Journal of the Japan Society for Marine Surveys and Technology

The performances of in situ turbidity meters that are widely used in monitoring turbid water were compared. Five commercially available models of different measurement systems were calibrated with the turbidity standard material, and the performances of the instruments were investigated with various test waters.
When the turbidity-causing material is mainly inorganic suspended particles, all tested models of different measurement systems were adaptable to the measurement. However, the models of the back scattering system and side scattering system were strongly affected by the particle size distribution of suspended particles. Thus, it was necessary to prepare calibration curves for the expected turbidity of suspended particles. When the turbidity-causing material contains organic particles in addition to inorganic particles, precise measurements could be achieved with the models of the integrating sphere system or beam attenuation system.

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To collect bathymetric data and bottom material information, multi narrow beam echo sounder and/or side scan sonar are usually used over wide area. The autonomous underwater vehicle (AUV) has the advantage of getting closer to the sea surface as compared with a survey from the vessel. URASHIMA is a 3000 m class AUV, is loaded with multi narrow beam depth sounder, side scan sonar and sub-bottom profiler. Using URASHIMA, side scan sonar images and sub-bottom profiles were collected around off Hatsushima Island where mud flow was generated with some large earthquakes occurrence. We could obtain many enough quality side scan sonar images and sub-bottom profiles. Mosaic image was constructed by obtained side scan sonar images, and shows distinctive surface structure. Some irregular patches are detected around the mudflow area 7km off Hatsushima Island. This structure do not relate to the bathymetry obtained by SeaBat8160 on R/V Natsushima. Therefore, these are interpreted as debris generated with earthquakes. Sub-bottom profiles could detect very clear sedimentary structure.

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