Development of an underwater gravity measurement system using autonomous underwater vehicle for exploration of seafloor deposits

It is known that there are mineral deposits below seafloor. Gravity survey is one of powerful methods for exploration of underground deposits. A new system is needed to survey a wide area quickly with a higher resolution. Recent technology of an autonomous underwater vehicle (AUV) enables us to measure gravity in underwater environment. We have developed an underwater gravity measurement system for exploration below a seafloor using an AUV. Our system consists of an underwater gravimeter and an underwater gravity gradiometer. The resolution of approximately 0.1 mgal for gravity and 10 Eötvös for gradient is required for the seafloor exploration. 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 has two gravimeters aligned vertical at a distance of 44 cm. We developed a gravimeter itself for the gradiometer. Both gravimeters are contained in a vacuum capsule and a gimbal system is also used to keep them vertical. For practical observation in the sea, we choose a large AUV Urashima. All the power is supplied from the Urashima and acoustic communication system on the Urashima enable real-time monitoring during observation. The first observation was carried out in September 2012 in Sagami-Bay, Japan. The Urashima was navigated at a constant speed and constant depth on the same profile. We succeeded in obtaining gravity data and it is found that our system has an accuracy or repeatability of 0.1 mgal for the gravity measurement. In August 2014, we carried out the second gravity survey using our underwater gravity measurement system in Izena caldera, the middle Okinawa Trough, where seafloor deposits were found. We obtained the data from both gravimeter and gradiometer with good quality for all the profiles.