Post on 31-Jan-2018
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Applied Geophysics – Seismic reflection IV
Seismic reflection - IV
Reflection reading: Sharma p130-158; (Reynolds p343-379)
Seismic methods:
Applied Geophysics – Seismic reflection IV
Seismic reflection processingFlow overview
These are the main steps in processing
The order in which they are applied is variable
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Applied Geophysics – Seismic reflection IV
Dipping layers
For horizontal reflectors the reflection point is vertically below the source/receiver
For dipping layers the reflection comes from a point up dip
Therefore, a traveltime section will always show a reduced dip
δδ sin'tan =
Applied Geophysics – Seismic reflection IV
Migration
Intended to deal with• Dipping interfaces• Curved interfaces• Diffractions• Reflections from the
3rd dimension
The process of trying to move reflections back to their point of origin
Time migration: restore the geometrical relationships between seismic events on the time section
Depth migration: generates a true depth section. Necessary when strong lateral velocity gradients
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Applied Geophysics – Seismic reflection IV
Diffraction hyperbolae
Traveltime plot
A to B: normal incidence reflections
B to C: diffractions from P Huygens’ principle
Equation of the diffraction hyperbolae:
Note that a diffraction hyperbolae has a greater curvature than a reflection hyperbolae
Recall the equation for a reflection hyperbolae:
( )221
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2 xzt +=α
221
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2 xzt +=α
Applied Geophysics – Seismic reflection IV
A non-migrated section
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Applied Geophysics – Seismic reflection IV
Anticlines and synclines
Anticlines broaden Synclines produce a bow-tie
Applied Geophysics – Seismic reflection IV
Anticlines and synclines
Pre-migration stack
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Applied Geophysics – Seismic reflection IV
Anticlines and synclines
Migrated stack
Applied Geophysics – Seismic reflection IV
Migration procedure
For a zero offset seismic section (ie stacked data):
Diffraction hyperbolae are identified by scanning adjacent traces and collapsed back to the point of origin
Kirchhoff migrationuses hyperbola
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Applied Geophysics – Seismic reflection IVStacked section
Applied Geophysics – Seismic reflection IVStacked section
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Applied Geophysics – Seismic reflection IVMigrated section
Applied Geophysics – Seismic reflection IV
Velocity distortions
Velocity relates the time section recorded to a depth section
…horizontal and vertical velocity variations distort the section
Example: vertical velocity gradient
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Applied Geophysics – Seismic reflection IV
Fault distortionsVelocity distortions:
Applied Geophysics – Seismic reflection IV
Lateral velocity variationsVelocity distortions:
Produce pull-upsand pull-downs
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Applied Geophysics – Seismic reflection IV
Source, reservoir and trapOil and gas
Applied Geophysics – Seismic reflection IV
Salt domeCommon traps in Gulf of Mexico
Examples
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Applied Geophysics – Seismic reflection IV
Faults and flat spotsExamples
Flat spots and bright spots: indicate oil or gas
Applied Geophysics – Seismic reflection IV
Delineating bedrockExamples
Why would we want to know this?
Bedrock valley
Sediments
Correlating seismic with well log data• Drilling a well provides “ground truth” to a seismic interpretation• Borehole provides velocities for depth migration• Synthetic seismograms generated from the well log can be tied
to the seismic
Borehole
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Applied Geophysics – Seismic reflection IV
Delineating bedrockExamples
Over-interpretation?
Look for additional data• Local geology• Other surveys• Boreholes
Applied Geophysics – Seismic reflection IV
GroundwaterExamples
Objective:
Map subsurface location of aquifer for the purpose of drilling a well
Surface geology
permeable
impermeable
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Applied Geophysics – Seismic reflection IV
GroundwaterExamples
Objective:
Map subsurface location of aquifer for the purpose of drilling a well
Applied Geophysics – Seismic reflection IV
Locating faultsExamples
Migrating fluidsSeismic hazard
Identified as discontinuities in reflection surfaces
None of these faults visible at
the surface –suggests recent
inactivity
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Applied Geophysics – Seismic reflection IV
Locating voidsExamples
Reflection survey above an abandoned mine
Voids indicated by absence of the otherwise continuous reflector
What other methods could we use?