Changes

Of the many processes applied to seismic data, seismic migration is the one most directly associated with the notion of imaging. Until the migration step, seismic data are merely recorded traces of echoes, waves that have been reflected from anomalies in the subsurface. In its simplest form, then, ''seismic migration'' is the process that converts information as a function of recording time to features in subsurface depth. Rather than simply stretching the vertical axes of seismic sections from a time scale to a depth scale, migration aims to put features in their proper positions in space, laterally as well as vertically.

Of the many processes applied to seismic data, seismic migration is the one most directly associated with the notion of imaging. Until the migration step, seismic data are merely recorded traces of echoes, waves that have been reflected from anomalies in the subsurface. In its simplest form, then, ''seismic migration'' is the process that converts information as a function of recording time to features in subsurface depth. Rather than simply stretching the vertical axes of seismic sections from a time scale to a depth scale, migration aims to put features in their proper positions in space, laterally as well as vertically.

All the issues in seismic migration reviewed here are treated in the collection of reprints found in Gardner<ref name=pt07r14>Gardner, G. H. F., ed., 1985, Migration of Seismic Data: Tulsa, OK, Society of Exploration Geophysicists Monograph Series, 462 p.</ref>.

All the issues in seismic migration reviewed here are treated in the collection of reprints found in Gardner.<ref name=pt07r14>Gardner, G. H. F., ed., 1985, Migration of Seismic Data: Tulsa, OK, Society of Exploration Geophysicists Monograph Series, 462 p.</ref>

==What migration accomplishes==

==What migration accomplishes==

file:seismic-migration_fig2.png|{{figure number|2}}Schematic depth section (top) and zero offset time section (bottom) for a dipping reflector at depth.

file:seismic-migration_fig2.png|{{figure number|2}}Schematic depth section (top) and zero offset time section (bottom) for a dipping reflector at depth.

file:seismic-migration_fig3.png|{{figure number|3}}Schematic depth section showing normal incidence ray paths for two-way travel between source-receiver positions and a dipping reflector.

file:seismic-migration_fig3.png|{{figure number|3}}Schematic depth section showing normal incidence ray paths for two-way travel between source-receiver positions and a dipping reflector.

file:seismic-migration_fig4-part1.jpg|{{figure number|4a}} CMP stack of data from the Santa Barbara Channel, offshore California.

file:seismic-migration_fig4-part1.jpg|{{figure number|4a}}CMP stack of data from the Santa Barbara Channel, offshore California.

file:seismic-migration_fig4-part2.jpg|{{figure number|4b}}Figure 4(b) Result of migration.

file:seismic-migration_fig4-part2.jpg|{{figure number|4b}}Result of migration.

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[[:file:seismic-migration_fig3.png|Figure 3]] shows another perspective on this mispositioning. Reflections recorded at zero source-receiver offset follow ray paths that are perpendicular to the reflector. As a result, the reflection from the point on the reflector beneath point P, for example, would be recorded by the geophone at location G, to the right.

[[:file:seismic-migration_fig3.png|Figure 3]] shows another perspective on this mispositioning. Reflections recorded at zero source-receiver offset follow ray paths that are perpendicular to the reflector. As a result, the reflection from the point on the reflector beneath point P, for example, would be recorded by the geophone at location G, to the right.

[[:file:seismic-migration_fig4-part1.jpg|Figure 4]] shows the application of migration to CMP-stacked field data. The superposition of diffraction curves evident in the unmigrated data of [[:file:seismic-migration_fig4-part1.jpg|Figure 4a]] gives rise to crossing reflections that can not plausibly be interpreted as structure. By correcting for lateral mispositioning of dipping reflectors and “collapsing” diffraction curves to zones defined by the diffraction apex, migration converts the recorded waves to a subsurface picture ([[:file:seismic-migration_fig4-part2.jpg|Figure 4b]]) depicting both broadly and tightly folded anticlines and synclines.

[[:file:seismic-migration_fig4-part1.jpg|Figure 4]] shows the application of migration to CMP-stacked field data. The superposition of diffraction curves evident in the unmigrated data of [[:file:seismic-migration_fig4-part1.jpg|Figure 4a]] gives rise to crossing reflections that can not plausibly be interpreted as structure. By correcting for lateral mispositioning of dipping reflectors and ''collapsing'' diffraction curves to zones defined by the diffraction apex, migration converts the recorded waves to a subsurface picture ([[:file:seismic-migration_fig4-part2.jpg|Figure 4b]]) depicting both broadly and tightly folded anticlines and synclines.

==How migration is accomplished==

==How migration is accomplished==