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

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.

Single mode transmission and acoustic backscattering measurements in a laboratory waveguide

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.

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

The reflections of transient pressure signals from hard (perfectly reflecting) finite width plane facets have been studied theoretically and experimentally. The experiments and theory used point sources, and sound pressures were observed in the specular direction. Theoretical solutions used the Helmholtz–Kirchhoff integral and incident spherical waves in the Fresnel approximation. The results of these laboratory experiments are compared to numerical evaluations of the integrals. Applications to acoustical oceanography and to architectural acoustics are: the specular reflection of waves from a reflecting facet on the seafloor and the specular reflection of sound waves from sound deflectors above a performer on a stage.

Specular reflections of transient pressures from finite width plane facets

Reflection, diffraction, and higher order diffractions from a finite width wedge construction on an effectively infinite plane were studied experimentally and theoretically. The construction consisted of a 12.6‐cm‐width angle iron on dry wall board or sheet rock. The experiments used a spark as a 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 experimental geometry is in the reflections and is different [Medwin et al., J. Acoust. Soc. Am. 72, 1005–1013 (1982)] transmission and shadow geometry. Medwin et al.’s Huygens based constructions were used to compute the double diffraction events.

Clarence S. Clay

Papers

Single mode transmission and acoustic backscattering measurements in a laboratory waveguide

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

Specular reflections of transient pressures from finite width plane facets

Time domain reflections and diffractions from facet‐wedge constructions: Acoustic experiments including double diffractions

Ocean Acoustics: Theory and Experiment in Underwater Acoustics