The 4D microgravity method for waterflood surveillance: Part 3 - 4D absolute microgravity surveys at Prudhoe Bay, Alaska

The 4D microgravity method is becoming a mature technology.Aprojecttodeveloppracticalmeasurementandinterpretation techniques was conducted at Prudhoe Bay,Alaska, from1994through2002.Beginningin2003thesetechniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a 150 km 2 area are reoccupied in each survey year with sub-5 Gal precision absolute gravity and centimeter precision Global Positioning System GPS geodetic measurements. The 4D gravity measured over epochs 2005‐2003, 2006‐2003, and 2007‐2003 has been successfullymodeledtotrackthemassofwaterinjectedsincelatein 2002.AnewandimprovedversionoftheA-10field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology.The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There aresubstantialdifferencesbetweena4Dabsolutemicrogravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the4Dgravitysignalineachepochislessthan10 Gal.