Geoscience Reference
In-Depth Information
is often possible to observe this effect on seismic sections, manifested as an amplitude
maximum at a particular apparent bed thickness. Sometimes it may be necessary to
model this effect and allow for it when interpreting amplitude variation. For bed thick-
ness less than the maximum of the tuning curve, it may be possible to map bed thickness
using the linear amplitude-thickness relation, although this will work only if there is no
lateral variation in acoustic impedance of the bed and the material encasing it. Accurate
thickness prediction also depends on using a correct tuning curve, which in turn depends
on having an accurate estimate of the wavelet present in the data.
5.4
AVO analysis
of
R
0
and
G
, the normal incidence reflectivity (sometimes referred to as
intercept
) and
gradient values defined in
section 5.2
.
Figure 5.8
shows typical responses for different
shale-sand interfaces (i.e. typical top reservoir in a clastic sequence); a shale-sand
interface usually exhibits a negative gradient, i.e. the reflection coefficient becomes
more negative with increasing offset. Class I response is characterised by an increase
in impedance downwards across the interface, causing a decreasing amplitude with
increasing incidence angle. Class II response has small normal incidence amplitude
Classes of AVO Response
Impedance
Contrast
+
Model originally based on
shale/brine sand interface
R
0
Class I
AVO curves can be
generated from:
seismic gathers
single interface models
synthetic gathers
Class IIp
θ
(deg)
10
20
30
Class II
Class III
Class IV
−
