Pitfalls Discussion Group 1

8/10/2019 Pitfalls Discussion Group 1

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Pitfalls in Seismic DataInterpretation

Julia Mulhern and Allie Jackson

1. Velocity Artifacts2. Fault Shadow Artifacts

3. Structural Feature Artifacts

4. Migration Artifacts

5. Other Artifacts


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Velocity Artifacts

1. Bad Velocity Model

2. Basinward Thinning

3. Reefs

4. Channels

5. Velocity Pull up and Pull Downs caused by salt

6. Dim Spots


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Bad Velocity Model Original Data - unclear how thechannel features and pay

interval are related.

Reprocessed Data -improved velocity model

shows that there is a

channel structure below well

H-2.

(Nanda et al. 2008)

Pitfall: Poor velocity

estimate leads to aninaccurate interpretation.


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Basinward Thinning

Thinning from left to right Actually time interval, not bed, thinning from left to right

Pitfall:Assuming that all seismic

section thinning must mean thinning

in the beds.

(Tucker and Yorston, 1972)

-Increase in compaction basinwards

-Increase in interval velocity with depth (basinwards)


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Channels


Check: Does the purported syncline

maintain the same size and shape with

depth?

Pitfall: Not consideringthat a water filled

channel can increase

travel times below the

cut, leading to an

apparent syncline

beneath the channel.


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(Okere and Toothill 2012)

Velocity Pull up and Pull Downs caused by salt

Pitfall: Salt (white boxes) causes velocity pull ups (orange arrows) and pull downs

(white arrows).


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Dim Spots

dim spot = reduction in

amplitude caused by

hydrocarbon

Pitfall: Normally when hydrocarbon replaces water in a sand reservoir a bright spot

is created, however at depth after the impedance curves have cross over,

replacing water with hc decreases the velocity contrast, creating a dim spot.

(Brown, 2005)


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Fault Shadow: Normal Fault

Fault Shadow: An area near a fault where seismic wave propagation is distorted,

generally under the footwall zone of the fault.

Fault shadows of reverse faults tend to be more severe as these areoften associated with more displacement.


Pitfall: Interpreting the rollover as

a drag fold into a normal fault.


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Vertical Faults


Pitfall: Layer thickness can appear to change across

vertical faults depending on the velocity distribution.

Zones thinner on right No fault shadow

Still potential abrupt velocity

changes across the fault


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Anticline


Pitfall: In addition to the fold complications, additionalpitfalls come when faults occur at the apex of a fold.


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Syncline

(Tucker and Yorston, 1972)Migrated Section

Unmigrated Section

Pitfall: Synclines create bowtie features that are

removed with migration.


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Thrust Belt Complications

High surface velocities can scatter waves and can be detrimental to wave propagationfurther into the subsurface.

Additional complications associated with thrust belts involve structural complexity(complicated fold geometry, steep dips, faults, etc.).

(Alaei, 2012)

Pitfall: Thrust belts can place high velocity material over low velocity

material, distoring the image below the thrust.


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Migration Artifacts1. Comparing time vs. depth sections

2. Migration Algorithm

3. Pre Stack vs. Post Stack Migration


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Circled area: enhanced sub-salt resoltion in the depth migrated section

Orange Arrow: artifact within the salt that does not exist in the depth migrated section.

(Okere and Toothill 2012)

Comparing time vs. depth sections

Pitfall: Time vs. Depth migration can alter the data. In this case depth migrationimproved the data.

Time Migration Depth Migration


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Post Stack Depth Migration

Pre Stack vs. Post Stack Migration

Pre Stack Depth

Migration (1st

Iteration )

Pre Stack Depth Migration

(2nd Iteration )

Pitfall: Pre vs. Post stacking and numberof migrations can alter the data signature.

(Herron 2000)


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Other Artifacts

1. Data Polarity

2. Lateral Amplitude Changes


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Data Polarity

(Brown 2005)

- European and American seismic data have different polarity

- For zero phase data the pulses are co-located.

Pitfall: Polarity must be known before interpretation.


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The polarity ofthe data makes

one of these

strong reflectors

prospective while

the other is likelya hard bed.

Data Polarity

(Brown 2005)

Pitfall: Polarity must be taken into account when assessing reflectors.


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Lateral Amplitude Changes

(Brown 2005)

Pitfall: Inconsistent coverage leads to a misinterpretation of prospect size.


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References:

Alaei, B., 2012, Seismic Modeling of Complex Geological Structures, Seismic Waves - Research and Analysis, Dr.

Masaki Kanao (Ed.), ISBN: 978-953-307-944-8, InTech, DOI: 10.5772/29423.

Brown, A. R., 2005, Pitfalls in 3D seismic interpretation: Keynote presentation at the 11th Annual 3-D Seismic

Symposium, Denver: The Leading Edge, v. 24, no. 7, p. 716-717.

Herron, D. A., 2000, Pitfalls in seismic interpreataion: Depth migration artifacts: The Leading Edge, v. 19, no. 9.

Nanda, N., Ram Singh, and Satinder Chopra, 2008, Seismic Artifacts - a case study: CSEG Recorder.

Okere, D., and Toothill, S., 2012, New insights into hydrocarbon plays in the Caspian Sea, Kazakhstan: Petroleum

Geoscience, v. 18, no. 3, p. 253-268.

Reasnor, M. D., 2007, Salt interpretation practices for depth imaging in the Gulf of Mexico: The Leading Edge, v. 26, no.

11.

Sain, K., and Kaila, K. L., 1996, Ambiguity in the solution to the velocity inversion problem and a solution by joint

inversion of seismic refraction and wide-angle reflection times: Geophysical Journal International, v. 124, no. 1, p. 215-

227.

Tucker, P., and Yorston, H., 1972, Pitfalls in Seismic Interpretation, from Esso Production Research Company, 40 p.

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Pitfalls Discussion Group 1