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.
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.