Changes

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 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 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 (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.

[[file:three-dimensional-seismic-method_fig1.png|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_fig2.png|thumb|{{figure number|2}}(a) Selected time slices. Copyright: a marine 3-D survey and (b) a time-structure map of a marker horizon derived from the 3-D volume of migrated data. (Data courtesy of Western Geophysical, Division of Western-Atlas International.]]

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.

==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 (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==