3d avo and migracion_gray
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3-D AVO and migration David Gray*, Veritas GeoServices Ltd.; Taiwen Chen and Bill Goodway, PanCanadian Petroleum Ltd.
IntroductionThe use of AVO is being extended from pure
stratigraphic into combined stratigraphic-structural areas like the North Sea. As a result,
migration is being used in conjunction with
AVO (Tura and Rowbotham 1996; Lindsay and
Ratcliff, 1996; Boztas et al, 1997; Nickerson et
al, 1997). The purpose of this presentation is to
evaluate various migration methods to determine
which are suitable for AVO.
Method
The effects of different migrations on AVO are
tested using a relatively unstructured data set. In
these data, the differences between the migrated
and unmigrated data are minimal because the
migration does not move data very far.Therefore, any significant differences in AVO
must be primarily due to the amplitude
preserving effects of the migration. Results are
compared to AVO derived from NMO only
gathers, which have good correlation to well
control.
Two 3-D data volumes meeting these criteria
were provided for these tests. These are the
Shorncliffe 3-D (Gray, 1997) and the Crowfoot
3-D (Chen et al., 1997). In both volumes, images
of channels cut into flat-lying sediments are used
to determine how well the migration methods arepreserving the AVO.
The Shorncliffe 3-D is used to test different
migration algorithms, following the work of
Gray (1997). The effectiveness of the migrations
is tested by the delineation of a channel at 0.7 s.
The Crowfoot 3-D is used to test the order to
best apply pre-stack interpolation (PSI), pre-
stack time migration (PSTM) and AVO to the
data. Simple Kirchhoff PSTM (KM) and
common offset PSTM (COM) are used. The
targets are Lower Cretaceous, Glauconitic andLithic channels containing gas. This 3-D has
extensive well control (Chen et al, 1997, 1998).
Discussion
Shorncliffe 3-D - Time slices of stacks (Fig. 1)
show little difference between the migrations.
However, the AVO gradients show significant
differences (Fig. 2). The COM gradient displays
the channel better than the NMO only gradient.
The gradients derived from the other gathers
display the channel very poorly. There is an
insignificant difference between the COM withand without interpolation suggesting PSI is not
responsible for the improved results.
a) b) c) d)
Fig. 1. Shorncliffe amplitudes: Time slices
showing the channel. The methods are a) NMOonly, b) KM (N-S stripe was unmigrated), c)
common offset migration with interpolation
(COMI) and d) COM.
a) b) c) d)
Fig. 2. Shorncliffe gradient: slices showing thechannel. The methods used are a) NMO only, b)
KM, c) COMI and d) COM.
Crowfoot 3-D – Horizon slices (Figure 3) show
subtle differences depending on the PSI-
migration-AVO flow used. The delineation of
the oval shape just above the center of the
section and anomaly just below and to the left of
it in the necktie shape running NNW-SSE is the
goal. These locations are known to contain gas.
The COM PSTM show better imaging of these
features in the amplitude slices. These
differences are also evident in the Fluid Factor(Smith and Gidlow; 1987) in Figure 4. The Fluid
Factor discriminates between the high
amplitudes caused by gas in the channel and
those caused by shales in the SW of the survey.
The upper part of the necktie shaped anomaly
and the oval anomaly are better defined when
PSI is applied prior to the migration.
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Conclusions
Better AVO attributes are derived after COM.
Interpolation of the data prior to COM appears to
make subtle improvements in the AVO for theCrowfoot 3-D. Kirchhoff PSTM is to be avoided
in AVO analysis unless it is explicitly amplitude
preserving.
a) b)
c) d)
Fig. 3. Crowfoot P-wave amplitude slices at the
zone of interest: a) AVO, interpolation; b)
interpolation, COM, AVO; c) COM,
interpolation, AVO and d) AVO, interpolation,
post-stack migration.
a) b)
c)
Fig. 4. Crowfoot - Fluid Factor slices at the zone
of interest: a) AVO, interpolation, b)
interpolation, COM, AVO and c) COM,
interpolation, AVO. AVO, interpolation,
migration was not performed on these data.
Acknowledgements
The authors wish to thank PanCanadian
Petroleum Ltd. for permission to use their data
and PanCanadian and Veritas GeoServices formaking this presentation possible.
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