Clarence S. Clay

Acoustic models of fish: The Atlantic cod (Gadus morhua)

Acoustic fish models should represent the fish body form. The Atlantic cod were used to model the acoustic scattering function of teleost fish. The model provides a basis for choices of sonar carrier frequencies. Anesthetized live Atlantic cod ranging from 156 to 380 mm (SL) were ‘‘soft’’ x‐rayed to image inflated swimbladders and skeletal elements. Maximum body heights and widths were 0.18 and 0.13 of fish lengths. Lengths and diameters of swimbladder were approximately 0.25 and 0.05 of the fish lengths. A series of short‐length fluid‐filled cylinders were used to represent body flesh. For carrier frequencies above the breathing mode resonance, swimbladders were modeled as a series of short gas‐filled volume elements of cylinders. A Kirchhoff‐ray approximation was used to compute the high‐frequency acoustic scattering. A low mode solution for a gas‐filled cylinder was used to compute the low‐frequency ‘‘breathing mode resonance.’’ All contributions were added coherently. The scattering lengths L, or targ...

Average echoes from randomly oriented random‐length finite cylinders: Zooplankton models

By heuristically extending the previously developed ray solution [Stanton et al. J. Acoust. Soc. Am. 94, 3454–3462 (1993)] to predict the scattering by cylinders over all angles of incidence, approximate expressions are derived which describe the echo energy due to sound scattered by finite cylinders averaged over orientation and length. Both straight and bent finite length cylinders of high aspect ratio are considered over the full range of frequencies (Rayleigh through geometric scattering). The results show that for a sufficiently broad range of orientation, the average echo is largely independent of the degree of bend—that is, the results are essentially the same for both the straight and bent cylinders of various radii of curvature (provided the bend is not too great). Also, in the limit of high frequency (i.e., the acoustic wavelength is much smaller than the cross‐sectional radius of the object), the averages are independent of frequency. The resultant formulas derived herein are useful in describi...

Ray representation of sound scattering by weakly scattering deformed fluid cylinders: Simple physics and application to zooplankton

Data indicate that certain important types of marine organisms behave acoustically like weakly scattering fluid bodies (i.e., their material properties appear fluidlike and similar to those of the surrounding fluid medium). Use of this boundary condition, along with certain assumptions, allows reduction of what is a very complex scattering problem to a relatively simple, approximate ray‐based solution. Because of the diversity of this problem, the formulation is presented in two articles: this first one in which the basic physics of the scattering process is described where the incident sound wave is nearly normally incident upon a single target (i.e., the region in which the scattering amplitude is typically at or near a maximum value for the individual) and the second one [Stanton et al., J. Acoust. Soc. Am. 94, 3463–3472 (1993)] where the formulation is heuristically extended to all angles of incidence and then statistically averaged over a range of angles and target sizes to produce a collective echo ...

Composite ray‐mode approximations for backscattered sound from gas‐filled cylinders and swimbladders

The normal mode solution for sound backscattered by a finite fluid (gas)‐filled cylinder in water is replaced by a Kirchhoff approximation for ka>0.15 and the normal mode solution for ka<0.15 where a is cylinder radius and k is the wave number. The composite solution was tested for density and sound‐speed ratios g and h in the ranges of 0.0012<g<0.05 and h=0.23. The simplification and approximations have been tested over the range of 0.0005<ka<20. Expressions that include straight and bent cylinders are given. A derivation of the amplitude and phase factor for finite cylinder lengths is given. The amplitude factor is proportional to the length of the cylinder for lengths much less than the Fresnel zone diameter. At larger lengths, the amplitude factor oscillates above and below 1 as the increasing length passes through the Fresnel zones. The phase is −π/4 for very small lengths and oscillates above and below 0 as the increasing cylinder length passes through Fresnel zones. In experimental (laboratory) mea...

Backscattering cross sections of live fish: PDF and aspect

The probability density function (PDF) of the peaks of the envelopes of sonar echo from live fish were measured at beam aspect. The measurements were made at 220 kHz and in a waveguide. The fish was the common shiner (Notropis cornutus) and was about 120 mm (about 18 acoustic wavelengths) long. The PDF of the echoes was approximately Rayleigh when the fish was moving gently. The backscattering cross section equaled 4.2×10−5 m2. Transformation of the PDF’s to a target strength display in decibels displaced the maximum of the PDF to the target strength equaling 10 log10 (σbs/A0)+3 dB where σbs is the mean backscattering cross section and A0=1 m2. The target strengths of the common shiner (120 mm) and the mummechog (Fundulus heteroclitus, 100 mm) were measured as a function of aspect angle. Comparison of the experimental measurements and Love’s empirical target strengths for any aspect showed that the measured target strengths at broadside aspect were about the same and the target strengths at other aspect a...

Acoustic measurements of the anatomy of fish at 220 kHz

Many measurements of sound backscattered by fish have been made, but most of these are farfield measurements from whole fish. Some of the acoustic properties of a fish can be inferred from the dependence of scattering on aspect and removal of organs such as the swim bladder. However, farfield measurements do not give the detailed structure needed to construct a Huygen’s scatterer acoustic model of a fish. We built a 220‐kHz focused sonic scanner. The focal distance was 10 cm and the axis of the fish was placed at 10 cm from the transducer for measurements. We measured the peak amplitudes of backscatter sound along the length of yellow perch (Perca flavescens) (12–14 cm) and hog sucker (Hypenteeium nigricans) (30 cm). Repeatability of results was examined in three different yellow perch and gave a good agreement between the resulting backscattering curves. Preservation (freezing) of the fish resulted in approximately 30% reduction of the overall backscatter but did not affect the shape of the backscatterin...

Specularly scattered sound and the probability density function of a rough surface

The coherent component of specularly scattered underwater sound is sensitive to the probability density function (PDF) of displacements of the rough surface. For the specular reflection of diverging waves, the coherent component and the PDF are shown to be related by the Fourier transformation. Laboratory measurements of sound scattered at a partially shadowed nearly Gaussian model sea surface show the coherent component is much larger than would be expected for a Gaussian PDF. Fourier transformations of the measured PDF, on inclusion of a shadowing correction, gave the coherent component. Fourier transformation of the coherent component yields a surface PDF similar to the measured PDF with shadowing correction.

Distribution of peaks of 70 kHz acoustic scattering in relation to depth and temperature during day and night at the edge of the Gulf Stream—EchoFront 83

Abstract The depth and temperature distribution of acoustical scattering peaks in the near-surface frontal zone of the Gulf Stream was examined with a 70 kHz downward-looking sonar to 130 m. A peak was defined as a layer with relatively more scattering than layers just above or below it. Data were collected during the day and night in July 1983, approximately 115 km ENE of Cape Hatteras in a front with sharp thermal gradients above 130 m, and in the Gulf Stream with rather homogeneous temperatures above 130 m. “Peaks” in the front occurred more frequently both day and night above 60 m and at temperatures warmer than 15°C; in the Gulf Stream peaks occurred more frequently only above 20 m and at temperatures warmer than 24.5°C and then only during the day. The distribution of peaks was more homogeneous with depth and temperature in the Gulf Stream than in the front. Day and night distribution also differed with temperature in the front but with depth in the Gulf Stream. Acoustical scattering intensity in the peaks generally increased at night in the Gulf Stream at all depths above 130 m; in the front scattering levels appeared to increase both above and below the gradient but decrease in the gradient. Differences in the nocturnal increase in scattering between the front and Gulf Stream supports the concept of thermoclines as barriers to vertical migrants.

Single mode transmission and acoustic backscattering measurements in a laboratory waveguide

Our purpose was to make ’’farfield’’ sound scattering measurements in a limited space. We used a thick waveguide, about 35 acoustic wavelengths deep, and excited it in the first mode. The waveguide had pressure release surfaces on the top and bottom. To excite and receive in the first mode, we used a vertical array that was amplitude shaded to match the eigenfunction of the first mode. Experimental measurements verified that 90% of the energy was in the first mode. A pair of omnidirectional transducers were used as a probe transmitter and receiver to calibrate the system for scattering measurements. At 220 kHz, 24‐cm depth, and 100‐cm range, the cross section of the scattering region was about 6 cm high. At this range the first Fresnel zone was 10 cm wide. Scattering measurements of the target strength of a steel sphere, diameter 0.8 cm, gave −54±1 dB compared to −54 dB theoretical target strength.

Interpretation of Sea Beam backscatter data collected at the Laurentian fan off Nova Scotia using acoustic backscatter theory

The purpose is to use acoustic scattering theory and Sea Beam measurements to estimate seafloor roughness parameters. The Sea Beam backscatter data are from a test area in the Laurentian fan, a relatively flat region. Sidescan sonarlike images were reconstructed from the multibeam data. These images in the test area show two distinctly different types of areas (A) and (B). The backscatter model uses the Helmholtz–Kirchhoff formulation of scattering theory and correlation function C(r)=exp[−‖r/l‖n], where r is the displacement, l is the correlation length, and n is the exponent. A single rough interface model fits the backscatter data in (A). The root‐mean‐square roughness was 6–8 cm and the correlation lengths were 140–270 cm. The exponent n ranged from 0.95 to 1.5. The type (B) areas required a two‐layer model: the interfaces in the first type of areas (A) is covered by a sediment having a smoother surface. The rms roughness of the covering sediments were about 3 cm, the exponent n was nearly 2 and corre...