Geology Reference
In-Depth Information
OWC - Oil sand/brine sand
Sin
2
-0.5
-0.25
0
0.25
0.5
0.75
1.0
1.25
Range of linear
observations
Shale/brine sand
Range over which sin
2
θ
is physically defined
Shale/oil sand
Figure 5.52
Linearized (two-term) AVO (modified after Whitcombe
et al
.,
2002
).
5.5.1 AVO projections, coordinate rotations
and weighted stacks
Linearised AVO enables the construction of a seismic
dataset at any angle using Shuey
linearised (two-term) approximations (i.e. based on
Shuey
sequation)(
Chapter 2
). Linear combinations
of intercept and gradient describe a continuum of
rock and fluid effects, forming the basis for a set of
readily useable tools that aid fluid and rock dis-
crimination from seismic. In the 1980s and 1990s
various workers combined intercept and gradient
sections in different ways to create a variety of
AVO attributes. Examples of these included the
'
'
'
s equation R
¼
R
0
+
G sin
2
(
Fig. 5.52
). For example, an intercept/gradi-
ent projection at an angle of incidence of 30° (i.e. R
θ
¼
R
0
+ G sin
2
30°) will look similar to an angle stack
with an effective angle of 30°, although they will not
be exactly the same owing to the different effects of
noise in the two datasets. The value of projections,
however, is in generating seismic displays beyond the
acquired angles in order to exploit differences in
AVO.
Figure 5.52
illustrates the concept. Three separate
AVO responses are shown, typical of a Class IIp oil
sand scenario. The amplitude difference of the pro-
jected responses for the oil sand and wet sand
increases with increasing angle. It also shows how
the difference diminishes with increasing negative
angle (of course negative angles do not exist in reality;
they are simply a graphical construct). A simple 2D
attribute (Smith and Gidlow,
1987
),
pseudo-Poisson
fluid factor
'
s reflectivity (Verm and Hilterman,
1995
), R
p
-R
s
(Castagna and Smith,
1994
). At the
time it was thought that there may be a universal
AVO indicator for fluid identification from seis-
mic. However, over time it was realised that most
of the
'
products can be understood as
representing particular angle projections or coord-
inate rotations on the AVO crossplot (e.g. Smith,
2003
) and that AVO is an adaptive technique. This
section describes the mechanics of linearised AVO
in terms of both reflectivity and angle dependent
'
'
hard wired
'
93
impedance
'
.
