The final stage in earth modeling and imaging is the verification of the accuracy of the model itself (model updating). For an earth model in depth to be acceptable, it has to pass the following two tests:
- Image gathers from prestack depth migration using the earth model in question must exhibit flat events (Figures 10.1-10a and 10.1-11a). Events associated with multiples and converted waves (4-C seismic method) are not expected to be flat. Nevertheless, even with good models, usually, there also are some primary events that do not meet the flatness criterion. Violation of this criterion may occur because of erroneous layer velocities or 3-D effects that are not accounted for by 2-D modeling and imaging. Other sources of departures from flatness include strong lateral velocity variations that are much less than a cable length and effect of anisotropy on layer velocities.
- For an earth model in depth to be an acceptable representation of the subsurface geological model, it must be consistent with the seismic data used to estimate the model in question. To check for consistency, perform ray-theoretical modeling of zero-offset traveltimes associated with the layer boundaries included in the model (Figure 10.1-14). Then, superimpose the modeled traveltimes on the CMP-stacked section and observe any discrepancy between the modeled and the actual traveltimes (Figure 10.1-15). Here, we are assuming that the reflection traveltimes observed on a CMP-stacked section can be closely approximated by two-way zero-offset traveltimes. Actual traveltimes interpreted from the unmigrated CMP-stacked section are shown in Figure 10.1-3.
Figure 10.1-10 The Southern Gas Basin line: (a) selected image gathers from the left-half of the line as in Figure 10.1-1 from prestack depth migration using the final velocity-depth model in Figure 10.1-9; (b) stack of the image gathers — depth image from prestack depth migration for the left-half of the line as in Figure 10.1-1. See text for the labeled events.
Figure 10.1-11 The Southern Gas Basin line: (a) selected image gathers from the right-half of the line as in Figure 10.1-1 from prestack depth migration using the final velocity-depth model in Figure 10.1-9; (b) stack of the image gathers — depth image from prestack depth migration for the right-half of the line as in Figure 10.1-1. See text for the labeled events.
Aside from these two criteria, the estimated earth model needs to be validated by examining it for consistency with the structural model applicable to the area of interest. For instance, the faulting and folding implied by the the model must have the same patterns as in the true subsurface situation. Calibration to well tops is also part of the model validation procedure (model building).
- Subsalt imaging in the North Sea
- Estimation of the overburden model
- Estimation of the substratum model