Development of a High-Resolution Underwater Gravity Measurement System Installed on an Autonomous Underwater Vehicle

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.