Broadband sonar for mapping turbulent and double-diffusive microstructure and plankton 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. Using standard narrowband sonar 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.The problem is that we have been limiting ourselves by "viewing" the ocean in black and white. Traditional narrowband sonar uses only a single acoustic frequency. This is like viewing the world in only one colour, since colour is determined by the frequencies of light that are preferentially reflected by an object. Restricting our eyes to observing only one frequency of light is like observing the world in black and white. Clearly, we have a much easier time identifying objects when we see them in full colour.For that purpose I propose to purchase a broadband sonar system. This emerging technology allows the observation of a wider frequency spectrum of acoustic scatter in the ocean. This sea-going instrumentation will be the cornerstone of my research program. It will turn the broadband acoustic methods I have been developing in the laboratory-extracting information on plankton size and variability of fluid properties from spectral data-into useful oceanographic measurements. My colleagues at Dalhousie and elsewhere in Canada are keen to use this novel instrument. This broadband sonar will give colour to oceanographic research in Canada.