Many kinds of hydroacoustic devices are used in the field of ocean development of resources and observations. The effects of sound reflection from the sea surface are often evaluated in estimating the performance of these devices and in analyzing acoustic data obtained in actual seas. When sea surface waves are sufficiently calm, the assumption of specular reflection of sound waves is valid for the estimation of acoustic propagation characteristics. On the other hand, actual surface waves change randomly over time because of wind and gravity, and consequently reflected sound waves also fluctuate randomly. In this study, we evaluated the variability characteristics of reflected sound waves from the sea surface by acoustic simulation using the finite-difference-time-domain method. Our results clarify the effect of the wave height and wavelength at the sea surface on the variability characteristics of reflected sound waves.
To acquire a received signal with broadband width and a high signal-to-noise ratio (SNR) by using a time reversal (TR) method for target ranging, we had developed a sensitivity-compensated amplitude- and frequency-modulated (SC-AFM) signal. The SC-AFM signal has two-order sensitivity compensation to ensure the TR reference transmitting a signal with one-order sensitivity compensation. In that previous work however, the phase distortion of sensitivity was not taken into account in the calculation of the second-order compensation. In the present study, considering the possibility that the phase distortion could unexpectedly influence the shape of the time-domain TR reference signal, we propose a phase-aware SC-AFM signal and studied its ability to enhance the transmitting energy of the TR reference signal and thus improve the accuracy of target ranging. The experimental results show that by using the phase-aware SC-AFM signal,(i) the normalized power of a received signal of its direct wave is enhanced to approx. 0.41 dB,(ii) the SNR of compressed signals for distance measurement is improved to an average of >0.4 dB, and (iii) the accuracy of distance measurement is improved by >3%. Although these quantitative results would depend on the experimental set-up and conditions, they indicate that the phase-aware SC-AFM signal’s efficiency can be expected for higher-accuracy target ranging.
In the past, the multibeam-echosounder (MBES), which performs seafloor depth measuring, detection, and hydrographic surveying, has been used only for observation targets that far exceed the error level. However, AUV and sea surface platform surveys, which have high positioning accuracy, is beginning to be used to the increasing needs for high-accuracy monitoring, e.g., hydrographic surveys, harbor surveys, seafloor detection such as shipwrecks, and exploration of resources, volcanoes, and faults. The significance of properly recognizing and evaluating errors of the MBES is increasing. In this study, we describe a method of acquiring the MBES observation data in the tank space precisely measured by the total stations to verify the accuracy. According to the method proposed in this study, it is feasible to evaluate the accuracy of a device itself on the order of cm. We also report the results of the actual experiment as follows; the ranging accuracy directly under the MBES was about 1–1.5 cm regardless of the distance and depends on the pulse length (T=30µs): the accuracy was worse than expected when the beam angle changed: the seafloor detection accuracy, which depends on the angular resolution, was about twice the footprint. The above experimental findings are not completely consistent with theoretical predictions and require further research, which contributes to the prediction of accuracy and detection performance.
It is well known that juvenile salmon tend to swim near the surface, and it is difficult to detect them using echo sounders whose beams are directed vertically downwards. Thus, an upwards looking transducer might be effective. In this case, it is vital to investigate the characteristics of ventral aspect target strength (TS). Here, we examined the relation of TS to length, as well as variations of average TS for juvenile salmon (Oncorhynchus keta). TS was predicted at four frequencies (38, 70, 120, and 200 kHz) using a prolate spheroid modal-series scattering model that described the swimbladder as a vacant prolate spheroid. The fish morphological parameters required for model calculations were obtained by digitizing soft X-ray images of fish. Model predictions were verified by TS measurements in a laboratory indoor tank. Averaged TS on different tilt-angle distribution was calculated. Normalized ventral TS by the squared standard length were predicted as -64.5 dB at 38 kHz, -65.2 dB at 70 kHz, -66.0 dB at 120 kHz, and -66.6 dB at 200 kHz, assuming normal tilt-angle distribution with a mean 0 deg and standard deviation of 20 deg. The variation of average TS decreased, and the average TS increased, with decreasing frequency. Among the four frequencies, it is advantageous to use 38 kHz for the acoustic survey of juvenile salmon due to high signal-to-noise ratio and insensitivity to the variation of tilt-angle distribution.
We constructed a portable target strength measuring system for small targets like zooplankton. An acrylic water tank (60×60×180 cm) was used for the experiment. This was a bistatic system using two transducers at 400 kHz. The target strength of two cylinders was measured and compared with a sound scattering model to confirm that the system could measure target strength exactly. The model predictions were compared to measurements, and we confirmed that precise measurements of target strength were possible. In addition, we measured the side-aspect target strength patterns of Euphausia pacifica, which inhabit many of the ocean areas around Hokkaido. We confirmed that measurement was possible in an area with a noise level of approximately −90 dB.
Passive acoustic monitoring (PAM) has been introduced to monitor rockfish populations and distributions. We conducted water tank experiments to determine whether the white-edged rockfish (Sebastes taczanowskii) is soniferous and to confirm whether its sound properties are appropriate to adopt the PAM method. We observed two types of sound production by white-edged rockfish. The pulse durations of the Type 1 and Type 2 sounds were 0.017±0.003 s and 0.071±0.034 s, respectively, and their peak frequencies were 456.0±54.9 Hz and 116.3±28.1 Hz. These results suggest that white-edged rockfish produce sounds specific to their species and that it would therefore be possible to estimate the fish population by PAM.
We measured the density and sound-speed contrasts of Euphausia pacifica off the coast of Kushiro, Hokkaido from 2010 to 2014. These experiments were carried out in May–June (2010) and September–October (between 2011 to 2014). The density contrast was measured using the density bottle method, and the sound-speed contrast was measured by the time-of-flight method using a T-tube in each year. There were no significant differences in the mean values of the measured density and sound-speed contrasts by year, and the mean density and sound-speed contrasts through all years were 1.043 and 1.040, respectively. We also estimated the dorsal aspect mean target strength (TS) using the distorted-wave Born approximation-based deformed-cylinder model (DWBA model), finding that the relationships between total length (TL) [mm] and the mean TS at each frequency were TS=60.9 log TL−177.2 at 38 kHz, TS=57.2 logTL−163.6 at 70 kHz, TS=52.9 log TL−151.5 at 120 kHz, and TS=44.2 log TL−135.5 at 200 kHz. Since there was less annual variability in the density and sound-speed contrasts during the period examined in the present study, we recommend that the acoustic survey be conducted in September or October.
Recently, there has been a worldwide movement toward the reduction of underwater noise from shipping. In Japan, research has been conducted on the behavioral reaction of whales when exposed to underwater noise from shipping in the sea around the Ogasawara islands. In shallow water such as that sea, sound propagation is affected by environmental parameters of the path between the sound source and receiver. Additionally the sound source and receiver both move continuously. Therefore, it is impossible to estimate an accurate SEL (sound exposure level) for the receiver with the commonly used equation of spherical spreading loss. The objective of our research is to examine the estimation method for accurate SEL, taking into account the influence on sound propagation of environmental parameters and movement of the sound source and receiver. We simulated the sound field at every two minutes within a circle area of the sea when a ship was navigating, and made a SEL map that integrated these sound fields. In addition, we compared the SEL maps based on the simulation and on the equation of spherical spreading loss. Finally, we illustrated the differences in SEL estimated by the two maps.
