In a modern marine 3-D seismic survey, typically, more than 100 000 traces per square km are recorded. This imaging requirement means that the areal extent of a 3-D survey almost always is larger than the areal extent of the objective.Ī few hundred thousand to a few hundred million traces normally are collected during a 3-D survey. The size of the survey area is dictated by the areal extent of the subsurface target zone and the aperture size required for adequate imaging of that target zone. This dense coverage requires an accurate knowledge of shot and receiver locations. In contrast to 2-D surveys in which line spacing can be as much as 1 km, the line spacing in 3-D surveys can be as small as 25 m. The direction that is perpendicular to the inline direction in a 3-D survey is called the crossline direction. In marine 3-D surveys, the shooting direction (boat track) is called the inline direction for land 3-D surveys, the receiver cable is along the inline direction. A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to each other and placing the shotpoints in the perpendicular direction (swath shooting).įigure 7.0-1 A 2-D CMP-stacked section from an area with a subsurface structure associated with overthrust tectonics. On the other hand, 3-D migration of 3-D data provides an adequate and detailed 3-D image of the subsurface, leading to a more reliable interpretation.Ī typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). These misties are caused by inadequate imaging of the subsurface resulting from the use of 2-D rather than 3-D migration. Although out-of-plane reflections (sideswipes) usually are recognizable by the experienced seismic interpreter, the out-of-plane signal often causes 2-D migrated sections to mistie. Despite the fact that a 2-D section contains signal from all directions, including out-of-plane of the profile, 2-D migration normally assumes that all the signal comes from the plane of the profile itself. A two-dimensional (2-D) seismic section is a cross-section of a three-dimensional (3-D) seismic response. Examples include salt diapirs, overthrust and folded belts, major unconformities, reefs, and deltaic sands. Subsurface geological features of interest in hydrocarbon exploration are three dimensional in nature.
8 Appendix G: Mathematical foundation of 3-D migration.6 7.5 Interpretation of 3-D seismic data.5.4 Aspects of 3-D prestack time migration - a summary.5.1 3-D DMO correction combined with 3-D common-offset migration.4.5 The phase-shift-plus-correction method.4.4 Explicit schemes combined with the mcclellan transform.4.3 Two-pass versus one-pass implicit finite-difference 3-D migration in practice.4.2 Impulse response of the one-pass implicit finite-difference 3-D migration.3.5 Aspects of 3-D DMO correction - a summary.3.2 Azimuth dependence of moveout velocities.
2 7.1 3-D survey design and acquisition.1.1 The need for imaging in three dimensions.
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