Geoscience Reference
In-Depth Information
distance
Depth Model
distance
Stacked Section
distance
Migrated Section
Fig. 2.32
Seismic migration moves events from their recorded position to their true subsurface
position. This diagram shows the effect of migration on a syncline. The rays are reflected at right
angles to the surface. It can be seen that the blue rays from the right hand of the syncline are
sometimes recorded to the left of the green rays reflected from the left side of the syncline. This
gives a complicated bow tie pattern in the corresponding time section. The effect of migration is to
move the events back to their true subsurface location.
earliest forms of migration were based on an approximation to the wave equation and
ignored ray-bending at velocity boundaries. Today such techniques are known as time
migration methods. In areas of lateral velocity contrasts the straight ray approximation
of time migration can seriously misposition events. The solution to this problem is depth
migration, which correctly handles velocity variation. Of course, there is the additional
complication of deriving the velocity model needed to drive depth imaging and this
is another active area of research and development. Most methods require an iterative
approach. An assumed velocity-depth model is used, the data are migrated pre-stack
using this model, and the images across the migrated CMP gather are compared. If
the model velocity is too high then the further offsets will have the event positioned
too deep compared with the near offsets, and conversely if the velocity is too low the
events will be imaged shallower on the far offsets than on the nears. Only if the velocity
