Lecture 5
Mitchum et al., 1977b
AAPG©1977 reprinted with permission of the AAPG
whose permission is required for further use.
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F W Schroeder
‘04
L 5 – Seismic Method
Basic Exploration Workflow
Identify
Opportunities
Acquire
Seismic Data
Interpret
Seismic Data
Capture
Prime Areas
Drill
Wildcats
Process
Seismic Data
Assess
Prospects
Economic
Analysis
Confirmation
Well
Uneconomic
Drop
Prospect
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‘04
Success
Failure
Success
To D/P
L 5 – Seismic Method
The Seismic Method
Energy
Source
An Explosion!

.4 s
Listening Devices
.8 s
.1
.2
0
.3
.4
.5
.6
.7
.8
0 sss
Some Energy is Reflected
Most Energy is Transmitted
Some Energy is Reflected
Most Energy is Transmitted
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‘04
L 5 – Seismic Method
Raw Seismic Data
Time
Device
#1
Device
#2
For the explosion we just considered ...
0.0
0.1
0.2
0.3
0.4
0.5
Listening device #1 records a reflection
starting at 0.4 seconds
0.6
0.7
0.8
Listening device #2 records a reflection
starting at 0.8 seconds
To Image the Subsurface, We Use Many Shots (explosions)
and Many Receivers (listening devices)
Arranged in Lines either on Land or Offshore
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‘04
L 5 – Seismic Method
Seismic Acquisition
• A 3D survey is designed based on:
– Imaging Objectives: image area, target depth, dips, velocity,
size/thickness of bodies to be imaged, etc.
– Survey Parameters: survey area, fold, offsets, sampling,
shooting direction, etc.
– Balance between Data Quality & $$$$$
Land Operations
Vibrators Generate a Disturbance
Geophones Detect Motion
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Marine Operations
Air Guns Generate a Disturbance
Hydrophones Detect Pressure
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‘04
L 5 – Seismic Method
Raw Data - Marine
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‘04
L 5 – Seismic Method
Seismic Processing
Data Processing
Stream
Field Record
(marine)
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Subsurface ‘Image’
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‘04
L 5 – Seismic Method
Shot Gather
For Shot 1
Source
S1
Receivers
Direct Arrival
R1
R2
R3
R4
Offset (Distance)
R5
R1 R2 R3 R4 R5
1
2
Reflections
3
For each shot, reflections are
recorded in 5 receivers
2 Way Travel Time
Direct Arrival
Reflection
There are 5 ‘bounce’ points
along interface 3
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‘04
L 5 – Seismic Method
Common Midpoint Gather
For Point A
Sources
Receivers
CMP Gather
S5
S4
S3
S2
S1
R1
R2
R3
R4
R5
A
We sort the shot-receiver pairs so
that data from the same ‘bounce’
point (e.g., at ‘A’) is captured
CMP = common mid point
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‘04
Offset Distance
L 5 – Seismic Method
CMP Gather
CMP Gather
The travel times differ since
the path for a near offset trace
is less than the path for a far
offset trace
With the correct velocity, we
can correct for the difference
in travel time for each trace.
The curvature of this hyperbola is a
function of the average velocity
down to the depth of the reflection
Offset Distance
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‘04
L 5 – Seismic Method
With Correct Velocity, Gather is Flat
CMP Gather
Velocity
Too Slow
Velocity
Correct
Velocity
Too Fast
Curves
Down
Flat
Curves
Up
Offset Distance
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‘04
L 5 – Seismic Method
A Stacked Trace
CMP Gather
Moveout Corrected
Midpoint Gather
Stacked
Trace
We stack several
offset traces
(# traces = fold)
The geologic
‘signal’ will be
additive
The random
‘noise’ will tend
to cancel
Stacking greatly
improves S/N
Offset Distance
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10 Fold
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‘04
(signal-to-noise)
L 5 – Seismic Method
Positioning Problems
Energy
Source


0.4 s Bounce
Point
The seismic ray hits an inclined
surface at 90º and reflects back
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‘04
The reflection is
displayed beneath the
source-receiver midpoint
L 5 – Seismic Method
Time for an Exercise
1
2
3
4
5
6






90º
Where would the reflection lie?
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‘04
L 5 – Seismic Method
Time for an Exercise
1
2
3
4
5
6






Compass
Where would the reflection lie?
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F W Schroeder
‘04
L 5 – Seismic Method
Time for an Exercise
1
2
3
4
5
6






Where would the reflection lie?
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F W Schroeder
‘04
L 5 – Seismic Method
Exercise Answer
1
2
3
4
5
6






The reflection is downdip and its
dip is less than the interface
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‘04
L 5 – Seismic Method
Migration – Correcting for Location
Unmigrated energy on single trace...
S
R
Sweep Ellipse
S
R
...spread to all possible locations of origin
Sweep Ellipse
S
R
Sweep Ellipse
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‘04
L 5 – Seismic Method
Migration – Power of Correlation
Two reflections on unmigrated data
After spreading to all possible locations
Reflections are not positioned
in the subsurface correctly
since they have dip
Constructive interference occurs
where the reflections are properly
positioned
Destructive interference dominates
where the reflections are NOT
properly positioned
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‘04
L 5 – Seismic Method
Seismic Migration
Unmigrated Image
Positioning
Problems ‘Blur’
the Image
Migrated Image
Migration Reduces
Positioning
Problems, which
Improves the
Image
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‘04
L 5 – Seismic Method
Seismic Interpretation
Mitchum et al., 1977
AAPG©1977 reprinted with permission of the AAPG
whose permission is required for further use.
Determine the local geology from the subsurface images
• Map faults and other structural features
• Map unconformities and other major stratal surfaces
• Interpret depositional environments
• Infer lithofacies from reflection patterns & velocities
• Predict ages of stratal units
• Examine elements of the HC systems
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F W Schroeder
‘04
L 5 – Seismic Method