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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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