© 2010 The Authors. This article is distributed under the terms of the Creative Commons Attribution-Noncommercial License. The definitive version was published in ICES Journal of Marine Science: Journal du Conseil 67 (2010): 365-378, doi:10.1093/icesjms/fsp262.

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New broadband methods for resonance classification and high-resolution imagery of fish with swimbladders using a modified commercial broadband echosounder

The low-frequency target strength of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300–1200Hz range using (1) a low-frequency ocean acoustic waveguide remote sensing (OAWRS) system, (2) areal population density calibration with several conventional fish finding sonar (CFFS) systems, and (3) low-frequency transmission loss measurements. The OAWRS system’s instantaneous imaging diameter of 100km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to c...

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Low-frequency Target Strength and Abundance of Shoaling Atlantic Herring (Clupea harengus) in the Gulf of Maine during the Ocean Acoustic Waveguide Remote Sensing 2006 Experiment

A new method is developed in which a single standard target is used to calibrate the system response of broadband monostatic active acoustic systems. Spheres have commonly been used to calibrate narrowband systems. A major challenge involves the resonances of the scattering by the sphere. For narrowband systems, the sphere is designed so that the acoustic frequency is between the resonances. However, this cannot be done for broadband systems as the frequencies will span multiple resonances. Our method completely eliminates the source of the resonances through pulse-compression processing of the signal. Through this processing, the echo from the front interface of the sphere is isolated from other echoes which cause the resonances. The echo from the front interface is generally weakly dependent upon frequency, making it ideal for calibrating a broadband system. The method is a generalization of what was proposed by Dragonette et al. (J. Acoustic. Soc. Am. 69,1186-1189 (1981)). The advances in this new method involve incorporation of signal processing to better isolate the echo, as well as a more accurate treatment of the scattering physics. As a result, the approach is applicable over a wide range of conditions.

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Calibration of broadband active acoustic systems using a single standard spherical target

To date most sonars use narrow band pulses and often only the echo envelope is used for object detection and classification. This paper considers the advantages afforded by bio-inspired sonar for object identification and classification through the analysis and the understanding of the broadband echo structure. Using the biomimetic dolphin based sonar system in conjunction with bio-inspired pulses developed from observations of bottlenose dolphins performing object identification tasks, results are presented from experiments carried out in a wave tank and harbor. In these experiments responses of various targets to two different bio-inspired signals are measured and analyzed. The differences in response demonstrate the strong dependency between signal design and echo interpretation. In the simulations and empirical data, the resonance phenomena of these targets cause strong notches and peaks in the echo spectra. With precision in the localization of these peaks and dips of around 1 kHz, the locations are very stable for broadside insonification of the targets and they can be used as features for classification. This leads to the proposal of a broadband classifier which operates by extracting the notch positions in the target echo spectra.

Analysis and classification of broadband echoes using bio-inspired dolphin pulses.

Acoustic methods are widely used in fisheries research, often providing vital information that can be obtained in no other way. In reviewing active methods, phenomena of sound scattering are first described. The means of ensonification and detection, the generic sonar, is described. Examples include the traditional scientific echo sounder and the following six classes of sonar: multibeam, sidescan, acoustic lens-based, parametric, synthetic aperture, and conventional low-frequency sonars. Methods of data processing, quantification, and data interpretation are addressed. In reviewing passive methods, sounds produced by organisms are exemplified. The traditional means of detecting sound, the hydrophone, is then described together with various configurations of hydrophones. Methods of data analysis and classification are outlined. Calibration is addressed separately for sonars and hydrophones. Applications of the various methods are cited. Potential applications of new, improved, or refined acoustic methods to outstanding problems in fisheries and fisheries habitat research are indicated.

Acoustic Methods: Brief Review and Prospects for Advancing Fisheries Research

A new echosounder that exploits the resonance frequency of swimbladder-bearing fish is described. The system is a custom configuration of a commercial device originally developed for studying the seafloor and adapted for studying fish. The frequencies span the range 1.7 kHz to 100 kHz, with some gaps, with the lower portion of that range corresponding to the resonance frequency of common adult fish. Through pulse-compression processing of the broadband signals, the range resolution is greatly improved to about 20 cm at the lowest frequencies and about 3 cm at the highest frequencies. As a result of the resonance classification at these low frequencies, significant ambiguities that are normally associated with interpretation of high frequency echosounder data are eliminated. Summarized in this paper are the design and calibration of the system, as well as results from using the system to study schools of fish in the ocean.

A broadband echosounder for resonance classification of swimbladder-bearing fish

The Simrad EK500 has been the state-of-the-art scientific echo sounder for surveying marine fish stocks; the EK60 is its successor. Both echo sounders have been calibrated with the same 38-kHz, 12-deg-beamwidth, split-beam transducer by the standard-target method at the acoustic calibration facility on Iselin Dock at the Woods Hole Oceanographic Institution. The principal measurements were on-axis target strengths and the two-way directivity patterns of the main lobe, measured with a 60-mm-diameter copper sphere. For each echo sounder, the respective split-beam-determined and directly measured angles of the standard target are compared. The directivity pattern as approximated by Simrad firmware is fit to the experimental data, and both the split-beam-determined and newly compensated values of target strength are expressed through histograms. Target strength distributions are compared for two spheres: a 60-mm-diameter aluminum and 38.1-mm-diameter tungsten carbide with 6% cobalt binder spheres.

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Calibrating two scientific echo sounders

The Simrad EK500 and EK60 scientific echo sounders have been operated with the same 38-kHz, 12-deg-beamwidth, split-beam transducer, with alternate pinging by means of an external triggering-and-switching system. The respective performances of the two systems are compared by means of standard-target calibrations conducted at the acoustic calibration facility on Iselin Dock at Woods Hole Oceanographic Institution with a 60-mm-diameter copper sphere. Principal comparisons include those of time series of echo strength and target strength, split-beam-determined angles, and derived measures of directivity.

Comparing two scientific echo sounders

Measurements of broadband acoustic backscattering by swimbladder-bearing fish have recently been made over Georges Bank, a shallow-water region off of Cape Cod, Massachusetts. A seismic source, towed by a ship just above the schools of fish, was used to transmit chirp (linear frequency modulated) signals downward toward the fish. The system, which is most sensitive over the range 2-4 kHz, swept through the range of frequencies, 1-10 kHz. The echoes were especially strong at frequencies near the swimbladder resonance of 3 kHz. The measurements were done in conjunction with net tows in order to ground truth the echo data. The echoes were processed with a matched filter, achieving high signal-to-noise ratios and range resolutions of approximately 20 cm. As a result of this resolution, fish very near the seafloor were resolved. The filtered echoes were partitioned into various spatial windows and analyzed both in the time domain and frequency domain. Of particular interest was the statistical behavior of the echoes. The schools were highly patchy and the statistics of the echoes varied according to whether a set of echoes were from within a patch, or spanned across the patches. The echoes are shown to be strongly non-Rayleigh when the sonar is sweeping across the patches. Net tows and swimbladder-resonance data indicate that all targets are mono-size and mono-specific. Thus, the fluctuations in the echoes are a result of a combination of interference between random phase echoes from individual targets, local variability in numerical density of targets, and effect of beam pattern associated with individual targets or patches of targets smaller than a sonar footprint

Statistical Behavior of Echoes from Swimbladder-Bearing Fish at 2-4 kHz

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 swimbladder-bearing fish. These aggregations of fish produce highly non-Rayleigh distributions of echo magnitudes. The probability density functions of the echoes are accurately predicted by a computationally efficient, physics-based model that accounts for beam-pattern and waveguide effects as well as the scattering response of aggregations of fish. [Work supported by the U.S. Office of Naval Research.]

J.M. Jech

Papers

A broadband echosounder for resonance classification of swimbladder-bearing fish

Calibrating two scientific echo sounders

Comparing two scientific echo sounders

Statistical Behavior of Echoes from Swimbladder-Bearing Fish at 2-4 kHz

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