Sound scattering by several zooplankton groups. I. Experimental determination of dominant scattering mechanisms

Target strength measurements of individual sub-Arctic krill have frequency-dependent differences from scattering model predictions

Target strength (TS) is commonly used to convert acoustic backscatter from marine organisms to numerical abundance estimates. Shipboard, tank-based TS measurements were made on four sub-Arctic krill species (Euphausia pacifica, Thysanoessa spinifera, Thysanoessa inermis, and Thysanoessa raschii) from the eastern Bering Sea and Gulf of Alaska at discrete frequencies between 42 and 455 kHz. These measurements were compared to scattering model predictions parameterized with data from the same (when possible) individual krill. Statistically significant differences between modeled and experimental estimates at 42, 45, 120, and 131 kHz exceeded 2 dB on average. Variability in the signal-to-noise ratio, animal length, and measurements from two separate narrowband and broadband transducer pairs (at those frequencies) did not account for these differences. Scattering predictions at 120 and 131 kHz were consistent with the expected transition from Rayleigh-to-geometric scattering where models become increasingly sensitive to orientation and body shape variability. Disagreement between modeled and measured TS may be due to using scattering models developed for, and validated on, larger krill (i.e., Euphausia superba) rather than smaller species of krill. Acoustic surveys of smaller (15-30 mm) krill may require further validation of both the generalizability and parameterization of applied scattering models.

Exploring the promise of broadband fisheries echosounders for species discrimination with quantitative assessment of data processing effects

It remains an open question how well the increased bandwidth afforded by broadband echosounders can improve species discrimination in fisheries acoustics. Here, an objective statistical approach was used to determine if there is information available in dual channel broadband data (45-170 kHz) to allow discrimination between in situ echoes obtained from monospecific aggregations of three species (hake, Merluccius productus: anchovy, Engraulis mordax; and krill, Euphausiia pacifica) using a remotely operated vehicle. These data were used to explore the effects of processing choices on the ability to statistically classify the broadband spectra to species. This ability was affected by processing choices including the Fourier transform analysis window size, available bandwidth, and the method and scale of data averaging. The approach to normalizing the spectra and the position of individual targets in the beam, however, had little effect. Broadband volume backscatter and single target spectra were both used to successfully classify acoustic data from these species with ∼6% greater success using volume backscatter data. Broadband data were effectively classified to species while simulated multi-frequency narrowband data were categorized at rates near chance, supporting the presumption that greater bandwidth increases the information available for the characterization and classification of biological targets.

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.

Can the elongated hindwing tails of fluttering moths serve as false sonar targets to divert bat attacks?

It has long been postulated that the elongated hindwing tails of many saturniid moths have evolved to create false sonar targets to divert the attack of echolocation-guided bat predators. However, rigorous echo-acoustic evidence to support this hypothesis has been lacking. In this study, fluttering luna moths (Actias luna), a species with elongated hindwing tails, were ensonified with frequency modulated chirp signals from all angles of orientation and across the wingbeat cycle. High-speed stereo videography was combined with pulse compression sonar processing to characterize the echo information available to foraging bats. Contrary to previous suggestions, the results show that the tail echoes are weak and do not dominate the sonar returns, compared to the large, planar wings and the moth body. However, the distinctive twisted morphology of the tails create persistent echoes across all angles of orientation, which may induce erroneous sonar target localization and disrupt accurate tracking by echolocating bats. These findings thus suggest a refinement of the false target hypothesis to emphasize sonar localization errors induced by the twisted tails, and highlight the importance of physics-based approaches to study the sensory information involved in the evolutionary arms race between moths and their bat predators.

Arctic complexity: a case study on diel vertical migration of zooplankton

Diel vertical migration (DVM) of zooplankton is a global phenomenon, characteristic of both marine and limnic environments. At high latitudes, patterns of DVM have been documented, but rather little knowledge exists regarding which species perform this ecologically important behaviour. Also, in the Arctic, the vertically migrating components of the zooplankton community are usually regarded as a single sound scattering layer (SSL) performing synchronized patterns of migration directly controlled by ambient light. Here, we present evidence for hitherto unknown complexity of Arctic marine systems, where zooplankton form multiple aggregations through the water column seen via acoustics as distinct SSLs. We show that while the initiation of DVM during the autumnal equinox is light mediated, the vertical positioning of the migrants during day is linked more to the thermal characteristics of water masses than to irradiance. During night, phytoplankton biomass is shown to be the most important factor determining the vertical positioning of all migrating taxa. Further, we develop a novel way of representing acoustic data in the form of a Sound Image (SI) that enables a direct comparison of the relative importance of each potential scatterer based upon the theoretical contribution of their backscatter. Based on our comparison of locations with contrasting hydrography, we conclude that a continued warming of the Arctic is likely to result in more complex ecotones across the Arctic marine system.

Assessing the low frequency acoustic characteristics of Macrocystis pyrifera, Egregia menziessi, and Laminaria solidungula

The acoustic properties of kelp forests are not well known, but are of interest for the development of environmental remote sensing applications. This study examined the low-frequency (0.2-4.5 kHz) acoustic properties of three species of kelp (Macrocystis pyrifera, Egregia menziessi, and Laminaria solidungula) using a one-dimensional acoustic resonator. Acoustic observations and measurements of kelp morphology were then used to test the validity of Wood's multi-phase medium model in describing the acoustic behavior of the kelp. For Macrocystis and Egregia, the two species of kelp possessing pneumatocysts, the change in sound speed was highly dependent on the volume of free air contained in the kelp. The volume of air alone, however, was unable to predict the effective sound speed of the multi-phase medium using a simple two-phase (air + water) form of Wood's model. A separate implementation of this model (frond + water) successfully yielded the acoustic compressibility of the frond structure for each species (Macrocystis = 1.39 ± 0.82 × 10(-8) Pa(-1); Egregia = 2.59 ± 5.75 × 10(-9) Pa(-1); Laminaria = 8.65 ± 8.22 × 10(-9) Pa(-1)). This investigation demonstrates that the acoustic characteristics of kelp are species-specific, biomass-dependent, and differ between species with and without pneumatocyst structures.

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.

Measuring the target strength spectra of fish using dolphin-like short broadband sonar signals

Dolphins identify their prey using broadband sonar signals. The broadband spectrum of the target strength (TS) of fish is believed to be a key factor in target discrimination. In this study, the TS spectrum was measured using sonar signals generated by two different dolphin species: finless porpoise and bottlenose dolphin. First, the broadband form functions of a tungsten carbide sphere and a copper sphere were measured in a water tank, and a close agreement between measurements and theoretical values was confirmed. Second, the TS spectra of anesthetized fish from three species were measured in a water tank. Although the results showed characteristics similar to previous measurements, they varied among species, individuals, and tilt angles. Third, the TS spectra of live fish suspended and tethered by nylon monofilament lines were measured at sea. The dolphin-like sonar signals were effective in obtaining the broadband TS spectra of the fish. Cross-correlation processing of the echo from a tungsten carbide sphere showed a further advantage of using the dolphin-like sonar signals: the signal-to-noise ratio increased by more than 10 dB. The variation of TS spectra with fish behavior provides useful information for target identification.

Classification of live, untethered zooplankton from observations of multiple-angle acoustic scatter

A broadband, multiple-angle acoustic array was used to classify millimeter to centimeter sized live zooplankton in a laboratory tank. Reflections in the frequency range from 1.5 to 2.5 MHz were recorded from untethered 1-4 mm calanoid copepods and 8-12 mm mysids over an angular range of 0 degrees -47 degrees . A synchronized, coregistered video system recorded animal location and orientation. To highlight differences between animals, a frequency correlation matrix was computed from the observed wide-band power spectra of the scattered sound. Significant differences in the slopes and shapes of the eigenvalue spectra of this matrix were found for mysids versus copepods. These results support the idea that broadband, multiple-angle scatter can be used to classify organisms of different sizes and shapes.

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.

Determining dominant scatterers of sound in mixed zooplankton populations

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.

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.

Comparison of multifrequency acoustic and in situ measurements of zooplankton abundances in Knight Inlet, British Columbia

An investigation of midwater zooplankton aggregations in a coastal fjord was conducted in November 2002. This study focused on quantitative comparisons between a calibrated, three-frequency (38, 120, and 200 kHz) vessel-based echo-sounder, a multinet towed zooplankton sampler (BIONESS), and a high-resolution underwater camera (ZOOVIS). Daytime layers of euphausiids and amphipods near 70-90-m depth were observed in lower parts of the inlet, especially concentrated by tidal flows around a sill. Quantitative backscatter measurements of euphausiids and amphipods, combined with in situ size and abundance estimates, and using an assumed tilt-angle distribution, were in agreement with averaged fluid-cylinder scattering models produced by Stanton and Chu [ICES J. Mar. Sci. 57, 793-807, (2000)]. Acoustic measurements of physonect siphonophores in the upper inlet were found to have a strong 38-kHz scattering strength, in agreement with a damped bubble scattering model using a diameter of 0.4 mm. In relatively dense euphausiid layers, ZOOVIS abundance estimates were found to be a factor of 2 to 4 higher than the acoustic estimates, potentially due to deviations from assumed euphausiid orientation. Nocturnal near-surface euphausiid scattering exhibited a strong (15 dB) and rapid (seconds) sensitivity to vessel lights, interpreted as due to changing animal orientation.

Broadband acoustic backscatter and high-resolution morphology of fish: measurement and modeling

Broadband acoustic backscattering measurements, advanced high-resolution imaging of fish morphology using CT scans and phase-contrast x rays (in addition to traditional x rays), and associated scattering modeling using the images have been conducted involving alewife (Alosa pseudoharengus), a swimbladder-bearing fish. A greater-than-octave bandwidth (40-95 kHz) signal was used to insonify live, individual, adult alewife that were tethered while being rotated in 1-deg increments over all angles in two planes of rotation (lateral and dorsal/ventral). These data, in addition to providing the orientation dependence of the scattering over a continuous band of frequencies, were also used (after pulse compression) to identify dominant scattering features of the fish (including the skull and swimbladder). The x-ray and CT scan images of the swimbladder were digitized and incorporated into two scattering models: (1) Kirchhoff-ray mode (KRM) model [Clay and Horne, J. Acoust. Soc. Am. 96, 1661-1668 (1994)] and (2) conformal-mapping-based Fourier matching method (FMM), which has recently been extended to finite-length bodies [Reeder and Stanton, J. Acoust. Soc. Am. 116. 729-746 (2004)]. Comparisons between the scattering predictions and data demonstrate the utility of the CT scan imagery for use in scattering models, as it provided a means for rapidly and noninvasively measuring the fish morphology in three dimensions and at high resolution. In addition to further validation of the KRM model, the potential of the new FMM formulation was demonstrated, which is a versatile approach, valid over a wide range of shapes, all frequencies and all angles of orientation.

On the acoustic diffraction by the edges of benthic shells

Recent laboratory measurements of acoustic backscattering by individual benthic shells have isolated the edge-diffracted echo from echoes due to the surface of the main body of the shell. The data indicate that the echo near broadside incidence is generally the strongest for all orientations and is due principally to the surface of the main body. At angles well away from broadside, the echo levels are lower and are due primarily to the diffraction from the edge of the shell. The decrease in echo levels from broadside incidence to well off broadside is shown to be reasonably consistent with the decrease in acoustic backscattering from normal incidence to well off normal incidence by a shell-covered seafloor. The results suggest the importance of the edge of the shell in off-normal-incidence backscattering by a shell-covered seafloor. Furthermore, when considering bistatic diffraction by edges, there are implications that the edge of the shell (lying on the seafloor) can cause significant scattering in many directions, including at subcritical angles.

Three-dimensional modeling of acoustic backscattering from fluid-like zooplankton

Scattering models that correctly incorporate organism size and shape are a critical component for the remote detection and classification of many marine organisms. In this work, an acoustic scattering model has been developed for fluid-like zooplankton that is based on the distorted wave Born approximation (DWBA) and that makes use of high-resolution three-dimensional measurements of the animal's outer boundary shape. High-resolution computerized tomography (CT) was used to determine the three-dimensional digitizations of animal shape. This study focuses on developing the methodology for incorporating high-resolution CT scans into a scattering model that is generally valid for any body with fluid-like material properties. The model predictions are compared to controlled laboratory measurements of the acoustic backscattering from live individual decapod shrimp. The frequency range used was 50 kHz to 1 MHz and the angular characteristics of the backscattering were investigated with up to a 1 degree angular resolution. The practical conditions under which it is necessary to make use of high-resolution digitizations of shape are assessed.

Acoustic scattering by benthic and planktonic shelled animals

Acoustic backscattering measurements and associated scattering modeling were recently conducted on a type of benthic shelled animal that has a spiral form of shell (Littorina littorea). Benthic and planktonic shelled animals with this shape occur on the seafloor and in the water column, respectively, and can be a significant source of acoustic scattering in the ocean. Modeling of the scattering properties allows reverberation predictions to be made for sonar performance predictions as well as for detection and classification of animals for biological and ecological applications. The studies involved measurements over the frequency range 24 kHz to 1 MHz and all angles of orientation in as small as 1 degree increments. This substantial data set is quite revealing of the physics of the acoustic scattering by these complex shelled bodies and served as a basis for the modeling. Specifically, the resonance structure of the scattering was strongly dependent upon angle of orientation and could be traced to various types of rays (e.g., subsonic Lamb waves and rays entering the opercular opening). The data are analyzed in both the frequency and time domain (compressed pulse processing) so that dominant scattering mechanisms could be identified. Given the complexity of the animal body (irregular elastic shell with discontinuities), approximate scattering models are used with only the dominant scattering properties retained. Two models are applied to the data, both approximating the body as a deformed sphere: (1) an averaged form of the exact modal-series-based solution for the spherical shell, which is used to estimate the backscattering by a deformed shell averaged over all angles of orientation, and produces reasonably accurate predictions over all k1a(esr) (k1 is the acoustic wave number of the surrounding water and a(esr) is the equivalent spherical radius of the body), and (2) a ray-based formula which is used to estimate the scattering at fixed angle of orientation, but only for high k1a(esr). The ray-based model is an extension of a model recently developed for the shelled zooplankton Limacina retroversa that has a shape similar to that of the Littorina littorea but swims through the water [Stanton et al., J. Acoust. Soc. Am. 103, 236-253 (1998b)]. Applications of remote detection and classification of the seafloor and water column in the presence of shelled animals are discussed.

Sound scattering by several zooplankton groups. II. Scattering models

Mathematical scattering models are derived and compared with data from zooplankton from several gross anatomical groups--fluidlike, elastic shelled, and gas bearing. The models are based upon the acoustically inferred boundary conditions determined from laboratory backscattering data presented in part I of this series [Stanton et al., J. Acoust. Soc. Am. 103, 225-235 (1998)]. The models use a combination of ray theory, modal-series solution, and distorted wave Born approximation (DWBA). The formulations, which are inherently approximate, are designed to include only the dominant scattering mechanisms as determined from the experiments. The models for the fluidlike animals (euphausiids in this case) ranged from the simplest case involving two rays, which could qualitatively describe the structure of target strength versus frequency for single pings, to the most complex case involving a rough inhomogeneous asymmetrically tapered bent cylinder using the DWBA-based formulation which could predict echo levels over all angles of incidence (including the difficult region of end-on incidence). The model for the elastic shelled body (gastropods in this case) involved development of an analytical model which takes into account irregularities and discontinuities of the shell. The model for gas-bearing animals (siphonophores) is a hybrid model which is composed of the summation of the exact solution to the gas sphere and the approximate DWBA-based formulation for arbitrarily shaped fluidlike bodies. There is also a simplified ray-based model for the siphonophore. The models are applied to data involving single pings, ping-to-ping variability, and echoes averaged over many pings. There is reasonable qualitative agreement between the predictions and single ping data, and reasonable quantitative agreement between the predictions and variability and averages of echo data.