The horizontal target strength (TS) of fish is necessary to estimate the abundance of fish near the sea surface using a quantitative omnidirectional sonar. In this study, we measured the TS of Pacific chub mackerel by varying the yaw, pitch, and sonar tilt angles. We also estimated the theoretical TS using the prolate spheroidal model (PSM) to understand the horizontal TS of fish. The results revealed that the theoretical TS was very close to the measured TS for variation with respect to angle, indicating that PSM is a suitable model for Pacific chub mackerel. The measurements and theoretical prediction revealed that yaw angle distribution was the most important parameter for horizontal TS of Pacific chub mackerel. The influence of pitch angle and sonar tilt angle on horizontal TS was slight. Therefore, it is necessary to make the yaw angle distribution uniform to estimate average TS accurately during sonar surveys.

For construction machines operating underwater, the use of positioning technology is necessary to ensure safety and efficiency. However, underwater acoustic positioning systems often encounter significant errors in multipath environments, resulting from numerous signal reflections. To address this problem, we have developed a new underwater acoustic positioning technique with signal filtering technology to eliminate unnecessary reflected waves. In this technique, the measurement field is divided into meshs, and the propagation time from the sound source (position: unknown) in each mesh to each receiver (position: known) is calculated on a computer and stored as a database. This database is then compared with the signals actually observed in the field to determine the location of the sound source on a mesh scale, and only the time-of-flight of the direct wave is selected for accurate positioning. In this paper, we perform a trial experiment of the new positioning system mounted on an underwater backhoe in an actual harbor area in Miyakojima, Okinawa. In experiments, the positioning error and missing rate were about 0.2 m and almost 0%, respectively, when there is a direct line of sight between the sound source and hydrophones. The obtained results suggest that the proposed underwater acoustic positioning system can become a viable solution to navigate construction machines operating in shallow water.

An echo sounder employing acoustic cavitation noise as sound source was developed with the aim of realizing a broadband pressure reflection coefficient measurement. This study sought to verify the feasibility of broadband pressure reflection coefficient measurement with this echo sounder. The developed echo sounder used an off-axis parabolic acoustic mirror to shape the spherical shockwaves generated under the ultrasonic horn into plane shockwaves. The plane shockwave irradiated the target. The shockwave was reference-measured on the mirror, and the target range was obtained from the cross-correlation function between the reference signal and the echo. A frequency spectrum of the cross-correlation function was used to determine the pressure reflection coefficient of the target. The results of the verification experiment using an acrylic plate as the target showed that this echo sounder could accurately measure the range. It was also shown that the sound pressure reflection coefficient of the acrylic plate could be measured successfully in the frequency range from 20 kHz to 5 MHz. These results indicate that the echo sounder using acoustic cavitation noise can measure the broadband pressure reflection coefficient, but the current echo sounder setup still has the problem of low correlation between the reference signal and echoes.

Oceanic vertical sound speed profiles are important for acoustic measurement and acoustic communication, as sound propagation depends on variations in sound speed, particularly during long-distance propagation. XBT (expendable bathythermography) is often used to measure vertical temperature profiles because of its advantages of low cost and ease of operation. As a sound speed profile is calculated from this temperature profile and statistical salinity, there is necessarily some discrepancy between it and the real sound profile because of the lack of real salinity data. Although the use of a CTD (conductivity, temperature, and depth) sensor combined with a sound velocity meter provides a more accurate sound speed profile, it entails the time-consuming process of deploying and retrieving a lengthy cable. In this paper, the authors propose a concept of an expendable, low-cost sound velocity meter like XBT using Doppler shift. In the proposed method, sound speed near the probe can be estimated from its descent speed and the Doppler shift received at the sea surface from the source in the probe. A 1D model of this method was performed numerically, and a tank experiment showed good agreement between the sound speed estimation from Doppler shift and the conventional sound speed calculation from CTD.