Changes

==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.

After stacking, the 3-D data volume is sometimes (but not always) migrated in two stages. First, a 2-D migration is applied along the in-line or cross-line direction. Then the data are sorted, and a second pass of 2-D migration is applied along the orthogonal direction. Before the second pass of migration, the data sometimes need to be trace interpolated along the cross-line direction to avoid spatial aliasing.

After stacking, the 3-D data volume is sometimes (but not always) migrated in two stages. First, a 2-D migration is applied along the in-line or cross-line direction. Then the data are sorted, and a second pass of 2-D migration is applied along the orthogonal direction. Before the second pass of migration, the data sometimes need to be trace interpolated along the cross-line direction to avoid spatial aliasing.

==3-D versus 2-D migration==

==3-D versus 2-D migration==

Three-dimensional migration often produces surprisingly different sections from 2-D migrated sections (see “[[Seismic migration]]”). The example in [[:file:three-dimensional-seismic-method_fig1.png|Figure 1]] shows a no reflection zone on the 2-D migrated section, while the same zone contains a series of continuous reflections on the 3-D migrated section that are easily correlated with reflections outside that zone. When we do 2-D migration, we confine the movement of the energy into the plane of the line itself. So the energy contained in the unmigrated stacked section in Figure 1a is indeed the same as the energy contained in the 2-D migrated section in [[:file:three-dimensional-seismic-method_fig1.png|Figure 1b]], except that it has been moved somewhere else on the section. As a result of moving the energy during 3-D migration within the 3-D volume, some energy moved into the section ([[:file:three-dimensional-seismic-method_fig1.png|Figure 1c]]) from others and some moved out of the section and migrated into the others.

Three-dimensional migration often produces surprisingly different sections from 2-D migrated sections (see “[[Seismic migration]]”). The example in [[:file:three-dimensional-seismic-method_fig1.png|Figure 1]] shows a no reflection zone on the 2-D migrated section, while the same zone contains a series of continuous reflections on the 3-D migrated section that are easily correlated with reflections outside that zone. When we do 2-D migration, we confine the movement of the energy into the plane of the line itself. So the energy contained in the unmigrated stacked section in Figure 1a is indeed the same as the energy contained in the 2-D migrated section in [[:file:three-dimensional-seismic-method_fig1.png|Figure 1b]], except that it has been moved somewhere else on the section. As a result of moving the energy during 3-D migration within the 3-D volume, some energy moved into the section ([[:file:three-dimensional-seismic-method_fig1.png|Figure 1c]]) from others and some moved out of the section and migrated into the others.

From the field data example, we see that 3-D migration provides complete imaging of the 3-D subsurface geology. Specifically, 2-D migration cannot adequately image the subsurface and introduces misties between 2-D lines in the presence of dipping events. However, 3-D migration eliminates these misties by completing the imaging process.

From the field data example, we see that 3-D migration provides complete imaging of the 3-D subsurface geology. Specifically, 2-D migration cannot adequately image the subsurface and introduces misties between 2-D lines in the presence of dipping events. However, 3-D migration eliminates these misties by completing the imaging process.