Showing papers by "Andone C. Lavery published in 2003"

High-frequency acoustic scattering from turbulent oceanic microstructure : the importance of density fluctuations

Abstract: Acoustic scattering techniques provide a unique and powerful tool to remotely investigate the physical properties of the ocean interior over large spatial and temporal scales. With high-frequency acoustic scattering it is possible to probe physical processes that occur at the microstructure scale, spanning submillimeter to centimeter scale processes. An acoustic scattering model for turbulent oceanic microstructure is presented in which the current theory, which only accounts for fluctuations in the sound speed, has been extended to include fluctuations in the density as well. The inclusion of density fluctuations results in an expression for the scattering cross section per unit volume, σv, that is explicitly dependent on the scattering angle. By relating the variability in the density and sound speed to random fluctuations in oceanic temperature and salinity, σv has been expressed in terms of the temperature and salinity wave number spectra, and the temperature-salinity co-spectrum. A Batchelor spectrum...

Distributions of physonect siphonulae in the Gulf of Maine and their potential as important sources of acoustic scattering

Abstract: The distributions of siphonulae stages of physonect siphonophores were mapped in Wilkinson, Jordan, and Georges basins of the Gulf of Maine using a video plankton recorder. Siphonulae are often overlooked in net samples and our optical survey appears to be the first in situ investigation of these organisms. Siphonulae were distributed at mid-depths in narrow horizontal layers, suggesting potential control of their buoyancy. The siphonulae possessed gas- filled pneumatophores with diameters of 0.1-0.4 mm. Pneumatophore diameters appeared to be similar over their entire sampled depth range, suggesting that siphonulae may be capable of regulating the pressure of gas within the pneumatophore to maintain a constant volume. The dimensions of the siphonulae pneumatophores placed them near the acoustic resonance region for scattering at 43 kHz when near the surface and 120 kHz when at depth. Theoretical esti- mates of the acoustical target strengths of gas bubbles of sizes corresponding to the sizes of our measured pneumato - phores produced relatively strong backscatter. Layers of siphonulae corresponded to regions of high acoustical backscatter at 120, 200, and 420 kHz. Resume : Un systeme d'enregistrement video du plancton nous a permis de decrire les repartitions des stades siphonu-

Thermally enhanced neutralization in hyperthermal energy ion scattering.

Abstract: Neutralization probabilities are presented for hyperthermal energy $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated.

Inference of geometrical and behavioural parameters of individual fish from echo-trace-analysis

Abstract: The volume scattering strength of swim-bladdered fish is very sensitive to fish size, shape, and orientation. However, the appropriate geometrical and behavioral parameters necessary to determine these parameters are not always available. Inadequate knowledge of these parameters limits our ability to correctly estimate lengths and abundances of fish with acoustic technology. A method for extracting geometrical and behavioral parameters of individual fish using a scattering model-based echo-trace-analysis (ETA) is presented. The scattering model used in the ETA is based on the Kirchhoff approximation and a prolate spheroid is used to approximate the shape of the swimbladder. The geometric shape of a resolved echo from an individual fish is used in a least-squares algorithm to infer information about the swimming speed, direction, and orientation of the fish. The acoustic intensity distribution of an individual fish along a resolved echo trace is used in a least-squares algorithm to infer information about the size, aspect ratio, and orientation of the swimbladder. Results from the ETA were more sensitive to variations in vertical swimming speed and orientation of the swimbladder than to variations in horizontal swimming speed and aspect ratio of the swimbladder. The proposed ETA was applied to field data, a 38 kHz echogram of a school of Atlantic cod recorded with a Simrad EK500 echo sounder. The inferred sizes and orientations of a number of the resolved echo traces for individual fish were consistent with available observation data. One of the advantages of the ETA method presented here is that it can be easily extended to a dual beam, a split-beam, or a multi-beam acoustic system.

High‐frequency acoustic scattering from gas‐bearing zooplankton

Abstract: High‐frequency acoustic scattering is well‐suited to the synoptic investigation of marine organisms that inhabit the water‐column, such as zooplankton and fish. However, the scattering characteristics of the organisms can be highly complex, and one must look for ways to distinguish one type of organism from another when interpreting the received echoes. From an acoustic scattering perspective, zooplankton generally fall into one of three categories: gas‐bearing, fluid‐like, or elastic‐shelled. Scattering models, of different levels of sophistication are available for all three types of zooplankton. Gas‐bearing zooplankton, unlike the other zooplankton categories, have a strong scattering resonance, which can be exploited for their identification. Scattering from gas‐bearing zooplankton at frequencies close to the resonance frequency is much stronger than scattering from other zooplankton. Thus, in this frequency region, acoustic scatter from a small number of gas‐bearing zooplankton can overwhelm that fro...