Geology Reference
In-Depth Information
a)
b)
Figure 2.2
Marine seismic geometry 2; (a) acquisition; each shot is recorded at a variety of receivers depending on the depth and the angle
of reflection and (b) the common midpoint (CMP); if it is assumed that the Earth is flat the data can be arranged according to reflection
location, i.e. different source-receiver pairs sampling the same position in the subsurface. For more complex velocity overburden sophisticated
imaging solutions are required.
Offset
Offset
Apply gain function to remove effect
of wavefront divergence
Remove noise and mute out
unwanted data
Apply spatial geometric corrections
(pre-stack migration)
Offse
t
Offse
t
Outer mute
Calculate velocities to flatten gather
(Moveout)
Raw gather
Corrected and
processed gather
Figure 2.3
Key steps in the processing of seismic gathers.
because the next step will be to stack the data by
summing the traces of the gather along lines of con-
stant time, i.e. along horizontal lines in the display of
Fig. 2.3
. This has the important effect of enhancing
signal and suppressing noise. Accurate horizontal
alignment across the gather is also important for the
study of amplitude variation with offset
described later in this chapter and in
Chapter 5
. The
process of time-shifting to flatten the reflections is
called moveout correction. A commonly used term is
normal moveout (NMO), which refers to the specific
case where there is no dip on the reflector.
Figure 2.4
illustrates a stacking methodology that
is popular for seismic AVO analysis. Seismic sections
5
(AVO)
