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4: Migration

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Published:
January 01, 2001

4.0 INTRODUCTION

Migration moves dipping reflections to their true subsurface positions and collapses diffractions, thus increasing spatial resolution and yielding a seismic image of the subsurface. Figure 4.0-1 shows a CMP-stacked section before and after migration. The stacked section indicates the presence of a salt dome flanked by gently dipping strata. Figure 4.0-1 also shows a sketch of two prominent features – the diffraction hyperbola D that originates at the tip of the salt dome, and the reflection B off the flank of the salt dome. After migration, note that the diffraction has collapsed to its apex P and the dipping event has moved to a subsurface location A, which is at or near the salt dome flank. In contrast, reflections associated with the gently dipping strata have moved little after migration.

Figure 4.0-2 is an example with a different type of structural feature. The stack contains a zone of near-horizontal reflections down to 1 s. After migration, these events are virtually unchanged. Note the prominent unconformity that represents an ancient erosional surface just below 1 s. On the stacked section, the unconformity appears complex, while on the migrated section, it becomes interpretable. The bowties on the stacked section are untied and turned into synclines on the migrated section. The deeper event in the neighborhood of 3 s is the multiple associated with the unconformity above. When treated as a primary and migrated with the primary velocity, it is overmigrated.

Figure 4.0-3a shows a stacked section that contains fault-plane reflections conflicting with the shallow gently-dipping reflections. Note the accurate positioning of the fault planes and delineation of the fault blocks on the migrated section in Figure 4.0-3b. From the three examples shown in Figures 4.0-1, 4.0-2, and 4.0-3, note that migration moves dipping events in the updip direction and collapses diffractions, thus enabling us to delineate faults while retaining horizontal events in their original positions.

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Investigations in Geophysics

Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data

Öz Yilmaz
Öz Yilmaz
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Society of Exploration Geophysicists
Volume
10
ISBN electronic:
9781560801580
Publication date:
January 01, 2001

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