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into a single stacked trace which approximates a zero-offset
recording made at
0
1000
Metres
s midpoint ( Fig. 6.20c ) .
A time shift is applied to each trace so that the re ected
arrival is shifted to the equivalent zero-offset travel time
(T 0 ). Since the time shift is equal to the NMO (see Section
6.5.1.1 ) this is called the normal moveout correction. After
the appropriate corrections have been made to each trace,
they are summed together; this is the
the gather
'
a)
Trace number
1299
1320
1344
1364
1384
1404
1424
Source
0.0
Direct and
refracted arrivals
Noisy traces
0.5
'
'
process
itself ( Fig. 6.20d ) . Any random noise present in the indi-
vidual traces comprising the CMP gather will tend to
cancel (see Section 2.7.4.1 ) and the reflected arrival,
common to all traces, will be enhanced. Also, arrivals with
moveouts different to the primary reflections (noise) will
be suppressed.
stacking
Surface waves
1.0
Air wave
Raw data
1.5
b)
0.0
Correcting for normal moveout
Key to stacking is the accurate determination of the NMO
correction. Recall that the travel times of re ections can be
de ned in terms of a constant component (T 0 ) and a
normal moveout (NMO) component (see Section 6.5.1.1 ) .
The NMO component depends on T 0 , the depth to the
re ector (Z); the root-mean-square velocity (V rms ), which
is a function of the overlying interval velocities; and the
source
0.5
1.0
+Trace editing, muting, frequency filtering & deconvolution
1.5
detector offset (X). The problem when seeking to
make the NMO correction is that only the offset is known
(if the other parameters were known then the seismic
survey would be unnecessary, an example of the geophys-
ical paradox mentioned in Section 1.3 ) . This apparently
insurmountable problem is addressed by using a combin-
ation of two well-established scientific techniques:
-
c)
0.0
0.5
'
trial
and error
'
, and a preconception about the
'
right
'
answer
-
a
1.0
strategy that can be described as
.
Figure 6.21a and b shows a hypothetical CMP gather for
a subsurface comprising three (horizontal and planar)
re
' 'fitting a model to the data
'
+ Static corrections
1.5
ecting interfaces. The interfaces separate layers that
have constant velocity, increasing with each deeper layer,
i.e. V 3
d)
0.0
). As
shown in Section 6.5.1.1 , the moveout of the re ections is
less when the interface is deeper. In order to effectively
stack the traces making up the CMP gather, it is necessary
to flatten each of the three arrivals, but a different time
shift needs to be applied to different parts of each trace
because the various re ections have different moveout.
There is a variety of ways to determine the NMO cor-
rection. The method we describe here has the advantage of
being conceptually simple and so is a useful means of
demonstrating the concepts involved. The method is based
on the creation of constant velocity scans of the CMP
gathers. These are created by determining the NMO
>
V 2
>
V 1 , and different acoustic impedance (
ΞΆ
0.5
Reflections
Reflections
1.0
+ Muting of 1st arrivals
1.5
Figure 6.19 Pre-stack processing of a shot gather from the
Kristineberg area. (a) Raw data with labels indicating various types of
arrival constituting noise; (b - d) data after successive processing
operations. Redrawn, with permission, from Ehsan et al.( 2012 ) .
 
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