Sea Technology

APR 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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10 st / April 2017 designed to record multicomponent seismic data continu- ously for up to six months, during which time active seismic sources towed at the sea surface illuminate the subsurface with a dense grid (approximately 50 by 50 m) of source points (acoustic impulses generated from the sudden release of compressed air). The active source wavefield is reflected, refracted, and scattered in the earth, ultimately returning to the seabed, where the nodes detect and record the returning I n the last decade, deepwater ocean bottom node (OBN) systems have recorded high-quality 3D marine seismic data in many oil fields around the world at greater than 1-km depth. Deepwater OBN data are most often used to support field development rather than pure exploration. The seismic data are used to image and analyze producing forma- tions in the subsurface. Many ma- rine seismic surveys, towed streamer, ocean bottom cable and OBN have been repeated after a typical time interval ranging from a few months to a few years to monitor res- ervoir changes. By reservoir monitoring, seismic changes over time due to production or intervention, such as water flooding, provide a time lapse or 4D seismic signal that has great value for understanding the reservoir and optimizing field development. OBN Equipment, Practice A deepwater ocean bottom node is an autonomous seis- mograph that incorporates seismic sensors (hydrophone and three-component geophone), digital recording electronics, precision clock, data storage medium, and power source in a compact pressure vessel that will couple well with the seabed. A node capable of operating in water depths up to approximately 3,000 m, FairfieldNodal's Z3000 weighs about 200 lb. in air and about 100 lb. in water in a cylindri- cal form factor and is about 20 in. in diameter and about 10 in. high. Ocean bottom nodes are typically deployed on the sea- floor in a sparse receiver grid (approximately 400 by 400 m). After synchronization of each node's internal clock with a GPS reference time signal and starting the digital record- ing of sensor data, nodes are deployed by ROV. They are Semipermanent Reservoir Monitoring With Ocean Bottom Nodes 4D Seismic Improves Understanding of Reservoirs By David Hays (Top) An illustration of deepwater OBN operations depicting node handling and deployment by ROV vessel and active seis- mic source vessel. (Bottom) The ZLoF semipermanent reser- voir monitoring node.

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