Subsalt imaging in the Gulf of Mexico
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.
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.