Acoustic scattering by benthic and planktonic shelled animals

Abstract

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.

Estimating Gulf of Maine zooplankton distributions using multiple frequency acoustic, video and environmental data

“…2-9), which could be explained by Lamb wave speeds varying due to changes in shell thickness (Stanton et al, 2000a). However, there is a sharp peak in the distribution at 18.5 p,sec.…”

“…Both of these animals have been successfully modeled with ray-based methods previously (Stanton et aL., 1998b(Stanton et aL., , 2000a. Ray-based methods are an approximation that is valid only in the geometrical scattering region (ka 2: 1), where k is the acoustic wavenumber and a is a characteristic size of the animal, generally its radius.…”

Acoustic Strength of Green Turtle and Fish Based on FFT Analysis

“…The scattering process of the animals was observed to be quite complex as the echoes were strongly dependent upon both frequency and angle of orientation [25] [26].…”

Broadband acoustic backscatter and high-resolution morphology of fish: Measurement and modeling

“…The experimental setup, similar in concept to that described in Stanton et al (2000), included the use of a power amplifier, signal generator, a pair of transducers, pre-amplifier, band-pass filter, oscilloscope and personal computer. All of these components, except for these particular transducers, were used in the experiment described in Stanton et al (2000). During the measurements, the individual fish were secured in an acousti-cally transparent harness in the center of the acoustic beam and rotated.…”