Gravity changes associated with underground injection of carbon dioxide at the Sleipner storage reservoir in the North Sea, and other marine geodetic studies

Three studies involving high precision measurements of gravity and water pressure on the seafloor are presented. In the first, baseline gravity and pressure measurements have been made over the Sleipner project in the North Sea. At this site, CO2 is being separated from recovered natural gas and sequestered by injection into a saline aquifer about 1000 m below the seafloor for environmental reasons. Ultimately, time-lapse gravity and pressure measurements will be used to constrain the density of CO 2 within the reservoir, and improve estimates of captured CO2 mass to insure the long-term safety of the technique. Modeling of time-lapse 3-D seismic data and reservoir simulation models indicates a maximum expected change in gravity of 2--8 muGal/yr, depending on the reservoir temperature (hence CO2 density). Data from the baseline gravity survey in 2002 show a repeatability of 4.3 muGal. A repeat survey is expected in the summer of 2005, allowing a three-year signal accumulation.; The second study makes use of high precision water pressure measurements made on seafloor benchmarks as a proxy for seafloor height to monitor volcanic inflation associated with the movement of magma within Axial Seamount since its 1998 eruption. Five years of vertical deformation data obtained from campaign style pressure measurements show inflation of up to 20 cm/yr. Modeling of the deformation data indicates an inflationary source located 1.7 km SSE of the caldera center and located at a depth of 3--5 km. If inflation continues at the current rate, the caldera will fully re-inflate to its pre-eruption level by 2014, suggesting recurrence interval of ∼16 years.; The final study is a spatial gravity survey of the Atlantis Massif oceanic-core complex. Modeling of the gravity measurements indicates that the massif has wedge-shaped core of increased density, 3150--3250 kg/m3 on average, consistent with seismic data suggesting that partially serpentinized to unaltered peridotite exist less than 1 km below the seafloor in this area. The observed wedge-like geometry of the core is consistent with the unroofing of deep-seated rock by extension and rotation along a detachment fault.