There are many important maritime facilities and ships including petroleum tankers operating along the Japanese coasts that could become the targets of terrorism. In addition, Japan has been suffering from continual crimes such as illegal dumping and smuggling in the water spaces. For the purpose of preventing terrorism and regulating underwater crimes, we have conducted a 3-years study, begun in 2005, entitled "Development of an Underwater Security Sonar System", based on which an integrated underwater monitoring system has been realized, with the support of the Special Coordination Funds for the Promotion of Science and Technology of MEXT. In this paper, bearing practical implementation of this system in the actual monitored sea areas in mind, optimum methods of underwater acoustic image monitoring are considered. Furthermore, to develop a more effective monitoring system, we explored monitoring techniques that involve putting together information from infrared cameras, marine radars or AIS, and tried to develop a complex display monitoring technique in which the sonar images are simultaneously projected together with map information. We confirmed the effectiveness of this system through various operational tests and demonstration experiments in the actual sea area.

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In port areas, a wide-angle underwater imaging system has been in strong demand for wide-range inspections of undersea structures for periodic maintenance and for rapid checks in case of disasters. However, seawater in such areas is often turbid due to urban activities, which makes underwater imaging more difficult. We developed a three-dimensional imaging sonar system (hereinafter called System 08) for surveying of portside underwater constructions based on a prototype named System 07, which was for off-line viewing of 3-D images made in tank experiments. System 08 could be printed on-line 3-D acoustic image at 1/8 screen/0.125 sec. and surveyed height of a slope within ± 5 % or less in water tank measurement.

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In the sound propagation in shallow water, the influence of an internal wave is worthy of attention. We investigated the influence of the shallow water internal wave on the sound propagation on the continental shelf. The internal wave was assumed to be 100 m wide, with an amplitude of 22.5 m. The relation between the position of the internal wave and the turning ray was examined at 50 m in sound source depth in the thermocline. Because the turning position and the internal wave overlap, a strong scattering is caused at the rear and underside of the internal wave. Moreover, the width of the influence to the turning position is ±400 m regardless of the position of the internal wave. We also determined that the influence increases according to the extent of the internal wave width.

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For underwater port surveillance, it is important to develop a technique that can automatically detect slow-moving targets such as divers. It is difficult to detect such targets since their echoes are masked by undesirable signals such as random noises, reverberations, wake bubbles, echoes of static objects, and crosstalk noises generated by multibeam processing. To detect moving targets effectively, a signal processing method that can eliminate only these undesirable signals is required. However, conventional methods that deal with only signal amplitude cannot remove these undesirable signals easily since random temporal fluctuations of amplitudes are incorrectly identified as moving targets, leading to many false detections. We propose a new signal detection method that uses an interferometric technique that can effectively eliminate only the undesirable signals, and detect moving targets. The proposed method deals with temporal fluctuations of phase difference measured by the split-beam method independently from amplitude-based methods. The spatiotemporal fluctuation of phase difference is closely related to the movement of a target, and it is possible to detect moving targets effectively by evaluating the spatiotemporal variance of the phase difference. In this study, the authors developed an algorithm for the proposed target detection method. By applying the proposed method to experimental data, we have confirmed that it has high detection performance compared to conventional methods.

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In order to improve the accuracy of abundance estimation of Japanese anchovy (Engraulis japonicus) by acoustic surveys using a quantitative echosounder, the shape of the swimbladder and the relationship between the target strength (TS) and the body length L_b were investigated. Specimens were soaked in formalin solution just before the experiments to prevent the gas in the swimbladder from leaking out through the anus. The body length ranged from 5.6 to 11.2 cm (44 specimens). The body length was measured from the anterior margin of the snout to the posterior margin of the scales. The swimbladder lengths of the Japanese anchovy obtained from the soft X-ray images were 1.4 to 1.9 times longer than those of walleye pollock given the same body length. The TS patterns according to the orientation angle of the body at 38 kHz were measured in a freshwater tank after the soft X-ray imaging and were in good agreement with the model predictions. The TS-L_b relationship, TS = 20 log₁₀ L_b - 64.0, was obtained by averaging the TS patterns over the orientation distribution (mean = 0° and s.d. = 10°). Also the depth-dependent TS according to Boyle's law was TS = 20 log₁₀ L_b - (20/3) log₁₀ (1 + z/10) - 63.4, where z represents the depth.

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