Dip and offset together

Gulf of Mexico example

Recall the Gulf of Mexico dataset presented in chapter . We did a reasonably careful job of NMO velocity analysis in order to produce the stack shown in Figure . But is this the best possible stack? To begin to answer this question, Figure 8.14 shows some constant-velocity stacks of this dataset done with subroutine velsimp() . This figure clearly shows that there are some very steeply-dipping reflections that are missing in Figure . These steep reflections appear only when the NMO velocity is quite high compared with the velocity that does a good job on the horizontal reflectors. This phenomenon is consistent with the predictions of equation (8.12), which says that dipping events require a higher NMO velocity than nearby horizontal events.

cvstacks Figure 14. Stacks of Gulf of Mexico data with two different constant NMO velocities. Press button to see a movie in which each frame is a stack with a different constant velocity.

Another way of seeing the same conflict in the data is to look at a velocity-analysis panel at a single common-midpoint location such as the panel shown in Figure 8.15 made by subroutine velsimp() . In this figure it is easy to see that the velocity which is good for the dipping event at 1.5 sec is too high for the horizontal events in its vicinity.

velscan Figure 15. Velocity analysis panel of one of the panels in Figure 8.14 before (left) and after (right) DMO. Before DMO, at 2.2 sec you can notice two values of slowness, the main branch at .5 sec/km, and another at .4 sec/km. The faster velocity is a fault-plane reflection.

Dip and offset together