GEOP 415

Spring 2011

Part 1

Essentials of Seismic Interpretation

Chapter 6

Interpretation Pitfalls

Geophysical concepts

Interpretation pitfalls

Seismic sections resemble geologic cross sections only in areas of simple geology.

Direct interpretation of seismic sections in complex areas may result in serious errors due to interpretation pitfalls (i.e., false effects).

Interpretation pitfalls can be produced by:

I. Geometrical effects: They are present on unmigrated time seismic sections and can produce the following false effects:

1. Sharper and narrower anticlines due to updip migration of dipping events. Opposite is true for synclines.

2. Bowties over synclines.

3. Nonconformable dip sets due to sideswipes (offline reflections) on 2-D sections.

Validity check for all geometrical effects: Migrate seismic sections (using 3-D migration, if possible) before interpretation.

Geophysical concepts

Interpretation pitfalls

II. Near-surface effects: due to lateral velocity and/or thickness changes in near-surface layers producing false:

1. Lows below low-velocity and/or thick near-surface zones

2. Highs below high-velocity and/or thin near-surface zones.

Validity check: Anomaly follows near-surface velocity and/or thickness profiles.

III. Deep effects: due to velocity changes in overburden caused by structural and/or stratigraphic features producing false:

1. Downdip thinning of reflections due to velocity increase with depth.

Validity check: Thinning effect increases with burial depth.

2. Lows beneath normal faults and highs beneath reverse faults due to juxtaposition of different-velocity layers.

Validity check: Anomaly only exists beneath the fault.

3. High or low beneath reefs or channels due to high or low velocity of reef or channel.

Validity check: Anomaly coincides with reef or channel.

Geophysical concepts

Interpretation pitfalls

III. Deeper effects (continued):

4. Lows beneath shale diapirs due to low velocity of shale and highs beneath salt diapirs due to high velocity of salt.

Validity check: Anomaly only exists beneath the diapir.

5. Thinning of reflections on downthrown side of vertical and normal faults due to velocity increase with depth.

Validity check: Thinning is confined to downthrown side of fault.

IV. Acquisition and Processing effects: due to improper selection of acquisition and/or processing parameters producing false:

1. Conformable and nonconformable dip sets due to multiples generated in the near-surface layer.

Validity check: Velocities of primary and multiple are the same.

2. Bedding over unconformity due to multi-cycle wavelet (e.g., bubble effect).

Validity check: Estimate source wavelet from clean part of section.

Reading Assignment

Sharper and narrower anticlines due to updip migration of dipping events.

Bowtie associated with a syncline on an unmigrated seismic section

Sideswip associated with 2-D data

Sheriff and

Geldart

(1995)

Sheriff and

Geldart

(1995)

Downdip thinning of reflections due to velocity increase with depth.

Velocity low associated with a normal fault

Sheriff and Geldart (1995) Velocity high associated with a reverse fault

http://perso.orange.fr/brcgranier/gmeop/Edgell_2001.htm Velocity high associated with a reef

Sheriff and Geldart (1995) Velocity low associated with a channel

Sheriff and

Geldart

(1995) Velocity high associated with a salt diaper

Thinning associated with a normal fault