Challenges in the acoustic quantification of marine life
Kenneth G. Foote
Woods Hole Oceanographic Institution
Woods Hole, MA 02543
The context for considering challenges in the acoustic quantification of marine life is the Census of Marine Life, with interests in the distribution, abundance, and diversity of organisms, and ultimately in understanding ecosystems. To realize the Census, the numerical density of organisms must be determined by taxon throughout regions of interest by a program of synchronous, systematic observation and sampling of a spectrum of organisms. One of the principal means of acquiring such information is active acoustics. Exemplary devices are the echo sounder, sonar, and acoustic Doppler current profiler (ADCP). Echo counting and echo integration are methods for quantifying organisms. These also provide a basis for elucidating challenges.
One class of challenges is aimed at making good acoustic measurements. In situations of high signal-to-noise ratio (SNR), the challenge is to calibrate the acoustic system as it is configured for use. Situations of marginal SNR may be remedied by noise reduction, designing transducer directivity patterns, digitizing the echo signal near the transducer, and moving the transducer to the vicinity of the target, all recognized generic challenges.
Another class of challenges is that of making good biological measurements. In particular, the acoustic data require conversion to biological measures of concentration. The challenges are to determine the backscattering cross section, or target strength, and its dependences on behavior through the orientation distribution, condition or biological state, depth, and frequency of insonification.
Exploiting acoustics with respect to biology constitutes another class of challenges. Achieving acoustic classification and bandwidth, developing models, and learning how to use midwater-column multibeam-sonar data to measure the concentration of pelagic organisms are major challenges. Determining representativity of sampling, whether by acoustics or other means, is a deliberate but vital matter that may be susceptible to acoustic investigation.
A fourth class of challenges in the present enumeration is that of integrating acoustics and optics. This exceeds the bounds of the nominal subject, but deserves mention. The potential gains from combining high-frequency echo sounders or sonars with video cameras on the same underwater platform are profound.
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