To conduct efficient seismic surveys using the pop-up ocean bottom seismographs (OBSs), we attempted attaching a metal rod (counterweight) to the OBS. As a result, the floating speed and the floating posture of the OBS improved during the OBS recovery. Since this attachment of counterweight also improved the floating posture on the sea surface, this was also effective for efficiency of the recovery operation of the OBS from the sea surface to on deck. When conducting a seismic survey using many OBSs, this method proves to be inexpensive, works very effectively, and may avoid the influence of individual differences with respect to the center of gravity and balance of each OBS.
Seven years passed since Fukushima Dai-ichi Nuclear Power Station accident which was caused large amount of radionuclide release to the sea. Elucidation of behavior mechanism of radiocesium in the seabed is required for restarting fishing industry. We developed radiation detection system using the unmanned surface vehicle for in-situ measurement of radiocesium concentration in seabed sediment. This system is able to automatically navigate to measurement point and obtain the radiation data on the bottom sediment. The detector was calibrated by comparing the actual sediment samples. The periodical measurement off-shore the Fukushima Prefecture was performed using developed this system. As these results, distribution of radiocesium concentration was changed due to oceanographic condition. However, radiocesium inventory was tendency to decrease according to radiocesium half-life in measurement area. This system is effective for elucidation of behavior mechanism of radiocesium because it can easily measure the radiocesium concentration in the bottom sediment.
From April 2008 to July 2009, an acoustic telemetry study on crucian carp was conducted in Lake Biwa, Japan. Twenty Nigorobuna, ten gengoroubuna and one ginbuna captured during spawning seasons were released with ultrasonic transmitters in the South Basin. Signals from the tagged fish were detected by 28 ultrasonic-receiver stations installed around Lake Biwa. Seasonal preference of habitats was examined by distance-based analysis. Seasonal distributions of nigorobuna and gengoroubuna in Lake Biwa were observed including consecutive two spawning seasons. It was revealed that seasonal migration patterns of nigorobuna and gengoroubuna were different. Nigorobuna appeared mainly around the release site all the year round, while gengoroubuna migrated to the North Basin after their spawning season, where they stayed during the non-spawning season, and moved to the South Basin in the next midwinter, where they stayed until the end of the next spawning season. It was also revealed that gengoroubuna preferred lower water temperature than nigorobuna. Gengoroubuna moved faster than nigorobuna and ginbuna. There were high-degree correlations in distance ratios of each station during spawning seasons in the release year and the next year in both nigorobuna and gengoroubuna, suggesting a multiyear use for the same spawning site. Through these results, deeper understanding of these endangered fish species in Lake Biwa and information to help the development of effective conservation programs would be provided.
Wind speed data are used to estimate the global air-sea momentum, heat, and CO2 gas flux. The accuracy of wind speed data is clearly very important for these purposes. However, since global wind speed datasets are assembled from various sources of satellite and reanalysis data, the most-suitable dataset is often unclear. This study verifies the accuracy of available wind speed datasets by comparing various satellites and reanalyzed wind speed datasets using wind speeds measured by buoys. The global wind speed datasets, such as Cross Calibrated Multi-Platform (CCMP), NCEP/CFSR, NCEP-R1, NCEP-R2, European Center for Medium-Range Weather Forecasts (ECMWF)/ERA-Interim, and JRA-55 datasets were used. The root mean square error (RMSE) values of the six datasets against reference measurements are 1.05, 1.53, 2.16, 2.45, 1.42, and 1.64 (m/s), respectively. The accuracy of global datasets differs depending on the ocean area. However, the wind speed dataset in CCMP is the most accurate for any local ocean area. In the north Pacific and north Atlantic, all the datasets we tested are inaccurate. These results show that differences in the accuracy of the wind speed datasets may have a significant effect on the estimation of global air-sea momentum, heat, and CO2 gas flux.
Importance of scientific ocean drilling was becoming more widely known in the Japanese scientific community, based upon the results of DSDP/IPOD/ODP by late 80s. Many scientific and engineering discussions took place domestically and internationally, and finally those discussions led the Japanese government to decisively move forward with the construction of D/V Chikyu. Finally, Chikyu was delivered to the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in July 2005. Chikyu was designed and built as a scientific ocean drilling vessel capable of riser drilling in water depths of 2,500 m (future target is 4,000 m class). Chikyu can drill in deeper water and to deeper sub sea floor depths than any other scientific drilling platforms. Chikyu’s large hull makes it possible for a large onboard laboratory, with space for large analytical instruments and facilities. Since the first scientific operation conducted in the Nankai Trough in 2007, Chikyu made significant scientific achievements by IODP Expeditions. Those results are briefly introduced and show a future operation challenge with introducing Chikyu’s general features.