Development of an underwater gravity measurement system with autonomous underwater vehicle for marine mineral exploration

摘要

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 Eötvös (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.