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Basics of Seismic Migration
Seismically migrated data, or "migration data," can refer to data that has gone through a number of different types of reanalysis. Done properly, seismically migrated data accounts for the ways that rock formations under the surface interact with seismic waves and one another, allowing the waves to pass at different rates and at different angles. Correcting for this allows an expert viewing the migration data to determine where non-rock formations, such as hot water wells or oil, are located.
Methods of Seismic Migration
The basic principle behind seismic migration is that pockets of interesting substances -- such as hot water, oil reserves, or a gas vein -- can be detected in data collected from a seismic reflection survey. A seismic reflection survey is the type of survey described in the introduction -- sending seismic waves into the earth and recording where and how they return. Then a number of seismic migrations must be weighed to determine which would be most appropriate and effective for the degree of resolution and accuracy necessary.
Factors Affecting the Choice of Seismic Migration Technique
The complexity of the rock formations under the surface is the most important factor that determines how complicated a seismic migration technique should be. Because seismic migration principally shifts echoes heard at the surface to where they are most likely coming from, waves that transition between different types of rock will be altered in distinct ways. "Migration" refers to the fact that the echoes are "migrated" to more true positions in a cross-section of the earth. So, the more distinct rock formations that are in a locale, the less linear the interfaces between the formations; and the greater the differences in the density of the different formations, the more complicated the seismic migration algorithm required.
Types of Seismic Migration
Kirchhoff, phase-shift and full-wave migration are among the most common types of seismic migration. Major distinctions between different types of seismic migration include whether they are applied before or after other data analysis and adjustment techniques are applied, and whether they adjust in space or time for certain formations. Time migration techniques factor in how different rock formations would affect the speed at which a seismic wave travels, while space migration techniques mostly focus on how interfaces between rock formations affects the angle at which a seismic wave travels.