For the purpose of monitoring deep seafloor environmental changes associated with a submarine earthquake swarm, a cabled observatory consisting of multidisciplinary sensors including a hydrophone was deployed by JAMSTEC at a depth of 1200 m off Hatsushima Island in Sagami Bay. By using the data obtained by the observatory, the relationship between surface weather changes and underwater ambient noise, especially the origin of low frequency noise lower than 100 Hz, was examined. As a result, the site effects on the noise characteristics obtained by the hydrophone, consisting of the surrounding topography and the water depth, became clear. It also became clear that the low frequency noise caused by an earthquake and a typhoon can be discriminately observed by the observatory. These results can be utilized for detecting fluctuation phenomena or precursors on the seafloor that are associated with earthquakes.
The knowledge of bottom scattering feature in shallow water is required to improve the prediction of an active sonar performance. Nevertheless, the direct measurement of the scattering feature is generally difficult in shallow water, particularly for lower grazing angle. Thus, we conducted the experiment for bottom back scattering in the continental shelf of East China Sea by using sound source and receiver arrays to extract the signal that came from the targeted process of scattering. Moreover, the bottom back scattering strengths at incident grazing angles less than 20 ° were acquired by selecting the value of the parameters of the scattering model so that the agreement of the measured and calculated reverberation data may be maximized.
The jellyfish Nemopilema nomurai, which reaches up to 2 m in diameter and 200 kg in weight, has appeared in large numbers in the Sea of Japan during the last several years and has had a negative effect on coastal fisheries in this region. Data on the abundance and distribution of these jellyfish are needed to forecast when and where they will occur in coastal areas. Acoustic techniques are commonly used to study the distribution and abundance of fish and zooplankton because these techniques can survey large areas relatively quickly. Before such surveys can be conducted, the acoustic characteristics of the target species must be known. In this study, the density and speed-of-sound in live jellyfish were measured in order to clarify their acoustic characteristics using a theoretical scattering model. The density of the jellyfish was measured using the dual-density method, while the speed of sound was measured using the time-of-flight method. Their acoustic characteristics were estimated with the distorted-wave Born approximation (DWBA) model using these material properties and the shape of free-swimming jellyfish. The results demonstrate the feasibility of investigating the abundance and distribution of jellyfish using acoustic methods.