The characteristics of acoustic wave propagation in marine sediments are influenced by the temperature. It was reported that the longitudinal wave velocity in marine sediment varies with the temperature. However, the investigation using acoustic model for marine sediment has not been done in detail. Also, the study on the temperature dependence for the attenuation is not seemed to be sufficient. In this study, the temperature dependence for the longitudinal wave velocity and the attenuation of the water-saturated glass beads as the sediment model has been measured using UPER (Ultrasonic Pulse Echo Reflectometer). The measured results and the calculated results using the Biot-Stoll model are compared and investigated.
Because of the recent advances of the SAS research in the world, the coherency of sea bottom reverberation seems to become the key factor to realize the practical SAS motion compensation. By utilizing the principle of DPC, both the time and spatial coherency of bottom reverberation has been experimentally confirmed. This result significantly strengthens the robustness of the SAS motion compensation, because the existence of a prominent point-target in view will not be required.
To overcome the disadvantage of short time Fourier transform for analyzing non-stationary signals, possible applications of wavelet transform for the sonar signal processing are studied by many researchers. In this paper, we propose the fast numerical calculation method for the wideband active sonar by using the harmonic wavelet transform and it can be shown that it has a possibility to reduce the calculation time for sonar signal processing including the beamforming and the correlation compared with the conventional FFT method. The calculation example for the planar array is also presented.
Many people now enjoy marine sports such as skin diving and scuba diving in sallow water area. Acoustic signals through an underwater loudspeaker can be used as a simple and effective way of preventing diving accident. At this time, it is fundamentally important to research in advance the masking of objective signals by ambient noises. In our research, measurements of masking effects of auditory ambient noise provide data from which the excitation pattern of the masking stimulus is derived in air and under water, using a pure tone masker and a band-limited noise masker centered at 1200 Hz and 2500 Hz. The present study casts further light on underwater masking results by comparing masking effects in air. The underwater results of masking measured are almost the same as the results in air. Therefore, it can be considered that under water masking effects are obtained on referring to usual experimental masking results in air.
The Boundary Element Method (BEM) is applied to calculate the target strength (TS) and the total cross section of a scatterer. The scattering amplitudes of the prolate spheroids of four types, namely, the vacant, rigid, liquid-filled, and gas-filled prolate spheroids are calculated by the BEM. The comparison of the results of the BEM and those of Prolate Spheroid Model (PSM), which is a numerical calculation using the spheroidal wave functions, shows excellent agreement for the vacant, rigid, and liquid-filled prolate spheroids. The TS of a gas-filled prolate spheroid calculated by the BEM shows resonance at a low frequency.
Recently, destruction of the ecological environment caused by plankton in ballast water on oceangoing ships is one of the most crucial topics for ocean environmental protection. The author has proposed a technique for water treatment using ultrasonic cavitation induced by the squeeze-film effect and has made a prototype of an ultrasonic sterilizer. This sterilizer can efficiently inactivate oosysts of Cryptospridium purvum which is pathogenic protozoan. In this paper, the effect of this ultrasonic water treatment on Heterocapsa circularisquama, which is phytoplankton and has sometimes caused red tide pollution, is experimentally investigated with the aim of applying to ballast water treatment. As a result, the number of killing H. circularisquama cells per unit input energy is almost constant independently of the thickness of the squeeze-film. The efficiency of inactivation of C. purvum oosysts are also rediscussed from the viewpoint of the number of inactivated oosysts per unit input energy.
A tomography array which consists of five 200 Hz transceivers were deployed in the Central Equatorial Pacific in December 1998 and measurements for two years were conducted. In our previous experiments, each transceiver transmitted sequentially every 30 minutes, and the total turn-around time was 2.5 hours. In this experiment, we intend to shorten a measurement time by simultaneous transmissions of five transceivers, which improves an accuracy of measurement for water current velocity due to reduce the effects of internal waves. In this case, however, each transceiver receives an overlapping signal of other transceivers. Multiple M-sequence codes are needed to separate mutual signals among transceivers. Simulation for separation of overlapping signals was conducted according to the arrangement of five transceivers and then five sets of M-sequences were selected carefully and written in the memory of the five transceivers. After recovery of all the equipment in August 1999, the observed data were read out from the transceivers and processed to separate overlapping signals successfully.