Acoustics, small-scale physics and bio-physics in the ocean

Acoustics opens a window revealing an underwater world. Light cannot reach the oceans' depths, so often the only way to "see" something at a distance is through the observation of reflected sound waves rather than light waves. For nearly three quarters of a century oceanographers have used sonar technology to learn about the ocean interior. This powerful tool does, however, have some limitations. It is difficult to distinguish between different sources of scatter. This makes it tricky to apply quantitative data analysis to determine concentrations of fish or plankton, or the amount of mixing between interacting ocean currents. These are not purely academic goals, as acoustic surveys are used to help make predictions of fish and zooplankton stocks and thus impact marine policy decisions. Being able to quantitatively measure the mixing of ocean currents using acoustics will dramatically increase our ability to make this type of measurement; the dearth of mixing information is one of the major factors hampering the reliability of climate and climate change models.The work proposed here is multifaceted. The crux of it, however, is that we will study how small-scale oceanic fluid flows interact with sound and zooplankton with the aim of improving acoustical surveying in the ocean. We will employ innovative techniques, such as broadband acoustics (akin to moving from "black-and-white" to "full-colour" acoustics) to help discriminate sources of sound scatter and a brand-new type of oceanographic profiler able to make coincident video observations of zooplankton and measurements of the small-scale flow field to determine whether plankton behaviour affects their bio-acoustical signature.