The horizontal acoustic backscattering strength of a fish school is an important factor for fisheries and for estimation of the size of a fish school using omnidirectional scanning sonar. However, the backscattering strength of a fish school is influenced by their behavior and orientation because the individual fish in the fish school have strong directivity of target strength. This paper discusses a method for estimating the directivity of the backscattering strength of the fish school by analyzing fish school behavior. Sonar data were collected by a purse seine fishing boat equipped with the quantitative scanning sonar FSV-30R in Barents Sea and on the Pacific coast of northern Japan. Swimming speed, swimming direction, volume backscattering strength (SV_SCH), and the target strength of the fish school (NTS) were analyzed for several swimming fish schools. The results showed strong directivity in the backscattering strength at the direction perpendicular to the swimming course; this result is similar to that of the directivity of the target strength. The directivity varied in relation to the distribution of the orientation of the fish, resulting from both the swimming speed and the direction of the fish school. These findings will enable the estimation of fish school abundance more precisely.
To apply multilayer piezoelectric actuators to broadband backscattering measurements of aquatic animals, we built a Langevin-type transducer using 10-mm-long actuators. The useful frequency band was 20–150 kHz, and the main lobes of the beam patterns were almost identical to those of an ideal circular piston source with a diameter of 110 mm and the beamwidths of 21.2° at 38 kHz and 6.6° at 120 kHz. The transducer and commercially available equipment were integrated as an echo-sounding system. A linear frequency modulated signal (20–150 kHz) was employed as the transmitted signal. The system was calibrated using a 20.6-mm-diameter tungsten carbide sphere. The broadband backscattering measurement method was verified using a 38.1-mm-diameter tungsten carbide sphere. The backscattering measurements of three aquatic animals were conducted: a swimbladdered fish, a decapod shrimp, and a benthic gastropod. The target strength (TS) spectra specific to the species were obtained and those of the swimbladdered fish and the decapod shrimp were reasonably consistent with the predicted TS spectra based on theoretical scattering models. That of the benthic gastropod was different from those of the other two species. The Langevin-type transducer using the multilayer piezoelectric actuators is applicable to the broadband backscattering measurements of aquatic animals.
The target strength (TS) spectrum of a marine organism is a key factor in the target discrimination of that organism. In this study, the side-aspect TS spectrum of Euphausia pacifica was measured as a function of the incident angle of the ensonified wave in a seawater tank using a broadband transducer with a bandwidth of 300–700 kHz. The measurements were compared with the theoretical predictions by the distorted-wave Born approximation-based deformed-cylinder model (DWBA model). The TS pattern measurements were in good agreement with the DWBA model predictions for the main lobe but not with those for the side lobe. The measurements of the relationship between the normalized mean TS spectrum and the total length divided by wavelength ratio were in good agreement with model predictions.
In studies of mass and heat transfers in the ocean, it is very important to determine the seafloor hydrothermal flows in mid-oceanic ridges. However, whole images of such hydrothermal flows and their internal structures are not easily captured or evaluated, and there is no established method for doing so. Here we report our development of a method for observing and delineating seafloor hydrothermal flows with an acoustic method. The proposed observation system consists of an acoustic video camera named “DIDSON,” a concentrator lens (1°), and a pan-and-tilt mechanism. DIDSON is an acoustic lens-based sonar that can substitute for an optical system in turbid or dark water where optical systems fail. The pan-and-tilt mechanism rotates the DIDSON with its concentrator lens to scan the hydrothermal flows discharging from the seafloor. We conducted a tank experiment to evaluate the utility of the proposed system for seafloor hydrothermal activity observation. We found that the system was able to delineate the image of an artificial seafloor hydrothermal flow; that is, a heated water flow that was pumped into the experimental tank through a narrow silicon tube. This result indicated that the proposed system will be of practical use.
A Fabry–Perot interferometer with chirped fiber Bragg gratings (CFBG-FPI) as reflectors has hundreds of resonance peeks in the transmittance spectrum and can be utilized as a sensor based on the dependency of the peak wavelengths on the influence applied to the fiber. Because the width of the peak is narrow, sensitive measurement can be achieved. In addition, one may expect a large dynamic range using multiple peaks. A CFBG-FPI sensor can achieve responsibility as high as a sensor based on a Fabry–Perot interferometer with fiber Bragg gratings and has the option to expand the dynamic range. In this paper, an application of a CFBG-FPI sensor is demonstrated experientially.