Benjamin A. Jones

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

Blainvilles 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 Blainvilles 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.

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.

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.

Echo statistics of individual and aggregations of scatterers in the water column of a random, oceanic waveguide

The relative contributions of various physical factors to producing non-Rayleigh distributions of echo magnitudes in a waveguide are examined. Factors that are considered include (1) a stochastic, range-dependent sound-speed profile, (2) a directional acoustic source, (3) a variable scattering response, and (4) an extended scattering volume. A two-way parabolic equation model, coupled with a stochastic internal wave model, produces realistic simulations of acoustic propagation through a complex oceanic sound speed field. Simulations are conducted for a single frequency (3 kHz), monostatic sonar with a narrow beam (5° -3 dB beam width). The randomization of the waveguide, range of propagation, directionality of the sonar, and spatial extent of the scatterers each contribute to the degree to which the echo statistics are non-Rayleigh. Of critical importance are the deterministic and stochastic processes that induce multipath and drive the one-way acoustic pressure field to saturation (i.e., complex-Gaussian statistics). In this limit predictable statistics of echo envelopes are obtained at all ranges. A computationally low-budget phasor summation can successfully predict the probability density functions when the beam pattern and number of scatterers ensonified are known quantities.

Observations of region-specific fish behavior using long- and short-range broadband (1.5—6 + kHz) active acoustic systems

Two broadband active acoustic systems, in concert with traditional narrowband systems and nets, were used to study distributions of fish in three regions within the Gulf of Maine. The long-range multi-beam broadband system detected fish out to 15 km range and the downward-looking short-range broadband system detected fish throughout the water column close behind the ship. The multi-year (2007–2011) study revealed distinct spatial patterns of fish and corresponding echo statistics in each region—diffusely distributed, sparsely distributed compact patches, and long (continuous) shoals. The broadband capabilities of the sonar systems (each spanning 1.5—6 + kHz) uniquely allow observations of resonance phenomena of the local swimbladder-bearing fish. The observed resonances were consistent with the fish species, sizes, and depths that were concurrently sampled in each area from a second research vessel. Spectral peak analysis also interestingly revealed the presence of distinct modes, which may be useful indi...

A scattering analysis of echoes due to biosonar signals emitted by foraging beaked whales

Blainville’s beaked whales (Mesoplodon densirostris) hunt their prey by echolocation at depths of more than 500 meters. These whales use a FM upswept, ultrasonic click, of greater than an octave bandwidth to search for, localize, and close on individual prey which generally consist of mesopelagic fishes and squid. It is well known that acoustic scattering from organisms of varying morphology (e.g., swimbladder‐bearing or fluidlike) is strongly frequency dependent. However, it is unknown if the broadband nature of the whales’ outgoing signal, and the frequency dependence of the echoes, is a key component in the classification and selection of their prey. Non‐invasive, acoustic ‘‘Dtags,’’ which sample stereo acoustic data at a rate which satisfies the high‐frequency Nyquist criterion for the animal’s transmit signal, were affixed to beaked whales. The Dtags successfully recorded transmitted signals and associated echoes. Structure was observed in the frequency content of echoes from isolated targets in the ...

Broadband classification and statistics of echoes from aggregations of fish measured by long-range, mid-frequency sonar

For horizontal-looking sonar systems operating at mid-frequencies (1-10 kHz), scattering by fish with resonant gas-filled swimbladders can dominate seafloor and surface reverberation at long-ranges (i.e., distances much greater than the water depth). This source of scattering, which can be difficult to distinguish from other sources of scattering in the water column or at the boundaries, can add spatio-temporal variability to an already complex acoustic record. Sparsely distributed, spatially compact fish aggregations were measured in the Gulf of Maine using a long-range broadband sonar with continuous spectral coverage from 1.5 to 5 kHz. Observed echoes, that are at least 15 decibels above background levels in the horizontal-looking sonar data, are classified spectrally by the resonance features as due to swimbladder-bearing fish. Contemporaneous multi-frequency echosounder measurements (18, 38, and 120 kHz) and net samples are used in conjunction with physics-based acoustic models to validate this approach. Furthermore, the fish aggregations are statistically characterized in the long-range data by highly non-Rayleigh distributions of the echo magnitudes. These distributions are accurately predicted by a computationally efficient, physics-based model. The model accounts for beam-pattern and waveguide effects as well as the scattering response of aggregations of fish.

Broadband classification and statistics of long-range, mid-frequency sonar measurements of aggregations of fish

Scattering from fish can constitute a significant portion of the high-amplitude echoes in the case of a horizontal-looking sonar system operating at mid-frequencies (1–10 kHz). In littoral environments, reverberation from fish with resonant gas-filled swimbladders can dominate bottom and surface reverberation and add spatio-temporal variability to an already complex acoustic record. Measurements of sparsely distributed, spatially compact fish aggregations have been conducted in the Gulf of Maine using a long-range, broadband sonar with continuous coverage over the frequency band of 1.5–5 kHz. Concurrent downward-looking, multi-frequency echosounder measurements (18, 38, and 120 kHz), and net samples of fish are used in conjunction with physics-based acoustic models to classify and statistically characterize the long-range fish echoes. A significant number of echoes, which are at least 15 dB above background levels, were observed in the long-range data and classified as due to mixed assemblages of swimblad...

Acoustics as a tool in sub-species and population identification for endangered fin whales, Balaenoptera physalus

Identification of “stocks” (sub-species and independent populations) is important for understanding and mitigating potential sources of human-caused mortality. This is especially critical for endangered and protected species, such as the large whales. Stock identification for whales has typically been based on ecology, life history, morphology, and genetics. However, for many species, acoustic differences in whale call types may indicate population or sub-species structure. The potential role of acoustics in identifying species and sub-species has been identified in numerous publications; however, this role has yet to be realized for large whales. In an effort to include acoustic data in this process, we are contributing to current efforts to update the status of endangered fin whales, Balaenoptera physalus, in the North Pacific. An analysis of North Pacific fin whale populations based on identification of “song” provides hypotheses that can be tested with genetics. Strengths and limitations of acoustic m...

Modeling of acoustic reverberation from a diurnally migrating fish aggregation with depth‐dependent resonance.

The depth dependence of the resonance frequency of fish with swimbladders is affected by both the change in volume (physostomes only) and pressure of the swimbladder (physostomes and physoclists). Due to these physical processes, the migration of fish upward or downward the water column can cause significant variation in the magnitude of the echoes received by an active sonar, most notably in narrowband applications where the resonance frequency may pass through the sonar band during migration. This variation in received echo level is further complicated by waveguide effects in long‐range acoustic scenarios. The sonar simulator toolset, a ray‐based sonar simulator, is used to simulate the reverberation of a narrowband, FM pulse by a vertically migrating aggregation of fish over a range of conditions. We examine the results of varying range, depth, and other waveguide parameters on the matched filter output. The predictions distinguish the effect of swimbladder resonance from acoustic waveguide effects on ...