Development of an underwater gravimeter and the first observation by using autonomous underwater vehicle

We developed an underwater gravimeter for exploration of a seafloor hydrothermal deposit. Our hybrid gravimeter system consists of an underwater gravimeter and an underwater gravity gradiometer. We adopted Micro-g LaCoste S-174 as a gravity sensor. The sensor is mounted on a gimbal mechanism with a fiber gyroscope. A titanium sphere contains the sensor system. Maximum depth rating is 4,200 m. The data are sent to a recording system housed in another cylinder-shape capsule. The whole system is controlled and monitored via acoustic link of the AUV. In September 2012, the first practical measurement in marine area was carried out by using JAMSTEC's AUV URASHIMA to evaluate performance of the system. The gravimeter and gravity gradiometer were simultaneously mounted on the URASHIMA and the first measurement was performed in Sagami-Bay. From the survey, we obtained the gravity data and supplemental data for compensation of the gravity data with good quality. According to preliminary analyses, the resolution of the gravity data from the first practical measurement is estimated to reach 0.1 mgal.