AST explores theoretical sound scattering from marine organisms to predict target strength versus acoustic frequency, and animal size, shape, orientation, and morphology. For example, the Stochastic Distorted Wave Born Approximation model, recently developed and validated by AST, has highlighted the importance of knowing the orientation distribution of animals beneath a survey vessel, and indicates there may be more krill in the Scotia Sea than previously thought.
Sound Scatter Measurements In-situ
As sound scattering depends on the acoustic frequency, the morphology of the animal, as well as its size, shape, and orientation, and these parameters are frequently ill-defined, in-situ measurements of TS may provide the best estimates. AST has developed a novel technique for improving the accuracy and precision of in-situ target strength measurements, which also provides measurements of sound scatter from the individual targets. These multiple frequency target strength measurements can reduce error in both target identification and target strength estimation.
Sound Scatter Measurements Ex-situ
Even using multi-frequency techniques, in-situ target strength measurements of densely aggregated animals is not possible. Therefore, AST has refined and extended a new technique for measuring broad bandwidth sound scatter of live animals in highly reverberant tanks in a laboratory or aboard a boat. The data are used to validate scattering models for harvested and cohabitant species. The models are used to improve acoustical identification of species and sizes, and improve estimates of target strengths – thus improving the accuracy and precision of survey estimates. AST has already used this technique to measure the sound scattering spectra of many species such as anchovy and sardine, Antarctic krill, northern krill, and bocaccio rockfish.
The new multi-scattering technique can also be used in aquaculture to measure the number of fish in a tank, their growth rates, and their behaviors. Moreover, the method was developed by AST to measure absolute absorption cross-sections over a broad bandwidth, as highlighted in popular articles from Science News (http://www.sciencenews.org/20031115/fob3.asp).