GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.3 737 depressions (up to 40 m deep) with moderate to steep slope (up to 15°) in which the gas fluid rises from deeper horizons. Thanks to their limited size, these features are not critical since only a small fraction of the total amount of nodes falls in these depressions and hence need to be slightly moved. The final position of the nodes can be further refined at the time of acquisition if the location of any future facility that will lay on the sea floor is known. These adjustments of the final node positions further improve the 4D receiver repeatability for any future 4D seismic monitoring. Conclusions. A feasibility study was conducted for the design of a marine seismic survey over a geologically complex area where one of the main challenges is the seismic shadow area beneath the salt bodies, as observed in the vintage narrow-azimuth towed-streamer dataset. With these regards, a full-azimuth OBN survey was found to be the optimal solution. In addition to the achievement of the main scope for this study, an OBN survey provides also other advantages, among the others, the high 4D seismic repeatability and the access to obstructed areas as those typically encountered in developed hydrocarbon province. Acknowledgements. The authors thank Eni Upstream and Technical Services for permission to publish this work. Reference Grion S., Exley R., Manin M., Miao X., Pica A. L., Wang Y., Granger P., and Ronen S.; 2007: Mirror imaging of OBS data . First Break, 25 (11), pp. 37-42. Hanafy S. M., Huang Y., Schuster G. T.; 2015: Benefits and limitations of imaging multiples: Mirror migration . The Leading Edge, 34 , pp. 796-800. Lewis B., Brooks C., Pfister M., Michell S., and Astvatsaturov G.; 2016: Efficient acquisition of deepwater node surveys . SEG Technical Program Expanded Abstracts: pp. 92-96. Petersen D. P. and Middleton D., 1962; Sampling and Reconstruction of Wave-Number-Limited Functions in N-Dimensional Euclidean Spaces , Information and Control, 5 , pp. 279–323. Fig. 1 - Acquisition geometry layout. Left) Node triangular grid with a node spacing equal to a . Right) Zoom around a node where the shot point (spacing equal to b ) and midpoint (spacing equal to c ) grids can be better appreciated as well as the hexagonal bin.