path analysis. Attenuation in the porous media and reflection and transmission from the interface are also studied. [Work supported by NRL.]