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Recently, hydrothermal deposits below the seafloor are believed to be useful for human activity. Because the size of each deposit is much smaller than that of an offshore oil field, detailed exploration for the position and the estimated mass is needed. An underwater gravity survey just above seafloor can directly give localized mass distribution below the seafloor. Therefore, we have developed an underwater gravity measurement system onboard an autonomous underwater vehicle (AUV), which is suitable for wide area surveys close to the seafloor with high resolution. An improved gravimeter was mounted on a levelling platform and the system was contained in a spherical pressure vessel with a diameter of 50 cm. The gravimeter system was installed on a large AUV. All the power is supplied from the AUV and an acoustic communication system enables control and monitoring during observation. The first observation was carried out in September 2012 in Sagami Bay, Japan. The AUV was navigated at a constant speed and constant depth on the same profile. As a result, our system has a repeatability of 0.1 mGal. The developed underwater gravimeter has been used for mapping of gravity anomalies in seafloor deposits areas around Japan since 2013.

Gravity survey is one of powerful methods for exploration of mineral deposits. Recent technology of an autonomous underwater vehicle (AUV) enables us to quickly survey a wide area in underwater environment with high resolution. We have developed an underwater gravity measurement system for exploration of seafloor deposits using an AUV. Our system consists of an underwater gravimeter and an underwater gravity gradiometer. A model calculation shows that the resolution of approximately 0.1 mgal for gravity and 10 Eotvos (E) for gradient is required to detect a typical seafloor deposit. For the gravimeter system, we adopted an improved sea/air gravimeter and the sensor is mounted on a gimbal mechanism to keep vertical. A pressure capsule for the gravimeter has a diameter of 50 cm. The gradiometer comprises two identical gravimeters with a vertical separation of 44 cm. We developed the two gravimeters for the gradiometer. Both gravimeters are contained in a vacuum capsule and a gimbal system is also used to keep them vertical. We chose the AUV Urashima (JAMSTEC) to install both of the gravimeter and gradiometer. All the power is supplied from the Urashima and acoustic communication system on the Urashima enables real-time monitoring during sea experiments. So far, we have carried out four sea experiments in Sagami Bay, Izena Caldera in the middle Okinawa Trough, and Bayonnaise and Myojin knolls in the Izu-Ogasawara Arc. The Urashima was navigated at a constant speed and a constant depth or height above the seabottom. We successfully collected the data from both gravimeter and gradiometer with good quality in all experiments. We succeeded in obtaining good quality gravity anomaly data through processing of gravity data, water pressure data and AUV's navigation data including pitch and roll motion. It was found that our gravimeter system can attain an accuracy of 0.1 mGal at least in a good survey condition.

Deep manned submersible ldquoSHINKAI 6500rdquo was built in 1989, and has been operated by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). She has capability to dive down to 6500 m depth. Currently, she made 1097 dives until the end of July 2008. A round hatch on the top of the pressure hull is designed to constitute a part of the spherical pressure hull, and is very important structure to keep atmospheric pressure and waterproof in the hull. Therefore, we handle a hatch carefully in every operation, and moreover, we remove a hatch from the pressure hull at the time of the annual-maintenance and are doing the careful overhaul which involves the measurement and exchange of components. The number of her diving exceeded 1000 times, and she will reach 20 years old shortly. As consideration for that, an operation team, an engineering division and a ship builder implemented investigation for reconfirmation about soundness and safety of the pressure hull including a hatch. In this report, the result of investigation and evaluation about behavior of a hatch including the pressure hull is described.

Deep Submergence Vehicle Shinkai 6500, owned and operated by Independent Administrative Institution Japan Agency for Marine-Earth Science and Technology (hereafter JAMSTEC, and JAMSTEC was reorganized from Japan Marine Science and Technology Center on April 1, 2004), is the deepest manned submersible in the world. Shinkai 6500 has a depth capability of 6500 m and was developed in 1989 by JAMSTEC, and Mitsubishi Heavy Industries, Ltd. (hereafter MHI). She was commissioned in 1990 with her exclusive support vessel "YOKOSUKA" (length overall; 105 m, gross tonnage; 4,436 GT). Shinkai 6500 has dived 811 times until the end of March 2004. The major dives were carried out in the East Pacific Raise, the Mid Atlantic Ridge, the Indian Ocean and the western Pacific region, including the Mariana Trench and the Japan Trench. She has achieved many fruitful and important scientific results for the science in the world. The original power source of Shinkai 6500 was oil filled pressure compensated silver oxide zinc battery. However, the silver-zinc battery is very expensive, has short life, and is necessary complex work for maintenance. The operators needed to do full electric discharge every 15th dive, and to adjust electrolysis liquid after 30 dives. In order to supply electricity for the Shinkai 6500, two 400Ah battery pods, 800Ah in total, are installed. From 1997, the JAMSTEC Shinkai 6500 operation team, and MHI and Japan Storage Battery Co. Ltd started to develop the 400Ah capacity oil filled pressure compensated lithium-ion battery for Shinkai 6500 main batteries. And we completed development and sea trial in March 2004. The advantage of lithium ion battery in comparison with silver zinc battery is (1) maintenance-free, it is not necessary to do adjustment of electrolysis liquid. (2) long life time, the battery life time increased from 75 times or 1 year, to 180 times or 2 years, and (3) light weight, it is about 100 kg lighter than the silver zinc battery system. We will development and result of operation of introduce the first big capacity oil filled pressure compensated lithium ion battery for the Shinkai 6500.