Subsalt imaging in the North Sea
The first 2-D case study for structural inversion is from the Southern Gas Basin of the North Sea. Figures 10.1-1 and 10.1-2 show selected CMP gathers from the two segments of the line accompanied by the corresponding CMP stacked sections. The left segment (Figure 10.1-1) contains a salt diapir with relatively steep flanks and the right segment (Figure 10.1-2) contains a salt diapir which has a broader base. The Zechstein formation comprises a halite unit with a velocity of 4400 m/s and anhydrite-dolomite rafts of various sizes and shapes with a velocity of 5900 m/s. The strong velocity contrast across the top-salt boundary and the presence of the anhydrite-dolomite rafts within the diapiric formation itself give rise to raypath distortions and thus constitute a complex overburden structure. The effect of the complex overburden on the underlying target zone — base Zechstein and the underlying Carboniferous sequence, is evident on the CMP gathers and the CMP stacks. Specifically, note events with complex moveout on CMP gathers 481-881 below 1.5 s (Figure 10.1-1) and the traveltime distortions along the base-Zechstein reflection on the CMP stack (events A, B, C, and D in Figures 10.1-1, and events E, F, G, H, and K in Figure 10.1-2).
The subsurface geology in the Southern Gas Basin can be represented by a layered earth model that consists of distinct layer boundaries with significant velocity contrast. Also, there exist vertical velocity gradients in layers above the salt formation. A typical velocity-depth model for the Southern Gas Basin may be considered in two parts:
- An overburden above the salt layer with some faulting where time migration is applicable, and ray-paths associated with nonzero-offset traveltimes yield a moveout behavior that makes it possible to estimate layer velocities with sufficient accuracy using Dix conversion, stacking velocity inversion or coherency inversion, and
- a substratum that includes the salt layer and the layers beneath, where depth migration is imperative.
The boundary between the overburden and the substratum is defined by the top of the salt layer where the most severe ray bending takes place. To estimate a velocity-depth model for a typical Southern Gas Basin subsalt target, the following procedure composed from the list of inversion methods in Table 9-1 can be used:
- Coherency inversion to estimate layer velocities and 2-D poststack depth migration to delineate reflector geometries within the overburden, and
- constant half-space velocity analysis of image gathers from prestack depth migration to estimate the substratum velocity and stacking of image gathers to delineate the reflector geometry of the base-Zechstein (top-Rotliegendes) target horizon.
This procedure is applied layer-by-layer starting from the surface to resolve layer velocity and reflector geometry of one layer before moving onto the next (model building). Such an approach enables us to establish the accuracy of the model one layer at a time and minimizes accumulation of errors as we proceed down to the target zone.