3-D migration

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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

By using a smooth version of the 3-D velocity field (Figure 7.2-15) derived from the 3-D DMO-corrected data, the 3-D DMO-stack volume is then time-migrated. Figure 7.2-17 shows the selected crosslines as in Figure 7.2-16 before and after 3-D poststack time migration (see 3-D poststack migration for 3-D poststack migration). Following 3-D migration, the folded, imbricate structure has been delineated. Note, however, to the left of inline location 200, the left flank appears to terminate abruptly along a spurious fault line that dips down from left to right (see, for instance, crossline X-200). Recall the discussion on migration and line length (further aspects of migration in practice) and examine closely this zone on crossline X-260. Where there is no image of the folded structure to the left of inline location 100, there exist the trails of the migration paths — the wavefronts. So, the incomplete image of the left flank of the folded structure (the no-event zone to the left of inline location 200) is because of missing data — insufficient areal extent of the survey to the left of the imbricate structure.

We now turn our attention to the overthrust fault to the right of the imbricate structure. Refer to the left of inline location 200 on crossline X-260. Again, we observe a no-event zone — but, this one arises for a different reason. The culmination along the overthrust fault has caused significant ray bending to the extent that no data were recorded at the surface. Alternatively, we may be dealing with turning rays along the overthrust fault, which were not accounted for in migrating the data.

Figure 7.2-18 shows the selected inlines before and after 3-D poststack time migration. Note on inline I-80, there is an absence of events below 2 s after 3-D migration. This precisely corresponds to the missing data zone on the crosslines after migration (Figure 7.2-15, see for instance crossline X-260, left of inline location 200). We also observe a contrasting situation — events appear on migrated data in zones where no event was present on the unmigrated data (see for instance inline I-200, the zone below 3 s). As a result of 3-D migration, events move in and out of cross-sections in the inline and crossline directions. Lack of events after 3-D migration should not be alarming — they just moved to another location outside the plane of the cross-section of migrated data under consideration.

Figure 7.2-19 shows selected time slices from the unmigrated 3-D DMO-stack volume of data. Compare with the time slices from the 3-D poststack time-migrated volume of data shown in Figure 7.2-20. Note, for instance from the 2200-ms time slice of the migrated data, the delineation of the thrusted fault zone. Also, recall from migration principles that anticlines appear bigger than their actual sizes on unmigrated data. As such, compare the 2200-ms time slices from the unmigrated data (Figure 7.2-19) and the migrated data (Figure 7.2-20) and note that the contours in the latter bounded by the fault trend are narrowed, indicating an anticlinal, imbricate structure.

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3-D migration
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