Development and demonstration of a gravity gradiometer onboard an autonomous underwater vehicle for detecting massive subseafloor deposits

highlights
• We developed a gravity gradiometer for the detection of subseafloor deposits.
• Self-noise was sufficient to detect a typical ore deposit onboard an AUV.
• Trial observation performed at a depth of about 1300 m showed steady operation.
• Resulting gravity gradient was 3×102E, mainly limited by noise of recording system.

Gravity surveys are used to detect underground density structures. Hybrid survey configurations using a gravimeter and a gravity gradiometer are particularly effective in this regard. A gravity gradiometer was developed for use in hybrid gravity surveys of massive subseafloor deposits. Its performance was evaluated in an ocean-based trial using an autonomous underwater vehicle (AUV). The gradiometer consisted of two identical vertical long-period pendulums, acting as gravity sensors, with a separation of 0.44 m. The self-noise of the gradiometer was estimated to be 6 E (6×10−9/s2) at 2–50 mHz, with which the gravity gradient signal could be detected when the AUV was above a typical ore deposit. The gravity gradiometer was mounted on a forced gimbal vertically controlled by fiber optic gyroscopes and tiltmeters. The ocean-based trial observation performed at a depth of approximately 1300 m in Sagami Bay using the AUV Urashima (JAMSTEC) showed that the system operated steadily on a moving platform except during periods of extreme motion, such as turning and pitching. The resulting gravity gradient was highly affected by the motion of the platform and the estimated resolution was 3×102 E, which was mainly limited by the noise of the recording system.