Geology Reference
In-Depth Information
Zero offset reflectivity
Offset reflectivity
AI
2
-AI
1
AI
2
+AI
1
EI
2
-EI
1
EI
2
+EI
1
Rc=
Rc =
AI
EI
TWT
+
TWT
Amp
AI
0
EI
Sin
2
θ
TWT
TWT
-
Figure 5.61
The concept of elastic impedance.
Owing to the fact that the general magnitude of elastic
impedance as initially defined varies with angle,
Whitcombe (
2002
) formulated it by normalizing with
average values of V
p
, V
s
and density. Thus visual
comparisons can readily be made between EI logs
calculated at different values. Normalised EI
An illustration of the relationship between EI and
reflectivity is shown in
Fig. 5.62
. A synthetic gather
comprising five traces in the range
40° is
plotted together with the corresponding two-term
elastic impedance curves.
Whitcombe et al.(
2002
) subsequently approached
angle-dependent impedance from an AVO perspec-
tive (i.e. by using
θ ¼
0
-
is
defined as:
χ
for AVO projections rather
2
4
3
5
0
@
1
A
0
@
1
A
0
@
1
A
a
b
c
than
). Just as elastic impedance relates to two-term
Shuey reflectivity, there is a corresponding impedance
for the modification of Shuey
θ
V
p
V
p0
V
s
V
s0
ρ
ρ
0
EI
ðÞ¼
V
p0
ρ
0
,
ð
5
:
22
Þ
s equation (
Eq. (5.23)
).
Given that the modification was designed to extend
the angular range of Shuey
'
where
V
p0
¼
s equation this impedance
parameter is called extended elastic impedance (EEI)
and is defined as:
'
average V
p
,
V
s0
¼
average V
s
and
ρ
0
¼
average
ρ
.
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