This study tried to determine robust detection criteria for the croaker's pulse-structure sound, using field recordings in WAV, WMA, and MP3 formats. Acoustic analysis results identified four temporal parameters and one frequency parameter as sound features. These feature quantities depended on the season of the year, not the file format. We created an automatic filter for croaker-sound detection using fixed-width ranges of these five parameters, and automatically detected sounds from identical fish-sound files, or the same files with noise. We also examined the correct detection (CD) and false alarm (FA) rates of the automatic detection. Without any noise mixing, it showed fair performance, e.g., 0% FA. Once noise was mixed, however, the CD reduced in accordance with the signal-to-noise ratio. However, the FA level remained constant (no more than 4.5%).

This paper describes a self-motion compensation technique for a near-range synthetic aperture sonar. We are developing a SBSAS (sub-bottom synthetic aperture sonar), intended for chemical ammunition exploration. Since chemical ammunition is about 20 cm diameter×70 cm length in size, and buried in the sedimentary layers, we chose a low frequency (15 kHz) and applied a synthetic aperture technique to obtain high-resolution (<15 cm). As the SBSAS is mounted on a small ship (<10 t), the platform is greatly affected by wind and waves, making it very difficult for the SBSAS to maintain constant velocity and direction as is needed for synthetic aperture processing. Furthermore, the SBSAS must keep near range in order to avoid refraction of the deposition layer. Therefore, it is strongly affected by the motion displacement of the surge. Although the conventional DPCA (displaced phase center antenna)^1)–3) technique performs self-motion compensation using its own sonar echoes, it compensates only for the sway; the surge is negligibly small for long-range SAS.

We developed a new DPCA semi self-motion compensation technique to compensate the surge for near-range SAS, and call this technique the “MP (Multi Ping) DPCA”. The results of the simulation and sea trial are reported here.

Underwater acoustic (UWA) communication is a critical technology for underwater activities. However, UWA communication systems do not always operate in an ideal state due to impedance mismatching of an UWA transducer. To cope with this problem, we propose an adaptive impedance matching system. The proposed system consists of a tuning circuit with a variable inductive load, voltage and current probes, and an adaptive controller. By measuring the impedance state of the transducer and controlling the inductive load, the proposed system is able to always maintain an optimal impedance matching state of the communication system. The performance of the proposed system was evaluated in experiments, which confirmed that the proposed system can actively optimize the impedance matching state of the communication system, and can provide efficient UWA communication.

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.