A new reservoir management application uses precise time-lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3–5-year period. We have constructed and deployed a new instrument—the ROVDOG (Remotely Operated Vehicle-deployed Deep-Ocean Gravimeter) system—for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea-floor benchmarks. We have built an instrument based upon the Scintrex CG-3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties.
The instrument prototype was deployed in the North Sea during June 1998; 75 observations were made at 32 stations. The standard deviation of repeated gravity measurements was 0.026 mGal; the standard deviation of pressure-derived heights, for repeated measurements, was 1.4 cm. A refined instrument was deployed in August 2000 in a three-sensor configuration. Multiple sensors improved the precision by averaging more samples without incurring additional survey time. A total of 159 measurements were made at 68 stations. The standard deviation of repeated measurements was 0.019 mGal; the standard deviation of pressure-derived heights was 0.78 cm.
A ROVDOG pressure case rated to 4500 m depth has also been constructed. This system was deployed with the Alvin manned submersible in November 2000 to a depth of 2700 m.