Sound scattering from oceanic turbulence




Ross, Tetjana

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Co-located measurements of acoustic backscatter and temperature/velocity microstructure are used to confirm theoretical predictions of sound scatter from oceanic turbulence. The data were collected with a torpedo-shaped vehicle carrying four shear probes and two thermistors on its nose, and forward-looking 44.7 and 307 kilohertz echosounders (mounted 20 centimetres below the turbulence sensors). The vehicle was towed through the stratified turbulence that forms tidally over the lee side of a sill in a British Columbia fjord. Conventional downward-looking echosounder measurements were also made with a 100 kilohertz sounder mounted in the ship’s hull. Populations of amphipods, euphausiids, copepods and gastropods were present in the fjord (sampled with 335-micrometre mesh vertical net-hauls) and could be seen in the sounder data. These plankton net-hauls indicated that there were too few zooplankton in the turbulent regions to account for the scattering intensity. At both 44.7 and 307 kilohertz, scatter that is unambiguously correlated with turbulence was observed. Turbulent scatter is much stronger at the higher frequency, illustrating the mportance of salinity microstructure—long neglected in turbulent scattering models—and shedding some light on the form of the turbulent temperature-salinity co-spectrum. The turbulent temperature-salinity co-spectrum has never been measured directly. Although several models have been proposed for the form of the co-spectrum, they all produce unsatisfactory results when applied to the turbulent scattering equations (either predicting negative scattering cross-sections in some density regimes or predicting implausible levels of correlation between temperature and salinity at some scales). A new co-spectrum model is proposed and shown to be not only physically plausible in all density regimes, but also in reasonable agreement with the scattering data. At 307 kilohertz, the backscatter is mostly from salinity microstructure and, depending on the strength of the stratification, can be as strong as—or stronger than—the signal from a zooplankton scattering layer. This could easily confound zooplankton biomass estimates in turbulent regions. The two targets’ different natures (discrete targets versus a volume effect) often allow them to be distinguished even when they occur simultaneously. The key is sampling the same targets at multiple ranges. At long-range, discrete targets have a constant volume scattering strength proportional to their number density. The sampling volume, however, decreases as the targets approach the sounder. At some range there will be only one (or no) target in the sampling volume and the volume scattering strength will increase (or disappear) as the target continues to near the sounder. Turbulence, as a volume scattering effect, has no range dependence to its volume scattering strength. Thus, by examining the scattering nature at close range we can distinguish discrete targets (like zooplankton) from turbulence.



Acoustics, Volume scattering, Oceanography, Turbulence, Shear, Zooplankton, Knight Inlet (B.C.)