Geology Reference
In-Depth Information
Offset (m)
2
1
0
3710
0
10000
.5
0
R
0
1
-10000
1.5
15
20
5
-20000
10
2.0
-30000
2.5
Sin
2
θ
Figure 2.27
Deriving intercept and gradient. Offsets have been
converted into sin
2
3.0
using a velocity model prior to fitting a robust
linear regression through the data.
θ
Figure 2.25
A gather displayed with incidence angle as coloured
background.
calculation of intercept and gradient, using the two-
term AVO equation. As suggested in the previous
section, care must be taken that for the angle range in
question a two-term fit is appropriate. Before calculat-
ing intercept and gradient, some data conditioning
may be needed as it is important that reflections are
flat across the moveout corrected gather (
Chapter 6
).
An alternative to intercept/gradient calculation is
to create angle stacks (i.e. stack the data according to
angle ranges). For the purposes of seismic interpret-
ation the available angle range is usually divided into
two or three equal parts and separate stacks created
for each. This maintains reasonable signal-to-noise
ratio whilst maintaining the key elements of the
AVO response. Typically, therefore, the interpreter
will have near, mid and far sub-stacks in addition to
the conventional full stack. It should be noted that the
full stack is seldom the product of adding together
the partial stacks. In most instances the full stack is
the processor
Figure 2.26
Converting angle to offset using well velocity data;
the simplest case is for a horizontal layered model and a straight ray
assumption.
accuracy of the offset to angle conversion is often a
significant factor for generating partial stacks and for
AVO analysis in general.
Once the offsets have been converted to angles
then the data in the corrected gathers can be com-
bined in a number of ways.
Figure 2.27
illustrates the
s best data quality stack and frequently
has a harsher mute of the outer traces of the gather
than the far stack. There may be instances where
more than three sub-stacks are generated, for example
in simultaneous inversion (
Chapter 9
).
'
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