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Events with a slow moveout velocity (10004000 ft/s)
are typically unwanted noise such as surface waves.
Their velocity is usually well separated from the apparent velocity
of deep reflections so we can filter them out in a domain in which they are
well separated from one another, namely the FK (i.e.,
frequencywavenumber) domain.
As an example, assume the data d(x,t)
d(x,t) 
= 

(1.8) 
consist of two linear events, one moving out
with a slow velocity v_{slow} and the other with a fast velocity
v_{fast}. The xintercepts are denoted by x_{0} and x_{0}'.
In the (x,t) domain the two linear events cross one another
and so it is difficult to mute one entirely from the other.
However, under an FK Fourier transform the above equation becomes:

= 

(1.9) 
where a and b are phase terms. It is clear the
transformed data describe two slanted lines that do not cross
each other except at the origin. Thus muting one line
from the other based on their
different slopes (i.e., velocities) is trivial in the FK domain.
The above procedure is called velocity filtering.
As a field data example,
Figure 1.15 shows a CSG before and after velocity filtering
to remove the steep surface waves.
Figure 1.15:
CSG seismograms before (LHS) and
after (RHS) FK velocity filtering to eliminate surface waves.

Next: Normal Moveout Correction to
Up: Basic Processing Steps
Previous: Static Corrections to Remove
Gerard Schuster
19980729