A fundamental challenge for exploration is pushing seismic imaging to open new frontiers. Typical of these frontiers is complex overburden with varying lithologies and structure that give rise to strong lateral changes in velocity and anisotropy. Two examples are subsalt and subvolcanic imaging. It is widely recognized that rich- or full-azimuth data can provide superior imaging results in such complex areas, besides providing other additional information (e.g., through fracture characterization). Typical ocean-bottom node (OBN) seismic surveys are acquired with a regular grid of sparse node and dense shots, providing full-azimuth/offset data with uniform sampling in the azimuth/offset domain. Figure 1 illustrates this point, showing the trace distribution for one CMP bin for the OBN survey used in this test; it features uniformly spaced nodes and a dense shot carpet. Every CMP bin in the survey receives contributions of exactly one trace in each azimuth /offset sub-bin (grid cells drawn in Figure 1). This means that (a) there is no redundant fold, and (b) the full-azimuth/offset space is uniformly sampled, points previously discussed by Vermeer (e.g., 1994, 2010, and other publications). We note that the theoretical CMP (= surface) illumination does not necessarily translate into the same subsurface illumination depending on the complexity of the overburden geology and the asymmetric source-receiver depth.

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