Subsalt imaging in the Gulf of Mexico
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| 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 |
The second 2-D case study for structural inversion is from the Gulf of Mexico. Figure 10.2-1 shows the DMO-stacked section and Figure 10.2-2 shows the poststack time-migrated section of a 2-D data set from the Gulf of Mexico. The high-amplitude event (such as event A in Figure 10.2-1) with complex traveltime is the top-salt reflection. Conflicting dips associated with the fault blocks within the overburden and the rugose top-salt boundary are preserved by way of DMO correction (Figure 10.2-1), and accurate imaging of the suprasalt region can be achieved by poststack time migration (Figure 10.2-2). Nevertheless, accurate imaging of the base-salt boundary and the subsalt region is only possible by way of prestack depth migration.
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Figure 10.2-1 The Gulf of Mexico line: unmigrated DMO-stacked section. (Data courtesy Schlumberger Geco-Prakla.) -
Figure 10.2-2 The Gulf of Mexico line: poststack time-migrated DMO-stacked section.
Aside from the water layer, a Gulf of Mexico velocity-depth model is typically represented in two parts:
- A background velocity field with vertical velocity variations characterized by gentle variations in the gradient, the absence of distinct layer boundaries, and mild-to-moderate lateral velocity variations.
- Tabular and diapiric salt bodies with different shapes, but with a constant velocity of 4450 m/s, embedded into the background velocity field.
To estimate a velocity-depth model for a Gulf of Mexico structural target below the tabular salt bodies, the following procedure composed from the list of inversion methods in Table 9-1 is used:
- Dix conversion of stacking velocities to estimate the background velocity field,
- Model updating and verification of the velocity field within the suprasalt region (model updating),
- Poststack or prestack depth migration to delineate the top-salt boundary,
- Assignment of the salt velocity into the half-space below the top-salt boundary,
- Prestack depth migration to delineate the base-salt boundary,
- Assignment of the the background velocity into the half-space below the base-salt boundary (the subsalt region),
- Prestack depth migration to obtain and verify the final earth image in depth.
| Layer Velocities | Reflector Geometries |
| Dix conversion of rms velocities | vertical-ray time-to-depth conversion (vertical stretch) |
| stacking velocity inversion | image-ray time-to-depth conversion (map migration) |
| coherency inversion | poststack depth migration |
| image-gather analysis | prestack depth migration |
In this case study, we shall first test the procedure used for the Southern Gas Basin line to estimate a layered earth model in depth for comparison with the procedure outlined above.
See also
External links
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