Deep Ocean Measurements of Gravity

A group at Scripps Institution of Oceanography, in partnership with industrial sponsors (primarily StatoilHydro), has developed instrumentation over the past decade that provides new capabilities for deep ocean gravity measurements. The instrument is based on the commercially available Scintrex CG5 quartz spring sensor. We have packaged that sensor in a compact gimbal frame and housed it in a deep ocean pressure case. We have deployed the sensor in two different modes. In the first, we make repeated campaign observations at stationary positions on the seafloor with a Remotely Operated Vehicle (ROV) to monitor changes with time in reservoir density associated with production. In the second, we mount the instrument in an Autonomous Underwater Vehicle (AUV) to facilitate exploratory surveys in the deep ocean, closer to the source than could be experienced with ocean surface observations. The stationary time-lapse surveys have been underway for several years now and we have achieved a precision of about 3 microGal (3 × 10-8, m/s2, ). In a recent experiment we tested the AUV concept for the first time. Initial results are promising and we expect that with continued effort we should achieve sub-milliGal precision in AUV gravity surveys.