No abstract

The acoustic scattering properties of live individual zooplankton from several gross anatomical groups have been investigated. The groups involve ͑1͒ euphausiids ͑Meganyctiphanes norvegica͒ whose bodies behave acoustically as a fluid material, ͑2͒ gastropods ͑Limacina retroversa͒ whose bodies include a hard elastic shell, and ͑3͒ siphonophores ͑Agalma okeni or elegans and Nanomia cara͒ whose bodies contain a gas inclusion ͑pneumatophore͒. The animals were collected from ocean waters off New England ͑Slope Water, Georges Bank, and the Gulf of Maine͒. The scattering properties were measured over parts or all of the frequency range 50 kHz to 1 MHz in a laboratory-style pulse-echo setup in a large tank at sea using live fresh specimens. Individual echoes as well as averages and ping-to-ping fluctuations of repeated echoes were studied. The material type of each group is shown to strongly affect both the overall echo level and pattern of the target strength versus frequency plots. In this first article of a two-part series, the dominant scattering mechanisms of the three animal types are determined principally by examining the structure of both the frequency spectra of individual broadband echoes and the compressed pulse ͑time series͒ output. Other information is also used involving the effect on overall levels due to ͑1͒ animal orientation and ͑2͒ tissue in animals having a gas inclusion ͑siphonophores͒. The results of this first paper show that ͑1͒ the euphausiids behave as weakly scattering fluid bodies and there are major contributions from at least two parts of the body to the echo ͑the number of contributions depends upon angle of orientation and shape͒, ͑2͒ the gastropods produce echoes from the front interface and possibly from a slow-traveling circumferential ͑Lamb͒ wave, and ͑3͒ the gas inclusion of the siphonophore dominates the echoes, but the tissue plays a role in the scattering and is especially important when analyzing echoes from individual animals on a ping-by-ping basis. The results of this paper serve as the basis for the development of acoustic scattering models in the companion paper ͓Stanton et al., J. Acoust. Soc. Am. 103, 236-253 ͑1998͔͒.

Acoustic backscattering measurements and associated scattering modeling were recently conducted on a type of benthic shelled animal that has a spiral form of shell ͑Littorina littorea͒. Benthic and planktonic shelled animals with this shape occur on the seafloor and in the water column, respectively, and can be a significant source of acoustic scattering in the ocean. Modeling of the scattering properties allows reverberation predictions to be made for sonar performance predictions as well as for detection and classification of animals for biological and ecological applications. The studies involved measurements over the frequency range 24 kHz to 1 MHz and all angles of orientation in as small as 1°increments. This substantial data set is quite revealing of the physics of the acoustic scattering by these complex shelled bodies and served as a basis for the modeling. Specifically, the resonance structure of the scattering was strongly dependent upon angle of orientation and could be traced to various types of rays ͑e.g., subsonic Lamb waves and rays entering the opercular opening͒. The data are analyzed in both the frequency and time domain ͑compressed pulse processing͒ so that dominant scattering mechanisms could be identified. Given the complexity of the animal body ͑irregular elastic shell with discontinuities͒, approximate scattering models are used with only the dominant scattering properties retained. Two models are applied to the data, both approximating the body as a deformed sphere: ͑1͒ an averaged form of the exact modal-series-based solution for the spherical shell, which is used to estimate the backscattering by a deformed shell averaged over all angles of orientation, and produces reasonably accurate predictions over all k 1 a esr ͑k 1 is the acoustic wave number of the surrounding water and a esr is the equivalent spherical radius of the body͒, and ͑2͒ a ray-based formula which is used to estimate the scattering at fixed angle of orientation, but only for high k 1 a esr . The ray-based model is an extension of a model recently developed for the shelled zooplankton Limacina retroversa that has a shape similar to that of the Littorina littorea but swims through the water ͓Stanton et al., J. Acoust. Soc. Am. 103, 236-253 ͑1998b͔͒. Applications of remote detection and classification of the seafloor and water column in the presence of shelled animals are discussed.

No abstract

No abstract