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ABSTRACT: One of Clay's passions was modeling the scattering physics of marine organisms, a passion that has transcended into new generations of scientists. The focus of this presentation is thecosome pteropods, a widely and patchily distributed group of shelled zooplankton that are important members of pelagic ecosystems as they constitute important prey for a variety of other zooplankton and top predators. Acoustic techniques are well suited to sampling pteropods on relevant spatial and temporal scales as they secrete aragonite shells that make them highly efficient scatterers of sound. However, pteropod shells are complex and very susceptible to an increasingly corrosive seawater environment due to ocean acidification. Understanding the scattering physics is key to using acoustics as a quantitative remote sensing tool. Here we report on recent field measurements that combine the use of broadband (30-600 kHz) and narrowband (43, 120, 200, and 420 kHz) acoustic scattering techniques, as well as supporting in situ measurements (nets, optics, CTD and ocean chemistry) to investigate the distribution, abundance and size of pteropods in both the northwest Atlantic and the northeast Pacific in relation to the oceanic chemistry. Existing scattering models are tested, and improvements and modifications to the acoustic instrumentation and models are suggested.
The Journal of the Acoustical Society of America 09/2012; 132(3):1882. · 1.55 Impact Factor
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ABSTRACT: The statistics of echoes from active sonar systems can yield important information on aggregations of scatterers. This study explores the use of echo statistics for estimating the numerical density of scatterers in monotype aggregations. Here, "monotype" refers to scatterers with the same scattering amplitude distribution in the considered frequency range. The signals are broadband, and the geometry involves direct paths between the sonar and the scatterers without interference from boundaries. Model probability density functions (pdf's) of envelope amplitudes of matched-filter outputs are numerically generated by varying the number of Rayleigh scatterers randomly-distributed in a half-space shell while accounting for the frequency-dependent system response, scatterer response, and beampattern effects. The shape of the echo pdf as observed by the sonar receiver is highly non-Rayleigh when there are few scatterers in the beam, and gradually approaches the Rayleigh distribution when the number of scatterers increases. This model is applied to broadband fish echoes (30-70 kHz) collected in the ocean through a best-fit procedure. The inferred numerical density of fish is comparable to the density estimated using corresponding measurements of volume backscattering strength and modeled target strengths.
The Journal of the Acoustical Society of America 09/2012; 132(3):1882. · 1.55 Impact Factor
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ABSTRACT: One of the first papers that Clay gave to one of the authors (TKS) when he entered Clay's office in 1980 was one of Clay's papers involving echo statistics and, specifically, accounting for beampattern effects in using single beam echoes from resolved fish to estimate their target strength and abundance. That, and a multitude of conversations with Clay helped propel TKS, and later DC, into a career where echo statistics was an integral aspect of their work. We will review our work on echo statistics associated with a variety of scatterers--the seafloor, sea ice, fish, zooplankton, and machined objects. A key aspect of this work has been connecting the physics of the scattering process and sonar parameters with parameters of the statistical functions (such as shape parameter).
The Journal of the Acoustical Society of America 09/2012; 132(3):1882. · 1.55 Impact Factor
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ABSTRACT: 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.
The Journal of the Acoustical Society of America 06/2012; 131(6):4461-75. · 1.55 Impact Factor
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ABSTRACT: Mixed assemblages are defined to describe the cases in which more than one type of scatterer are present and are randomly located and spatially interspersed among one another in each sonar resolution cell. The probability density functions (pdfs) formed by the echo envelopes in such cases can be highly non-Rayleigh and possess heavy tails. The shape of the pdf curves contains information for characterizing and discriminating the composition of mixed assemblages. A general characteristic-function-based mixed assemblage pdf model is formulated in this study. The model, which takes into account beampattern effects, was validated using numerical simulations. Simulated data of two-component mixed assemblages with different relative scattering strengths, numerical densities, and spatial distributions were used to compare the performance of this new mixed pdf model and the widely used weighted multiple component mixture pdf model. It was found that use of the latter model can lead to orders of magnitude errors in estimating the composition of the mixed assemblages. This study is inspired in the context of acoustic studies of mixed biological aggregations in the ocean, but the theory is generally applicable to other types of mixed assemblages as well.
The Journal of the Acoustical Society of America 10/2011; 130(4):2337. · 1.55 Impact Factor
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ABSTRACT: Van Holliday had a vision for sensing marine organisms with active acoustics over a very wide range of frequencies. This inspired us at the Woods Hole Oceanographic Institution, Woods Hole, MA, to conduct a series of measurements, both in the laboratory and in the ocean, and associated modeling, over the range of 1.5 kHz to 3 MHz over the past 23 years. The organisms were as small as millimeter-size copepods and as large as 20-cm herring and squid. Broadband acoustic scattering measurements were conducted in the laboratory as a function of frequency (24 kHz 3 MHz) and angle of orientation (0-360 deg) of many species. Instruments were developed to measure broadband acoustic scattering in the ocean over the range 1.5 kHz 1.2 MHz with some gaps. Scattering models, based on the laboratory data, were also developed for several major anatomical groups of organisms and spanned a range of complexity, from low-resolution models that account only for length, width, and general shape to high resolution models that account for shape of the body and heterogeneities within the body in three dimensional at fine scale as well as including roughness. In this presentation, we review the laboratory measurements and scattering models, as well as development of the broadband ocean instruments and their use at sea.
The Journal of the Acoustical Society of America 10/2011; 130(4):2324. · 1.55 Impact Factor
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ABSTRACT: The statistical characteristics of sonar echoes from a randomly rough, randomly oriented prolate spheroid are studied through the use of a previously published exact analytical formula by Ehrenberg, which involves a convolution between the probability density functions (PDFs) associated with the sonar beam-pattern and scatterer. This is a direct-path scattering geometry in which only the echo from the scatterer is involved (no reflections from boundaries, such as in a waveguide). The scattering by the rough prolate spheroid is modeled approximately through the use of the Rayleigh PDF for each orientation angle with the mean value at each angle determined by the high-frequency (Kirchhoff) solution to a smooth prolate spheroid. Once the orientation is randomized, the echo from the rough spheroid (without beam-pattern effects) is non-Rayleigh. It had previously been established that random location in the beam pattern causes strongly non-Rayleigh echoes for resolved scatterers whose echoes were otherwise Rayleigh distributed (such as a rough spherical scatterer) before beam-pattern effects. The presence of the randomly rough, randomly oriented prolate spheroid that is randomly located in the beam increases the degree to which the echoes are non-Rayleigh beyond non-Rayleigh effects from the beam pattern.
The Journal of the Acoustical Society of America 10/2010; 128(4):2460. · 1.55 Impact Factor
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ABSTRACT: 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 the scattering. A key result illustrated is a 28-dB variation in the received echo due to the resonance frequency passing through the band of a 3-kHz sonar during diurnal migration of the Atlantic herring.
The Journal of the Acoustical Society of America 10/2010; 128(4):2279. · 1.55 Impact Factor
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ABSTRACT: Broadband echoes (30-70 kHz) were collected from fish aggregations with a downward-looking single beam echosounder over Georges Bank near Cape Cod, MA in the fall of 2008. The echoes were collected in a short-range, direct path geometry. Echoes from sequential pings occurring within sampling windows that lie entirely in a given patch were grouped to obtain the distribution of the magnitude of matched filtered echoes. Non-Rayleigh distributed echoes were observed in patches both near the surface and bottom. Two-component mixed probability density functions (pdf's) were formed by accounting for beampattern effects and assuming various proportions of sizes of fish with correspondingly different scattering strengths and are used to fit the measured echo distribution. For each patch, parameters of the best fitting pdf's for a series of sampling windows within the given patch were used to infer the fish size composition of the patch. The best-fit parameters for the surface patches were different from those from near the bottom. Results of the acoustically inferred fish assemblages are compared with the net-tow data and are used to investigate the biological significance of the fish aggregations.
The Journal of the Acoustical Society of America 10/2010; 128(4):2460. · 1.55 Impact Factor
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ABSTRACT: Pulse compression signal processing techniques are commonly used to identify dominant scattering features for weakly scattering marine organisms. In cases where the organism can be accurately represented as homogeneous (e.g., eupausiids), the separation of two distinct peaks in the compressed pulse output (CPO) envelope produced by the front and back water-tissue interfaces can be used to estimate the size of the organism at some angles of orientation. In order to investigate the effect of internal inhomogeneities on the scattering, a 3-D distorted-wave Born approximation-based numerical model has been used to model the CPO due to scattering from weakly scattering inhomogeneous objects. It has been found that when the organism is highly inhomogeneous (e.g., sea-water filled cavities in squid), the CPO envelope does not provide an accurate estimate of size. Characteristics of the replica signal can further complicate the issue by introducing spurious peaks with high side lobes or smearing out the CPO peaks from internal inhomogeneities when the main lobe is wide.
The Journal of the Acoustical Society of America 10/2010; 128(4):2460. · 1.55 Impact Factor
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ABSTRACT: Long-Term Goals: To understand and exploit the resonance scattering by swimbladder-bearing fish (typically in the 1-10 kHz frequency region). Exploitation of the resonances can significantly reduce ambiguities in interpreting acoustic scattering in terms of meaningful biological parameters compared with traditional higher frequency approaches and other lower frequency narrowband approaches. Objectives: To conduct a new class of quantitative acoustic studies of scattering by swimbladder-bearing fish utilizing new broadband-acoustic technology that is optimized for use in the resonance scattering region offish.
07/2010;
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Timothy K Stanton
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ABSTRACT: A general theoretical formulation is developed for echo statistics associated with a finite number of scatterers and patches of scatterers, each being randomly located in a directional sonar beam. This is the case of a direct-path sonar where there are no echoes from the seafloor and sea surface. The formulation is derived by combining the equations of Ehrenberg and colleagues (single scatterer randomly located in sonar beam) and Barakat (sum of finite number of random variables). Each scatterer or patch of scatterers is independent and can have its own echo probability density function (PDF) (before beampattern effects). Examples show that the PDF of the echo (as received by the sonar) is strongly non-Rayleigh due to beampattern effects and finite number of scatterers or patches. Numerical simulations are made for validation. [Work supported by ONR.].
The Journal of the Acoustical Society of America 03/2010; 127(3):2042. · 1.55 Impact Factor
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ABSTRACT: Broadband midfrequency acoustic measurements of fish backscattering were made with two complementary sonar systems in September 2008 at the northern slopes of Georges Bank in the Gulf of Maine. One system, towed near the surface, was downward-looking (short-range: < 0.2 km) and was used to image fish at high vertical resolution throughout the water column. The other system was horizontal-looking (relatively long-range: 1-10 km), consisting of a vertical source array and a horizontal receive array, and was used to image fish at high horizontal resolution. In this talk, the focus is on characterizing the longer-range data. Spectrogram analysis showed that the echoes had resonance frequencies in the 2-4 kHz band, consistent with scattering by Atlantic herring, while examination of normalized match-filter output revealed strong, spatiotemporally variable clutter. The associated probability-density functions (PDFs) exhibited variable non-Rayleigh behavior. Several PDF models were fit to the data, with the K and Poisson-Rayleigh PDF models providing not only good fits, but physical insights into the clutter process (such as estimates of the expected number of discrete scatterers per unit area). These results provide an improved understanding of both fish behavior and the characteristics of fish aggregations as long-range clutter fields. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2642. · 1.55 Impact Factor
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ABSTRACT: When a sonar beam sweeps across a field of scatterers, the echoes can fluctuate significantly from ping to ping. The fluctuations can be strongly non-Rayleigh because: (1) there can be a small number of scatterers in the beam; (2) the distribution of scatterers can be inhomogeneous or "patchy;" and (3) the echoes are weighted by the nonuniform response of the sonar beam. In this paper, a general formulation combining equations derived by Ehrenberg [Proc. Conf. Eng. Ocean Environ. 1, 61-64, (1972)] and Barakat [Optica Acta 21, 903-921, (1974)] is developed to account for a directional sonar beam involving an arbitrary finite number of scatterers, each with an arbitrary echo probably density function (PDF) and randomly located in the beam. Theoretical predictions are made, along with numerical simulations for validation, for a range of conditions, including: (1) different number of scatterers arbitrarily located in the beam and (2) different echo PDFs of the scatterers. Here, a "scatterer" could be an individual or a patch of scatterers whose dimensions are much smaller than the footprint of the sonar beam. Although the application is intended for volumetric patches, the formulation could be applied to areal patches under appropriate conditions. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2643. · 1.55 Impact Factor
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ABSTRACT: Squid are important organisms both ecologically and commercially. Acoustic scattering techniques can provide synoptic data on their distribution and abundance, and have the advantage of being efficient compared with traditional net sampling methods. However, knowledge of the scattering properties of squid is required to accurately convert the acoustic data into meaningful biological information. To better understand the scattering of sound by squid, and to facilitate future model development, a controlled laboratory backscattering experiment was conducted on live squid (Loligo pealeii) using broadband linear chirp signals (45-105 kHz) with data collected over the full 360 deg of orientation in the lateral plane, in 1-deg increments. The scattered spectra often showed significant structure over the frequency band available at different angles of orientation, in addition to high levels of ping-to-ping variability. Pulse compression signal processing techniques were also used, revealing the following dominant scattering features: (1) at normal incidence, the front and back interface of the animal were resolved and (2) at off-normal incidence, the anatomical features in the head region were found to dominate the scattering. This information can serve as the basis for an accurate acoustic scattering model for squid.
The Journal of the Acoustical Society of America 05/2009; 125(4):2550. · 1.55 Impact Factor
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ABSTRACT: 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.
The Journal of the Acoustical Society of America 02/2009; 125(1):73-88. · 1.55 Impact Factor
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ABSTRACT: When calibrating a broadband active acoustic system with a single standard target such as a sphere, the inherent resonances associated with the scattering by the sphere pose a significant challenge. In this paper, a method is developed which completely eliminates the source of resonances through isolating and exploiting the echo from the front interface of a sphere. This echo is relatively insensitive to frequency over a wide range of frequencies, lacking resonances, and is relatively insensitive to small changes in material properties and, in the case of spherical shells, shell thickness. The research builds upon the concept of using this echo for calibration in the work of Dragonette et al. [J. Acoust. Soc. Am. 69, 1186-1189 (1981)]. This current work generalizes that of Dragonette by (1) incorporating a pulse compression technique to significantly improve the ability to resolve the echo, and (2) rigorously accounting for the scattering physics of the echo so that the technique is applicable over a wide range of frequencies and material properties of the sphere. The utility of the new approach is illustrated through application to data collected at sea with an air-filled aluminum spherical shell and long broadband chirp signals (30-105 kHz).
The Journal of the Acoustical Society of America 07/2008; 124(1):128-36. · 1.55 Impact Factor
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ABSTRACT: Predicting acoustic scattering by deformed elongated objects remains a challenge as there are no exact analytical solutions. Numerical solutions are also challenged through various computational issues. In this research, we have developed and applied an approximate analytical approach for predicting scattering by various deformed elongated objects, including bent or rough finite cylinders and deformed edges. The formulation is based on a line integral in which the scattering or diffraction per unit length of the exact solution to an infinitely long, straight undeformed object is integrated along the length of the finite-length deformed object, accounting for variations along the length of phase and certain local properties of the object. Predictions concerning various types of deformed finite cylinders are first discussed (including Stanton, J. Acoust. Soc. Am. 86, 691-705 (1989) and subsequent papers), followed by recent predictions concerning deformed finite edges (Stanton et al., J. Acoust. Soc. Am. 122, 3167-3176 (2007)). These latter predictions using the approximate line integral compare favorably with predictions based on the T-matrix numerical method, which is formally exact. Also discussed are: dependences upon length, orientation, bend, and roughness; comparisons with experimental data concerning machined objects and marine life; and range of validity of the approach.
The Journal of the Acoustical Society of America 06/2008; 123(5):3896. · 1.55 Impact Factor
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ABSTRACT: 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.
The Journal of the Acoustical Society of America 06/2008; 123(5):3105. · 1.55 Impact Factor
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ABSTRACT: When calibrating a broadband active acoustic system with a single standard target such as a sphere, the inherent resonances associated with the scattering by the sphere pose a significant challenge. In this research, a method is developed which completely eliminates the source of resonances through isolating and exploiting the echo from the front interface of a sphere. This echo is relatively insensitive to frequency over a wide range of frequencies, lacking resonances, and relatively insensitive to small changes in material properties and, in the case of spherical shells, shell thickness. The research builds upon the concept of using this echo for calibration in the work of Dragonette et al. (J. Acoust. Soc. Am. 69, 1186-1189 (1981)). This current work generalizes that of Dragonette by 1) incorporating a pulse compression technique to significantly improve the ability to resolve the echo, and 2) rigorously accounting for the scattering physics of the echo so that the technique is applicable over a wide range of frequencies and material properties of the sphere. The utility of the new approach is illustrated through application to data collected at sea with an air-filled aluminum spherical shell and long broadband chirp signals (30-105 kHz).
The Journal of the Acoustical Society of America 06/2008; 123(5):3348. · 1.55 Impact Factor
