Geology Reference
In-Depth Information
situations (e.g. deep sea sedimentary environments)
bandlimited impedance can be a good tool to evalu-
ate net pay (see
Chapter 10
).
Figure 5.71
shows a
schematic model of how bandlimited impedance
relates to seismic reflectivity. Several features can
be noted:
V
sh
Phi
AI
AI
bandlimited
the horizon pick changes from a maximum or
minimum on the reflectivity to a zero crossing on
the bandlimited impedance data,
the bandlimited impedance signature of the peak/
trough doublet related to the topmost reflection is
proportional to the magnitude of the real
impedance contrast,
where there is no signal (for example in thick
homogenous units) the amplitude of the
bandlimited impedance is zero,
as the data are bandlimited they are affected by
reflector interference (see also
Fig. 5.72
),
seismic data with negative standard polarity needs
to be multiplied by
1 prior to applying the
operator.
Figure 5.73
shows an example from the Palaeocene
of the Central North Sea where a simple SAIL
attribute works well. The seismic data in
Fig 5.73a
have good bandwidth and the bandlimited imped-
ance has a lower frequency look compared to the
reflectivity data (
Fig. 5.73b
). Note that if the ban-
dlimited impedance simply looks like a phase
rotated version of the reflectivity it is likely that
the inversion attribute will not provide any add-
itional useful information. In this case, the ban-
dlimited impedance has helped in defining the
presence of hydrocarbon; red colours in
Fig. 5.73b
represent low impedance oil sands in the structural
culmination (Simm et al.,
1996
;Ahmadiet al.,
2002
).
An enhancement to the SAIL approach, known
as coloured inversion, was proposed by Lancaster
and Whitcombe (
2000
). The concept is that the
bandlimited impedance from seismic is matched to
the spectral characteristics of the impedances from
log data (
Fig. 5.74
). A critical step in the method is
the definition of the operator as it can involve a
shaping of the spectrum. The coloured inversion
operator effectively maps the frequency response
from the seismic to that of the impedance logs
(
Fig. 5.74
). This technique is simple and robust,
requiring few assumptions except that the input
Figure 5.70
A bandlimited impedance log. Low and
high frequencies of the impedance log have been removed by
convolution of the log with a seismic wavelet (vertical scale
is 100 ft).
as it presents the data in layers rather than boundary
reflections. On logs it is a useful tool to evaluate
resolution (
Fig. 5.70
). In basin fill sequences it often
clarifies stratigraphic relationships and emphasises
hydrocarbon contacts.
In particular geological
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