4D Ocean Bottom Node Decimation Study over the North Sea Golden Eagle Field

Abstract

Summary Ocean Bottom Node (OBN) surveys provide full azimuth coverage with long offsets and rich bandwidth. These attributes improve the resolution, stability, and steep-dip fidelity of seismic images derived from the data, which are desirable for 4D monitoring of a producing oilfield. However, acquisition of OBN data is expensive, and it is important to understand the impact of receiver density (which directly affects the acquisition cost) on the resultant seismic image. Here, using a dense North Sea 4D dataset, we demonstrate the impact of node density on both the 3D and 4D seismic image by migrating progressively sparser node configurations (including 50x300 m and 300x300 m cases) and comparing the results. It is shown that 4D image quality is more sensitive to changes in node density than is the 3D image. Furthermore, an acceptable sparse survey for 3D imaging may be inadequate for 4D applications. Attempts to mitigate the effects of reduced node density with processing methods show partial success for 4D imaging, but serve to highlight the importance of suitable node density in 4D survey design for North Sea OBN data. Our tests suggest a minimum node density of 100x300 m is necessary for this 4D example.