Geology Reference
In-Depth Information
Rock properties and AVO
Chapter
5
5.1 Introduction
This chapter is an overview of how rock properties
relate to seismic amplitude and AVO. Much of the
information presented is the basis for further discus-
sion of rock physics (
Chapter 8
), seismic inversion
(
Chapter 9
) and approaches to reservoir characterisa-
tion (
Chapter 10
). The principal idea is to show the
reader how different types of information fit together.
to describe the case where there is an increase in the
absolute value of the amplitude with increasing offset
(or angle). Thus, positive AVO is applied to either
positive or negative amplitudes which are increasing
in magnitude with angle. Accordingly, negative AVO
describes the case where the absolute value of the
amplitude decreases with increasing offset (or angle).
5.2.2 AVO classes and the AVO plot
At the next level of detail, AVO response is convention-
ally described in terms of a number of classes. Ruther-
ford and Williams (
1989
) first classified shale/gas sand
interface responses into three types (I, II and III). Class
I responses are characterised by a positive impedance
contrast (i.e. the sand impedance is larger than the
shale impedance), together with a negative AVO gra-
dient, so that the reflection coefficient is positive and
decreases with angle. Class II responses have small
normal incidence reflection coefficients (which may
be positive or negative) and a negative gradient so that
the AVO effect leads to large negative reflection coeffi-
cients at far offsets. Ross and Kinman (
1995
) suggested
that the small positive normal incidence coefficient
Class II responses should be termed Class IIp, owing
to the phase reversal that is inherent in the response,
and that the term Class II should be reserved for the
small negative normal incidence coefficient case. Class
III responses have large negative impedance contrasts
and a negative gradient, leading to increasing ampli-
tude with angle (
Fig. 5.2
). There appears to be no
precise definition of where the boundary lies between
Class II and Class III responses. It depends on what is
meant by
5.2 AVO response description
There are a number of terms that are generally used to
describe AVO responses. Unfortunately,
they can
easily create confusion for the non-expert.
5.2.1 Positive or negative AVO and the
sign of the AVO gradient
First of all, there is an important distinction to be made
between
and positive (or
negative) AVO gradient. As discussed in
Chapter 2
, the
AVO gradient is calculated as the slope of the change in
amplitude with sin
2
'
positive (or negative) AVO
'
. On the AVO plot, negative AVO
gradients are inclined from upper left to lower right,
whereas positive gradients are inclined from upper
right to lower left (
Fig. 5.1
). On the other hand positive
AVO (also sometimes called rising AVO) is a term used
θ
+ve AVO
+ve gradient
+
A
I
-ve AVO
-ve gradient
normal incidence reflection coeffi-
cient in the definition of Class II. Typically, Class II
might be applied to the case where the amplitudes on
the nearest traces of the moveout corrected gather have
virtually no amplitude. A further class of AVO
response, Class IV, was introduced by Castagna and
Swan (
1997
). This has a large negative normal
'
small
'
Rc
0
Sin
2
+ve AVO
-ve gradient
-ve AVO
+ve gradient
A
I
-
58
Figure 5.1
Some AVO terminology.
