Migration and random noise

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Seismic Data Analysis
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

Figure 4.6-32 shows a section that contains band-limited random noise uncorrelated from trace to trace and its migration using the phase-shift method. Velocity increases linearly from 2000 m/s at the top to 4000 m/s at the bottom of the section. The amplitude and frequency characteristics of the input section in Figure 4.6-32 are virtually unchanged in the interior portion of the migrated section. However, note the smearing of amplitudes at the bottom and side boundaries after migration.

Ambient noise commonly dominates the deep portion of a stacked section where velocities are high. Therefore, organization of random noise caused by migration generally is more severe in the deeper part of a stacked section. A field data example is shown in Figure 4.6-33. In addition to smearing effects, the migrated section also has smiles, which are caused by sparsely distributed bursts of amplitude in the input section. Keep in mind that a single spike on the time section migrates to a semicircle on the depth section.

We already have seen the adverse effect of an improper choice of aperture width in Kirchhoff summation (Figure 4.2-7). A narrow aperture can introduce strong smearing as spurious, nearly horizontal events. A similar effect occurs for all types of migration algorithms if the maximum dip to migrate is severely restricted (Figure 4.5-4). It is a misconception to imagine that migration attenuates random noise and improves signal-to-noise ratio. Instead, one must keep in mind that migration organizes random noise, it does not attenuate it. A dip-limited migration algorithm acts upon the random noise like a dip filter and removes the noise energy beyond the dip limit much like shown in Figure 6.2-2. The dip-limited algorithm also attenuates unaliased linear noise with a dip steeper than the dip limit.

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Migration and random noise
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