Geoscience Reference
In-Depth Information
6
Inversion
The fundamental idea of inversion to seismic impedance is very simple. A reflectivity
seismic section contains reflections that can be studied by the methods discussed in
chapters 3
-
5
. These reflections show where there are changes in acoustic impedance
in the subsurface. Inversion is the process of constructing from this reflectivity dataset
a section that displays the acoustic impedance variation in the subsurface directly. As
we shall see, this often makes it easier to interpret the data in geological terms, because
it focuses attention on layers and lateral variations within them, rather than on the
properties of the interfaces between layers that cause the seismic reflections. This is
has not been used very much until recently, probably because good results require
input reflectivity data of excellent quality; the availability of modern 3-D datasets has
triggered an upsurge of interest in the technique.
This chapter begins with a summary of the principles, then discusses some prac-
tical processing workflows with particular attention to the issues that are critical for
the quality of the results, continues with some practical examples to demonstrate the
benefits of the technique, and concludes with a summary of some specialised advanced
applications.
6.1
Principles
A simple model for zero-offset seismic response is illustrated in
fig. 6.1
. The subsurface
is represented as a number of layers, each with its own acoustic impedance
A
; the zero-
offset reflection coefficient at the interface between the
n
th and the (
n
+
1)th is given by
R
n
=
(
A
n
+
1
−
A
n
)
/
(
A
n
+
1
+
A
n
)
or for a small change in impedance
δ
A
, then
R
=
δ
A
/
2
A
=
0
.
5
δ
(ln
A
)
.
The reflection coefficient series is convolved with the seismic wavelet to give the
seismic trace. Inversion aims to start from the seismic trace, remove the effect of the
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