In order to reproduce changes of the dissolved oxygen concentration in summer in the vicinity of the Nanko thermal power plant, located in the inner part of Osaka Bay, a numerical simulation model was developed. According to the model results, daily fluctuation of dissolved oxygen concentration in the surface layer is high in June, the early stratification period, because of instability of stratification and after July. The dissolved oxygen concentration gradually decreases from a level of 6 to 17 mg/L in June to a level of 6 to 9 mg/L in July in the vicinity of the Nanko thermal power plant. Both the observations and model values near the depth of the thermal power plant outlet are 3 to 5 mg/L, which is considered not to affect the water quality of dissolved oxygen. Our simulation model should be able to reproduce the characteristics of dissolved oxygen concentration during the stratification period in the vicinity of the Nanko thermal power plant.
The authors have proposed an innovative method to navigate an autonomous underwater vehicle (AUV) for visual mapping of seafloor with high positioning accuracy without using any vision-based matching. The proposed method was implemented in the AUV Tri-Dog 1 and sea experiments were carried out at Tagiri vent field, Kagoshima bay in Japan (Maki et al., 2008). Based on the success of the experiments, a series of dives was carried out at the same place. The AUV Tri-Dog 1 succeeded in 12 fully autonomous dives with a total duration of 29 hours. The vehicle took 9,288 pictures of the seafloor, keeping the altitude of 1.2 m with a surge speed of 0.08 m/s. A photomosaic of the seafloor was created by mapping 7,289 pictures based on the real-time estimates of the AUV state, without any pictorial correlation. The distributions of detailed features such as tube-worm colonies and bacteria mats are clearly shown. The photomosaic covers around 3,000 square meters. To the knowledge of the authors, this is one of the largest underwater photomosaic ever reported. The mapping accuracy was estimated to be 0.3 to 0.8 m based on the comparison of the photomosaic between dives.