Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (“ground truth”) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (〈u′2〉, 〈w′2〉) and covariance (〈u′w′〉) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measure...

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Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters. Mesopelagic fishes dominate the global fishes biomass, yet there exist major uncertainties regarding their global biomass. Irigoien et al.analyse acoustic data collected during a circumglobal cruise and show that biomass estimates should be raised by an order of magnitude.

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Large mesopelagic fishes biomass and trophic efficiency in the open ocean

In situ measurements of fish target strength are selected for use in echo integrator surveys at 38 kHz. The results are expressed through equations in which the mean target strength TS is regressed on the mean fish length l in centimeters. For physoclists, TS=20 log l−67.4, and for clupeoids, TS=20 log l−71.9. These equations are supported by independent measurements on tethered, caged, and freely aggregating fish and by theoretical computations based on the swimbladder form. Causes of data variability are attributed to differences in species, behavior, and, possibly, swimbladder state.

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Fish target strengths for use in echo integrator surveys

The origin of computational and numericalacoustics coincides with the emergence of theoretical physics [1] as an intellectual endeavor. Pythagoras developed the theory of the (Western) musical scale in terms of a device called a monochord in which adjacent consonant notes of the musical scale were obtained by plucking two string segments whose relative lengths were ratios of the small integers 1, 2, and 3. He recognized that the lengths of these strings were inversely proportional to the frequency of sound generated when plucked. Since that time, computational methods in acoustics have expanded to use more numbers than these first three integers. Mersenne [2] in the seventeenth century added the irrationals as a numerical tool when he determined that the frequency of a vibrating string was proportional to the square root of its cross-sectional area. He further added to the quantitative tradition of acoustics with conclusions such as: “The velocity of sound is greater than the velocity of cannon balls and equals 230 six-foot intervals per second.” Although the former statement is also probably true for sound propagating in water, Mersenne’s contributions to the understanding of underwater acoustics are suspect judging from his speculation that sound travels more slowly in water than air because the density of water is greater than air.

Fundamentals of Ocean Acoustics

Seismic reflection studies recently conducted on ice stream B, part of the marine ice sheet of West Antarctica, show a metres-thick layer immediately beneath the ice in which both compressional (P) and shear (S) wave speeds are very low. These low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure. From this, and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice, we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation.

Seismic measurements reveal a saturated porous layer beneath an active Antarctic ice stream

Acoustical Oceanography : Principles and Applications

Ocean Acoustics: Theory and Experiment in Underwater Sound

The signal processing techniques for acoustically estimating fish abundance are echo counting and echo integration. The transducer beam pattern complicates these estimates. We (1) statistically model the effects of the beam pattern on scattered signals, (2) compare acoustic data to this model, and then (3) for echo from individual fish, calculate estimates of fish density and scattering function based on these comparisons. At low densities we determine the proportionality constant relating the average echo integral and fish densities. Then at high densities, this proportionality is used to estimate fish density. The theory is compared to experimental data for alewife in Lake Michigan, U.S.A.

Acoustic estimates of fish density and scattering function

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...

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

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...

Clarence S. Clay

Papers

Acoustical Oceanography : Principles and Applications

Ocean Acoustics: Theory and Experiment in Underwater Sound

Acoustic estimates of fish density and scattering function

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

Backscattering cross sections of live fish: PDF and aspect