In Situ Measurements of Sound Speed and Density of Live Zooplankton

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Applications of acoustic techniques to marine biological surveys provide a more efficient way to map the spatial distribution, abundance and temporal variation of marine organisms in the ocean compared with traditional sampling techniques involving pumps and nets. Since acoustical methods are remote sensing techniques, they are only indirect measurements. In order to correctly infer useful biological information from raw acoustic data, reliable, robust, and accurate acoustic models are required. To date, various acoustical models have been developed and much progress has been made both in laboratory experiments and field applications. To accurately interpret acoustic data, no matter what model is chosen and whether the model is simple or sophisticated, one needs to select appropriate acoustic properties of the biological scatterers, such as sound speed and density contrasts ( and ). We have found that even using the same model, the target strength or volume scattering strength can vary as much as 6 dB or a 2 fold bias for a 2% variation in sound speed and density contrasts. Differences in target strength predictions due to biases of sound speed contrast, , and/or density contrast, , could be as large as or greater than those due to the choices of different acoustic models. In addition, variations in and result in not only different scattering levels, but also different scattering patterns as a function of frequency and orientation angle. We propose to develop a reliable and robust method to conduct in situ measurements of acoustic properties co-registered with the acoustic data, i.e., to provide time varying and corresponding to the actual positions of the animal where the acoustic data are collected. A laboratory version of an in situ measuring system, Acoustic Properties Of Plankton (APOP), will be developed and evaluated. The proposed method involves measurement of the backscattering from a water-zooplankton mixture. By using a broadband backscattering acoustical setup, the frequency response due to the acoustical interferences from two interfaces bounding the mixture layer is directly related to the effective sound speed and density contrasts ( and ) in the mixture. With the assistant of conductivity measurement which provides volume fraction of the zooplankton and the use of the theory of `effective medium`, the sound speed and density contrasts ( and ) in the zooplankton can be inferred. The proposed research is divided into two phases: (1) initial laboratory development of the measuring system and preliminary system performance evaluation, and (2) more extensive laboratory measurements using a variety of different zooplankton species to test the robustness and consistency of the system performance, and some refinement of the system. The effort of phase two is very important and necessary to achieve a much smoother transition from the laboratory version to the one that will be used for the actual field applications. Throughout the proposed experiments, error analysis will be conducted to guide the experiments. In addition, the concept of the `effective medium` based on different theoretical considerations will be examined thoroughly.