Further aspects of migration in practice

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


In this section, we shall discuss the effects of spatial aliasing, random noise and profile length on migration, and migration from topography. Spatial aliasing is a direct result of undersampling of recorded data (the 2-D Fourier transform). Because of spatial aliasing, migration can perceive events with steep dips at high frequencies as different from the actual dips in ms/trace. As a result, migration mispositions the aliased frequency components of the dipping events.

Random noise usually is more prominent in the deep part of a stacked section, just where velocities also are generally higher. This results in random noise organized along wavefront arches, commonly referred to as smiles. This organized noise corrupts the migrated primary energy not just in the deep part of the section but also has detrimental effect on shallow data in a migrated section.

Figure 4.1-1  Migration principles: The reflection segment C′D′ in the time section as in (b), when migrated, is moved updip, steepened, shortened, and mapped onto its true subsurface location CD as in (a). Adapted from [1].

Line length and location of the line traverse at the surface relative to the location of the target in the subsurface have a direct effect on the useability of the a migrated section. Usually a line traverse longer than the spatial extent of the subsurface target is needed (Figure 4.1-1). Keep in mind that your target does not necessarily lie directly beneath the CMP location where the reflection from that target appears on your unmigrated stacked section.

Irregular topography associated with areas subjected to overthurst tectonics has to be accounted for during migration if surface elevation changes are rapid along the line traverse. Migration algorithms, with the exception of the Kirchhoff summation and the constant-velocity Stolt method, are all based on wave extrapolation from one flat depth level to another. A CMP-stacked section is assumed to be equivalent to a zero-offset wavefield and usually is referenced to a flat datum. In the presence of severe topography, one needs to account for the difference between the elevation profile and the reference datum. Otherwise, if the reference datum is above the surface elevation, to a migration algorithm, events appear deeper than they are, and thus are overmigrated. If, on the other hand, the reference datum is below the surface elevation, events appear to a migration algorithm shallower than they actually are, and thus are undermigrated.

References

  1. Chun and Jacewitz, 1981, Chun, J.H. and Jacewitz, C., 1981, Fundamentals of frequency-domain migration: Geophysics, 46, 717–732.

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Further aspects of migration in practice
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