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Subsurface geological features of interest in hydrocarbon exploration are three-dimensional (3-D) in nature. Examples are salt diapirs, overthrust and folded belts, major [[Unconformity|unconformities]], reefs, and deltaic sands. A two-dimensional (2-D) seismic section is a cross section of a 3-D seismic response. 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 of the signal comes from the plane of the profile itself. Although out-of-plane seismic signals (sideswipes) are often recognizable by the experienced seismic interpreter, the sideswipe signal causes 2-D migrated sections to mistie. These misties are due to inadequate imaging of the subsurface resulting from the use of 2-D rather than 3-D migration<ref name=pt07r13>French, W. S., 1974, Two-dimensional and three-dimensional migration of model-experiment reflection profiles: Geophysics, 39, 265–277. [http://library.seg.org/doi/abs/10.1190/1.1440426 DOI:10.1190/1.1440426]</ref> (see [[Seismic data - mapping with two-dimensional data]]).

Subsurface geological features of interest in hydrocarbon exploration are three-dimensional (3-D) in nature. Examples are salt diapirs, [[overthrust]] and folded belts, major [[Unconformity|unconformities]], reefs, and deltaic sands. A two-dimensional (2-D) seismic section is a [[cross section]] of a 3-D seismic response. 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 of the signal comes from the plane of the profile itself. Although out-of-plane seismic signals (sideswipes) are often recognizable by the experienced seismic interpreter, the sideswipe signal causes 2-D migrated sections to mistie. These misties are due to inadequate imaging of the subsurface resulting from the use of 2-D rather than 3-D migration<ref name=pt07r13>French, W. S., 1974, Two-dimensional and three-dimensional migration of model-experiment reflection profiles: Geophysics, 39, 265–277. [http://library.seg.org/doi/abs/10.1190/1.1440426 DOI:10.1190/1.1440426]</ref> (see [[Seismic data - mapping with two-dimensional data]]).

==Performing 3-D surveys==

==Performing 3-D surveys==

[[file:three-dimensional-seismic-method_fig1.png|300px|thumb|{{figure number|1}}(a) A CMP-stacked section. Copyright: a marine 3-D survey. (b) The corresponding 2-D migrated section. (c) The 3-D migrated section. (Data courtesy of Amoco Europe and West Africa, Inc.]]

[[file:three-dimensional-seismic-method_fig1.png|300px|thumb|{{figure number|1}}(a) A CMP-stacked section. Copyright: a marine 3-D survey. (b) The corresponding 2-D migrated section. (c) The 3-D migrated section. (Data courtesy of Amoco Europe and West Africa, Inc.]]

A typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to one another and placing the shot points in the perpendicular direction (swath shooting). Other recording geometries have also been used in acquiring 3-D data. Shooting in circles has been done in the Gulf of Mexico to delineate salt domes. Shooting around a lake or a topographic high to achieve subsurface coverage under the surface obstacle has also been tried.

A typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to one another and placing the shot points in the perpendicular direction (swath shooting). Other recording geometries have also been used in acquiring 3-D data. Shooting in circles has been done in the [[Gulf of Mexico]] to delineate salt domes. Shooting around a lake or a topographic high to achieve subsurface coverage under the surface obstacle has also been tried.

In marine 3-D surveys, the shooting direction (boat track) is considered to be the ''in-line direction'', whereas in land 3-D surveys, the receiver cable is along the in-line direction. The direction that is perpendicular to the in-line direction in a 3-D survey is called the ''cross-line direction''. In contrast to 2-D surveys in which line spacing can be as much as [[length::1 km]], the line spacing in 3-D surveys can be [[length::50 m]] or less. This dense coverage requires an accurate knowledge of shot and receiver locations.

In marine 3-D surveys, the shooting direction (boat track) is considered to be the ''in-line direction'', whereas in land 3-D surveys, the receiver cable is along the in-line direction. The direction that is perpendicular to the in-line direction in a 3-D survey is called the ''cross-line direction''. In contrast to 2-D surveys in which line spacing can be as much as [[length::1 km]], the line spacing in 3-D surveys can be [[length::50 m]] or less. This dense coverage requires an accurate knowledge of shot and receiver locations.

==Processing of 3-D data==

==Processing of 3-D data==

The basic principles of 2-D seismic data processing still apply to 3-D processing. In 2-D processing, traces are collected as common midpoint (CMP) gathers, while in 3-D processing, traces are collected as common-cell gathers (bins). These gathers are used in velocity analysis, and common-cell stacks are generated. Typical cell sizes are 25 by [[length::25 m]] for land surveys and 12.5 by [[depth::37.5 m]] for marine surveys.

The basic principles of 2-D [[seismic data]] processing still apply to 3-D processing. In 2-D processing, traces are collected as common midpoint (CMP) gathers, while in 3-D processing, traces are collected as common-cell gathers (bins). These gathers are used in velocity analysis, and common-cell stacks are generated. Typical cell sizes are 25 by [[length::25 m]] for land surveys and 12.5 by [[depth::37.5 m]] for marine surveys.

Conventional 3-D recording geometries often complicate the process of stacking the data in a common-cell gather. Cable feathering in marine 3-D surveys can result in traveltime deviations from a single hyperbolic moveout within a common-cell gather. For land 3-D surveys, [[azimuth]]-dependent moveout within a common cell gather is an issue.

Conventional 3-D recording geometries often complicate the process of stacking the data in a common-cell gather. Cable feathering in marine 3-D surveys can result in traveltime deviations from a single hyperbolic moveout within a common-cell gather. For land 3-D surveys, [[azimuth]]-dependent moveout within a common cell gather is an issue.

==3-D interpretation==

==3-D interpretation==

The 3-D data volume is available to the interpreter as vertical sections in both the in-line and cross-line directions and as horizontal sections (time slices). The time slices allow the interpreter to generate contour maps for marker horizons ([[:file:three-dimensional-seismic-method_fig2.png|Figure 2]]). The interactive environment provides an effective and efficient means for interpretation of the sheer volume of 3-D migrated seismic data. Fault correlations, horizon tracking, horizon flattening, and some image processing techniques can be adapted to the interactive environment to help improve interpretation. Since the 3-D volume provides detailed constraints on interpretational decisions, drilling programs based on 3-D seismic usually have high success rates.

The 3-D data volume is available to the interpreter as vertical sections in both the in-line and cross-line directions and as horizontal sections (time slices). The time slices allow the interpreter to generate [[contour]] maps for marker horizons ([[:file:three-dimensional-seismic-method_fig2.png|Figure 2]]). The interactive environment provides an effective and efficient means for interpretation of the sheer volume of 3-D migrated seismic data. Fault correlations, horizon tracking, horizon flattening, and some image processing techniques can be adapted to the interactive environment to help improve interpretation. Since the 3-D volume provides detailed constraints on interpretational decisions, drilling programs based on 3-D seismic usually have high success rates.

==See also==

==See also==

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]