Geology Reference
In-Depth Information
a)
b)
Zoeppritz gather
Seismic gather
0º
55º
50ms
Figure 6.5
Synthetic and corresponding seismic gather. The synthetic shows a Class IIp AVO response in the upper part, in broad agreement
with the real data; in the lower part the reflections are nearly horizontal in the synthetic but show residual moveout in the real data.
(
Chapter 5
). For example,
can be estimated itera-
tively from the misties between a depth migrated
image and the true depth as measured in a well (Jones,
2010
), and
δ
X
2
V
nmo
X
4
V
nmo
T
0
V
nmo
+ 1+2
2
η
T
x
¼
T
0
+
,
ð
6
:
3
Þ
X
2
ð
η
Þ
η
'
where
is given in terms of Thomsen
s anisotropy
ε
can then be estimated from the combin-
parameters (see
Chapter 5
)by
ation of
functions
derived from seismic are usually quite generalised and
not usually applicable to the seismic amplitude mod-
elling of specific target horizons. There may also be
ambiguity regarding how much of the
η
and
δ
. In practice, the
ε
and
δ
η ¼
ε δ
1+2
ð
Þ
6
:
4
δ
and
estimation is
simply accommodating the effects of vertical velocity
gradients. The reader is referred to an excellent treat-
ment on velocity issues in migration by Jones (
2010
).
η
p
1+2
V
nmo
¼
V
P0
δ
,
ð
6
:
5
Þ
where V
p0
is the P wave vertical velocity.
In time processing
is usually derived from maxi-
mising stack semblance after initial velocity estima-
tion (e.g. Toldi et al.,
1999
).
Figure 6.6
shows an
example where the AVO response of gas sands is seen
at offsets large enough to require anisotropic moveout
corrections.
In principle, a combination of time and depth
migration can provide information on the anisotropic
parameters
η
6.2.6 Final scaling
The application of modern pre-stack migration and
demultiple techniques usually results in pre-stack
gathers that have reasonable amplitude scaling with
offset. In some instances, however, particularly in land
data, an evaluation of the moveout corrected gathers
reveals gross scaling differences between sets of traces
116
δ
and
ε
that can modify the AVO response
