Related to geophysics

1. 1
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 processing
Flow overview
These are the main
steps in processing
The order in which
they are applied is
variable

2. 2
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

3. 3
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:
( )2
2
1
1
2
2 xzt +=
α
22
1
1
2
xzt +=
α
Applied Geophysics – Seismic reflection IV
A non-migrated section

4. 4
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

5. 5
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 migration
uses hyperbola

6. 6
Applied Geophysics – Seismic reflection IVStacked section
Applied Geophysics – Seismic reflection IVStacked section

7. 7
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 distortions
Velocity distortions:
Applied Geophysics – Seismic reflection IV
Lateral velocity variations
Velocity distortions:
Produce pull-ups
and pull-downs

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Applied Geophysics – Seismic reflection IV
Source, reservoir and trap
Oil and gas
Applied Geophysics – Seismic reflection IV
Salt dome
Common traps in Gulf of Mexico
Examples

10. 10
Applied Geophysics – Seismic reflection IV
Faults and flat spots
Examples
Flat spots and bright spots:
indicate oil or gas
Applied Geophysics – Seismic reflection IV
Delineating bedrock
Examples
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 bedrock
Examples
Over-interpretation?
Look for additional data
• Local geology
• Other surveys
• Boreholes
Applied Geophysics – Seismic reflection IV
Groundwater
Examples
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
Groundwater
Examples
Objective:
Map subsurface
location of aquifer
for the purpose of
drilling a well
Applied Geophysics – Seismic reflection IV
Locating faults
Examples
Migrating fluids
Seismic hazard
Identified as discontinuities
in reflection surfaces
None of these
faults visible at
the surface –
suggests recent
inactivity

13. 13
Applied Geophysics – Seismic reflection IV
Locating voids
Examples
Reflection survey above
an abandoned mine
Voids indicated by
absence of the
otherwise continuous
reflector
What other methods
could we use?