post-stack seismic migration
TRANSCRIPT
8/6/2019 Post-stack Seismic Migration
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POST-STACK MIGRATION –TIME DOMAIN
What is migration?
Migration is a process which removes the effects of wave propagation from seismic
data.
Geological Model Seismic Section Section after migration
Migration is the process that moves the data on our stacked seismic section into itscorrect position in both space and time. Even after NMO and DMO corrections,reflections from dipping events are plotted on stack section in the wrong place. Theyneed to be moved "up-dip" along a hyperbolic curve in order to put them in the rightplace, the shape of this hyperbola depending on the velocity field. Before we start tolook at some of the methods of migration, let's look at the appearance of somesimple structures on our stacked section.
Seismic data is generated by waves propagating through a subsurface. The imagethat is obtained in this process is a distorted image that does not correctly reflect thetrue geometry of the subsurface structure. While a horizontal reflector in depth willappear as a horizontal reflector on the time section, a dipping reflector is alwaysincorrectly positioned on the seismic section. It is the task of migration to correct thismispositioning. This effect is demonstrated in Fig. 2. The true position of the reflectorin depth differs from its position on the time section.
correct
Wrong
correct
Wrong
Fig.2 The apparent dip of an event on a zero offset stack section is less than the true dip of the event.
8/6/2019 Post-stack Seismic Migration
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S S S R R R
CMP
Fig.3 showing flat horizon
S = Source
R = Receiver
CMP= Common Mid Point
We assume a horizontal reflector is flat with a homogeneous medium and constant
velocity. In this case the CDP is positioned on the CMP or zero offset two way travel
time. See Fig.3.
CMP
Fig.4 showing dipping horizon
At a dipping horizon, the CMP gather will have a spread of reflection point. Thelateral and vertical positions are different, as well as the dip of the reflector. A dippingreflector is always incorrectly positioned on the seismic section. See Fig.4.
The migration process removes the effect of wave propagation from the recordedseismic data, and as a result, seismic events move toward their correct subsurfaceposition.