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.

/pdf/large-mesopelagic-fishes-biomass-and-trophic-efficiency-in-1q1fmxkuue.pdf

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.

/pdf/fish-target-strengths-for-use-in-echo-integrator-surveys-2d56ahswnw.pdf

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

Underwater sound transmission and SI units [43.05.Ky]

The combination of time domain source imaging techniques and arrays to consider the localization of an impulsive source in a wedge waveguide, with a free surface and rigid bottom, is investigated. An impulsive signal is transmitted from an unknown location, received by an array of hydrophones, and stored. The signals are time reversed and transmitted into the model wedge by a transducer at each receiver location. Model transmission functions are used to compute the pressures at a set of locations. The field calculations use an image construction from D. Chu’s exact solution for a density contrast wedge [J. Acoust. Soc. Am. 86, 1883–1896 (1989)]. A map of the peak pressures gives an image of the source location. Particular attention is given to the peak amplitudes, sidelobe amplitudes, and spatial resolution as a function of the number and placement of receivers. Source localization is enhanced by the range dependency of the environment, which eliminates ‘‘range’’ sidelobes, even for a single hydrophone. Adding receivers along an arc eliminates ‘‘angle’’ sidelobes; but adding receivers along an arc or radial gives little reduction in overall background level. Adding receivers parallel to the wedge axis gives sharp, unambiguous azimuthal imaging.

Broadband source imaging in a shallow water wedge by an array of receivers

Coherent and Incoherent Components of Sound Scattered at a Time Dependent Rough Surface

The higher‐order diffractions of transient pressure signal from a combination of hard finite facet wedge of 45 deg and an infinite plane have been studied experimentally and theoretically. Experiments used a spark point source with a separated receiver at the same side of the wedge. The reflected pressure signals were well separated in time domain. Higher‐order diffractions from the apex and troughs of the wedge were observed. The studies were concentrated on the reflections from the facets and the diffractions. The experimental geometry is different from Medwin et al. [J. Acoust. Soc. Am. 83, 1794–1803 (1988)]. Biot and Tolstoy’s exact wedge solution [J. Acoust. Soc. Am. 29, 381 (1957)] and extensions were used to compare experimental data and theory.

Clarence S. Clay

Papers

Underwater sound transmission and SI units [43.05.Ky]

Broadband source imaging in a shallow water wedge by an array of receivers

Coherent and Incoherent Components of Sound Scattered at a Time Dependent Rough Surface

Reflections and diffractions from facet‐wedge constructions: Acoustic experiments and theory