Geology Reference
In-Depth Information
Seismic processing issues
Chapter
6
6.1 Introduction
It has long been recognised that AVO analysis makes
onerous demands of the seismic processing industry,
well beyond those typical of the requirement for
accurate imaging. What is needed are intercept and
gradient data for conventional AVO analysis, or
angle gathers and partial stacks for input into imped-
ance inversion, of high enough quality that the
results can be used for fluid and lithology discrimin-
ation or reservoir characterisation. A key require-
ment is that seismic amplitudes should be
proportional to synthetic amplitudes that would be
calculated from an accurate impedance model of
the subsurface. Achieving this to within satisfactory
tolerances is not always possible. To quote Cambois
(
2001
),
recovery of correct relative amplitudes
improvement of signal/noise ratio (e.g. removal of
multiples and other noise)
data enhancement for improved interpretability
(i.e. zero phasing, bandwidth improvement)
imaging the data correctly (i.e. pre-stack time or
depth migration)
aligning reflections correctly across the gather
(i.e. moveout and statics correction).
The critical issues for the interpreter include:
data quality
·
signal/noise
recognising bad data
'
Although preserved amplitude processing is
a clear requirement for AVO studies, such a process-
ing sequence is not uniquely defined
understanding the effect of noise on the
AVO gradient
One possible
definition could be: Any sequence that makes the
data compatible with Shuey
...
alignment
·
s equation (if this is
the model used for the AVO analysis). The simpli-
city of this definition should not hide the Herculean
nature of this task
'
the effect of residual moveout on the
gradient
differentiating residual moveout from
Class IIp AVO effects
.
Some aspects of seismic processing that are
important for AVO analysis have already been intro-
duced in
Chapter 2
. In this chapter they will be
discussed in more detail, considering first the steps
in common processing sequences that are most
important for AVO analysis, and then describing
how gathers can be conditioned to improve the qual-
ity of AVO results. This leads on to a consideration of
how AVO intercept and gradient can best be calcu-
lated. The chapter is not intended as a comprehensive
discussion of seismic processing (for which see e.g.
Yilmaz,
2001
; Jones,
2010
), but rather aims to high-
light the main issues that the amplitude interpreter
needs to recognise. From the point of view of ampli-
tude interpretation, the key aims of seismic process-
ing are:
'
moveout stretch
amplitude scaling
'
true
'
amplitude recovery
·
final adjustment to scaling with offset.
·
An appreciation of these issues requires that the inter-
preter is vigilant about the possible effects of various
seismic processing steps on the AVO response. Some-
times it may be impossible to carry out meaningful
AVO analysis because the AVO behaviour is severely
distorted. If at all possible the interpreter needs to
have access to the angle gathers to assess noise levels
in relation to partial stacks. Residual multiples that
may be easy to recognise on the gathers are usually
impossible to see by inspection of partial stacks. Here
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