Redwood W. Nero

A low‐frequency acoustic scattering model for small schools of fish

A new low‐frequency scattering model for small to moderately sized fish schools has been developed. The model, which uses a mathematical formalism based upon the harmonic solution of sets of coupled differential equations, allows a verified swimbladder scattering ‘‘kernel’’ for the individual fish to be incorporated. It includes all orders of multiple scattering interactions between fish, and calculates the aggregate scattering field by coherent summation. Application to simulated ensembles of closely spaced fish indicates significant deviations from incoherent scattering returns. Peak target strength reductions, and shifts in the resonance frequency, appear due to multiple scattering. The target strength also varies strongly with frequency as a result of interference effects. When applied to widely dispersed ensembles, the model reproduces the results of incoherent scattering. For larger ensembles, at greater depth, the model predicts sharply reduced target strength values around the main resonance. The ...

Boundary characterization experiment series overview

Ocean acoustic propagation and reverberation in continental shelf regions is often controlled by the seabed and sea surface boundaries. A series of three multi-national and multi-disciplinary experiments was conducted between 2000-2002 to identify and measure key ocean boundary characteristics. The frequency range of interest was nominally 500-5000 Hz with the main focus on the seabed, which is generally considered as the boundary of greatest importance and least understood. Two of the experiments were conducted in the Mediterranean in the Strait of Sicily and one experiment in the North Atlantic with sites on the outer New Jersey Shelf (STRATAFORM area) and on the Scotian Shelf. Measurements included seabed reflection, seabed, surface, and biologic scattering, propagation, reverberation, and ambient noise along with supporting oceanographic, geologic, and geophysical data. This paper is primarily intended to provide an overview of the experiments and the strategies that linked the various measurements together, with detailed experiment results contained in various papers in this volume and other sources

A field examination of acoustical scattering from marine organisms at 70 kHz

Acoustical scattering from aquatic organisms is an extremely complicated problem and can only be dealt with by empirical or semi-empirical models. In order to use sonars as a remote-sensing tool to study biological sound scatterers in the ocean, one must rely on such models. Our research at the Gulf Stream Boundary near Cape Hatteras, North Carolina, involves studying heterogeneous distributions of fish, crustaceans (e.g., shrimp), cephalopods (e.g., squid), and soft-bodied animals (e.g., salps) and their relationship to the front. To quantify the scattering from the animals, we use existing scattering models. However, the models were based on data from animals in artificial environments such as a laboratory tank or an underwater cage near a ship. Since scattering strengths depend on the animals behavior, depth, and whether it is alive or dead (and possibly preserved), data from the artificial environments may not accurately represent the scattering from the animals in their natural environments. Furthermore, the scattering models are mainly based on species not found in the area studied; therefore, we have considered them as general models for selected taxonomic groups. Working under this assumption, the present study examines the robustness of the general models to a field application. We used size frequency distribution of the various animal types from trawl data as input into the scattering models to produce a predicted value of the acoustic volume scattering strength. We also examined the field sonar data for potential problems in comparing trawl and sonar data (e.g., large animals which have high target strengths can avoid trawls). Our comparison between the predicted and field measured values demonstrates the potential usefulness of the general models in areas with different taxonomic assemblages. We find most data to lie within 5 dB of the predictions with some data systematically outside the 5-dB bars, which indicates animals at some stations were avoiding the trawl.

A highly reflective low cost backscattering target

A low cost biplanar reflector target has been developed and used in a system performance test of a near surface omnidirectional source and receiver. The target was constructed from sheets of closed cell plastic bubble sheeting. Theoretically, this material should have very high reflectance characteristics, and this was demonstrated by the experiment. The strongly reflecting target was located at the sea surface, and allowed the interpretation of data as the ship passed through a range of azimuthal angles. The higher than expected backscatter from the target was presumed to be due to the bubble sheeting behaving as a coherent reflector, like a thin layer of air, rather than an ensemble of individually resonating bubbles. This was verified by the data analysis. Lloyds mirror effects were strong, because practically all of the return signals from the 10-ms continuous wave pulses were overlapping. The target strength (TS) of the reflector was strongly reduced at ranges over 100 m. The experiment shows that studies of the statistical distribution of fish school TS must consider the effects of Lloyds mirror.

Changes in volume reverberation from deep to shallow water in the eastern Gulf of Mexico.

Scattering from fish is a primary cause of volume reverberation and, since fish populations change from deep to shallow water, the character of volume reverberation should also change. However, there are few data available to document expected changes. Therefore, an experiment was conducted in the eastern Gulf of Mexico to investigate possible changes in volume reverberation from deep to slope to shelf waters. Results showed that volume reverberation in outer shelf waters varied more rapidly with respect to both time and space than that in deeper waters. Day-time scattering was similar for deep, slope and shelf waters, total scattering strengths generally increased with frequency. Night-time scattering for the deep ocean and slope also increased with increasing frequency. Scattering modeling suggests that swimbladder-bearing fishes smaller than 10 cm were responsible for the observed volume reverberation. Night-time scattering at the outer shelf location was very different, with strong scattering peaks at...

Continuous wide area monitoring of fish shoaling behavior with acoustic waveguide sensing and bioclutter implications

Field measurements are used to show that the detailed behavior of fish shoals can be continuously monitored at roughly 1‐min intervals over wide areas spanning hundreds of square kilometers by long range acoustic waveguide sensing. The technique was used on the New Jersey Continental Shelf to produce unprecedented video images of shoal formation, fragmentation, and migration. Simultaneous line‐transect measurements show the imaged shoals to contain pelagic fish with densities of at least one individual per meter3. The technique relies upon acoustic waveguide propagation in the continental shelf. Here, trapped modes dominate propagation and suffer only cylindrical spreading loss rather than the spherical loss suffered in free‐space transmission or short‐range propagation in the ocean. In contrast, standard methods for fish surveyance involve line transect measurements from slow moving research vessels that significantly under‐sample fish distributions in time and space, leaving an incomplete behavioral pic...

A Lloyd‐mirror model to estimate the scattering strength of fish near rough ocean boundaries

It is well known that a point scatterer located near the sea surface creates an acoustic dipole and that one near the ocean bottom is in general described by a series of higher‐order moments (e.g., a double monopole in the simplest case of a hard, flat bottom). This Lloyd‐mirror effect has been coupled with a resonant fish scattering model of Love [R. H. Love, ‘‘Resonant acoustic scattering by swimbladder‐bearing fish,’’ J. Acoust. Soc. Am. 64, 571–580 (1978)] to develop a simple model which predicts the backscattering strengths of a variety of fish species known to occur (during at least part of a diurnal cycle) in near‐boundary layers. The model is applied to and compared with measurement results from Critical Sea Test 7 (CST‐7 Phase 2) to demonstrate that fish can be significant contributors to low‐grazing‐angle scattering. The general implications of the model are that observed enhancements in backscattering strength over levels predicted by first‐order rough‐surface scattering theories may, in some c...

Fish population dynamics revealed by instantaneous continental‐shelf scale acoustic imaging

Video images of fish population densities over vast areas of the New Jersey continental shelf have been produced from acoustic data collected on a long range bistatic sonar system during the Acoustic Clutter 2003 experiment. Areal fish population densities were obtained after correcting the acoustic data for two‐way transmission loss modeled using the range‐dependent parabolic equation, spatially varying beampattern of the array, source level and mean target strength per fish. The wide‐area fish density images reveal the temporal evolution of fish school distributions, their migration, as well as shoal formation and fragmentation at 50 s interval. Time series of the fish population within various density thresholds were made over the period of a day in an area containing millions of fish that at some instances formed a massive shoal extending over 12 km. The analysis shows that fish population in the area can be decomposed into a stable ambient population from lower‐fish‐density regions and a time‐varying...

Inferring fish school distributions from long range acoustic images: Main acoustic clutter experiment 2003

Long range scattering from fish schools and bottom reverberation in the New Jersey Continental Shelf environment are modeled using a unified, range‐dependent, bistatic scattering, and reverberation model based on the parabolic equation [Ratilal and Makris, J. Acoust. Soc. Am. 114, 2302 (2003)]. The fish swim bladder is approximated as an air‐filled bubble, while the bottom reverberation from volume inhomogeneities is modeled using the Rayleigh–Born approximation. The broadband scattered field, in the frequency range from 390 to 440 Hz, is beamformed and spatially charted using two‐way travel time. The model output is compared with scattered field levels from fish schools and background reverberation measured during the the Main Acoustic Clutter Experiment 2003 using a long range, bistatic sonar system. The fish school characteristics, such as size, distribution and density, are inputs to the model. These are obtained from measurements made by the fish finding sonar during the experiment. This calibrated m...

Fish schools are the dominant cause of long‐range active sonar clutter in the New Jersey Continental Shelf: Quantitative correlations

Long‐range underwater acoustic remote sensing data, acquired using a low‐ to mid‐frequency bistatic sonar system, is compared with data from a downward‐directed, fisheries‐standard high‐frequency fish‐finding sonar. These data were simultaneously measured in the New Jersey Continental Shelf environment during the Main Acoustic Clutter Experiment 2003. The long‐range sonar is capable of imaging extensive areas, spanning tens of kilometers in range, in near real‐time. This makes it possible to continuously observe the spatial and temporal variability of environmental returns over wide areas. In contrast, the fish‐finding sonar’s single, downward‐directed beam typically provides a swath of only 10 m width along the ship track. Dense populations of fish, indicated by prominent returns from the fish‐finding sonar, are overlain onto long‐range acoustic images. The outputs of the two systems are also directly correlated along the track of the fish‐finding sonar. High correlation was found between locations of pr...