Seismic Resolution 414v5x
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Lecture 8
Layer Thickness 1
10
*
20
top 20 ms
top
30
Courtesy of ExxonMobil
base
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Outline
Vertical Resolution • Resolution vs Detection • Thin Bed Response and Tuning
Lateral Resolution • Fresnel Zone
• Migration and Lateral Resolution
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Detection vs. Resolution - Analogy You are driving at night. You spot a light in the distance. Is it a car or a motorcycle???
Aha, it is a car!
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Resolution vs. Detection Detection:
Ability to identify that some feature exists
Resolution: Ability to distinguish two features from one another • Detection limit is always smaller than the resolution limit • Detection limit depends upon Signal-to-Noise
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Vertical Resolution
Shale Baseline
For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands? Courtesy of ExxonMobil
F W Schroeder ‘ 04
Sand
Sd
What is the minimum vertical distance between two subsurface features such that we can tell them apart seismically?
Shale
Gamma Ray
L 9 – Seismic Resolution
Thick Bed Response Question: What is a thick bed? Impedance
R. C.
Wavelet 1
Wavelet 2
Wavelet 1 ends before Wavelet 2 begins Composite
A B
Top of Bed Response
Dp
C
NO Interference
Base of Bed Response
Answer: A thick bed is one that has a TWT > Dp Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Partial Interference Wavelet 2 starts before Wavelet 1 ends
TWT thickness = 0.9 * Dp Impedance
R. C.
Wavelet 1
Wavelet 2
A B
Composite Top of Bed Response
Dp
C
Some Interference
Base of Bed Response
2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 form a trough doublet
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Maximum Interference - Tuning Wavelet 2 starts before Wavelet 1 ends
TWT thickness = ½ Dp Impedance
R. C.
Wavelet 1
Wavelet 2
A
B
Composite Top of Bed Response
Dp
Maximum Interference Base of Bed Response
C
2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 are completely in phase resulting in 2x amplitude Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Determining Vertical Resolution Input Parameters:
Pulse
Peak Frequency of the pulse at the zone of interest Computations: Period = 1/Peak Frequency
Period (ms)
Velocity at the zone of interest
wavelength = period X velocity
Wavelength = Period * Velocity Limit of Vertical Resolution = Wavelength/4
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
A Simple Exercise - 2 Zones
Calculating Vertical Resolution
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Typical Vertical Resolution Shallow Event Velocity = 2000 Meters / sec Pulse: Center Frequency = 50 Hz Period = 1 / 50 = .020 sec Wavelength = .020 x 2000 = 40 Meters Limit of resolution = 40 /4 = 10 Meters
Deep Event Velocity = 3000 Meters / sec Pulse: Center Frequency = 20 Hz Period = 1 / 20 = .050 sec Wavelength = .050 x 3000 = 150 Meters Limit of resolution = 150 / 4 = 37.5 Meters Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Summary: Vertical Resolution • Resolution is the ability to distinguish distinct events • Thin bed response occurs below tuning thickness • Short-duration seismic pulses are preferred – Broad bandwidth, zero-phase pulses are best – Pulses with minimal side-lobe energy enhance interpretability
• To Improve Resolution – Bandwidth can be increased by deconvolution – Frequencies to be included must have adequate S/N
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
What Is Lateral Resolution?
Would we image the narrow horst?
Would we image all three channel sands?
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Lateral Resolution What is the minimum horizontal distance between two subsurface features such that we can tell them apart seismically? Reflections from Reflector with Gaps Neidell & Poggiaglioimi, 1977
AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
The Fresnel Zone • An event observed at a detector is reflected from a zone of points
• The raypaths from source to detector which differ in length by less than a quarter wavelength can interfere constructively • The portion of the reflector from which they add constructively is the Fresnel zone • Changes that occur within this zone are difficult to resolve • The size of the Fresnel zone depends upon the wavelength of the pulse and the depth of the reflector
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Migration Reduces Lateral Smearing
Ideal / Model Response 800 m
Stack No Migration
Image After Migration
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Good Migration Enhances Resolution
Standard Migration Courtesy of ExxonMobil
F W Schroeder ‘ 04
High-end Migration L 9 – Seismic Resolution
Fresnel Zone Equations Pre-Migration
Post-Migration
Fd = Vavg T/F
Fd = λ /4 = Vavg /4 F
where:
Fd = Fresnel Diameter Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Another Simple Exercise - 2 Zones
Calculating Fresnel Zone Diameters
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Typical Lateral Resolution • Shallow Event – – – – –
Time = 1.0 s Vint = Vavg = 2000 m/s Pulse = 50 Hz PreMig Fresnel Diameter = 282 m PostMig Fresnel Diameter = 10 m
• Deep Event – – – – – – Courtesy of ExxonMobil
Time = 5.0 s Vint = 4600 m/s Vavg = 3800 m/s Pulse = 20 Hz PreMig Fresnel Diameter = 1900 m PostMig Fresnel Diameter = 47.5 m
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Graphical Answers
Fresnel Zone Circles
Shallow Window 282 m pre-migration 10 m post-migration
Deep Window
1900 m pre-migration 47.5 m post-migration
1 km
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Summary: Lateral Resolution • Migration enhances lateral resolution • Large aperture (receiver cable length) is needed for high lateral resolution • Fine spatial sampling is needed for high lateral resolution
• Prestack migration provides better lateral resolution than poststack migration • Depth migration provides better resolution than time migration
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Layer Thickness 1
10
*
20
top 20 ms
top
30
Courtesy of ExxonMobil
base
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Outline
Vertical Resolution • Resolution vs Detection • Thin Bed Response and Tuning
Lateral Resolution • Fresnel Zone
• Migration and Lateral Resolution
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Detection vs. Resolution - Analogy You are driving at night. You spot a light in the distance. Is it a car or a motorcycle???
Aha, it is a car!
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Resolution vs. Detection Detection:
Ability to identify that some feature exists
Resolution: Ability to distinguish two features from one another • Detection limit is always smaller than the resolution limit • Detection limit depends upon Signal-to-Noise
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Vertical Resolution
Shale Baseline
For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands? Courtesy of ExxonMobil
F W Schroeder ‘ 04
Sand
Sd
What is the minimum vertical distance between two subsurface features such that we can tell them apart seismically?
Shale
Gamma Ray
L 9 – Seismic Resolution
Thick Bed Response Question: What is a thick bed? Impedance
R. C.
Wavelet 1
Wavelet 2
Wavelet 1 ends before Wavelet 2 begins Composite
A B
Top of Bed Response
Dp
C
NO Interference
Base of Bed Response
Answer: A thick bed is one that has a TWT > Dp Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Partial Interference Wavelet 2 starts before Wavelet 1 ends
TWT thickness = 0.9 * Dp Impedance
R. C.
Wavelet 1
Wavelet 2
A B
Composite Top of Bed Response
Dp
C
Some Interference
Base of Bed Response
2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 form a trough doublet
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Maximum Interference - Tuning Wavelet 2 starts before Wavelet 1 ends
TWT thickness = ½ Dp Impedance
R. C.
Wavelet 1
Wavelet 2
A
B
Composite Top of Bed Response
Dp
Maximum Interference Base of Bed Response
C
2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 are completely in phase resulting in 2x amplitude Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Determining Vertical Resolution Input Parameters:
Pulse
Peak Frequency of the pulse at the zone of interest Computations: Period = 1/Peak Frequency
Period (ms)
Velocity at the zone of interest
wavelength = period X velocity
Wavelength = Period * Velocity Limit of Vertical Resolution = Wavelength/4
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
A Simple Exercise - 2 Zones
Calculating Vertical Resolution
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Typical Vertical Resolution Shallow Event Velocity = 2000 Meters / sec Pulse: Center Frequency = 50 Hz Period = 1 / 50 = .020 sec Wavelength = .020 x 2000 = 40 Meters Limit of resolution = 40 /4 = 10 Meters
Deep Event Velocity = 3000 Meters / sec Pulse: Center Frequency = 20 Hz Period = 1 / 20 = .050 sec Wavelength = .050 x 3000 = 150 Meters Limit of resolution = 150 / 4 = 37.5 Meters Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Summary: Vertical Resolution • Resolution is the ability to distinguish distinct events • Thin bed response occurs below tuning thickness • Short-duration seismic pulses are preferred – Broad bandwidth, zero-phase pulses are best – Pulses with minimal side-lobe energy enhance interpretability
• To Improve Resolution – Bandwidth can be increased by deconvolution – Frequencies to be included must have adequate S/N
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
What Is Lateral Resolution?
Would we image the narrow horst?
Would we image all three channel sands?
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Lateral Resolution What is the minimum horizontal distance between two subsurface features such that we can tell them apart seismically? Reflections from Reflector with Gaps Neidell & Poggiaglioimi, 1977
AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
The Fresnel Zone • An event observed at a detector is reflected from a zone of points
• The raypaths from source to detector which differ in length by less than a quarter wavelength can interfere constructively • The portion of the reflector from which they add constructively is the Fresnel zone • Changes that occur within this zone are difficult to resolve • The size of the Fresnel zone depends upon the wavelength of the pulse and the depth of the reflector
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Migration Reduces Lateral Smearing
Ideal / Model Response 800 m
Stack No Migration
Image After Migration
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Good Migration Enhances Resolution
Standard Migration Courtesy of ExxonMobil
F W Schroeder ‘ 04
High-end Migration L 9 – Seismic Resolution
Fresnel Zone Equations Pre-Migration
Post-Migration
Fd = Vavg T/F
Fd = λ /4 = Vavg /4 F
where:
Fd = Fresnel Diameter Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Another Simple Exercise - 2 Zones
Calculating Fresnel Zone Diameters
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Typical Lateral Resolution • Shallow Event – – – – –
Time = 1.0 s Vint = Vavg = 2000 m/s Pulse = 50 Hz PreMig Fresnel Diameter = 282 m PostMig Fresnel Diameter = 10 m
• Deep Event – – – – – – Courtesy of ExxonMobil
Time = 5.0 s Vint = 4600 m/s Vavg = 3800 m/s Pulse = 20 Hz PreMig Fresnel Diameter = 1900 m PostMig Fresnel Diameter = 47.5 m
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Graphical Answers
Fresnel Zone Circles
Shallow Window 282 m pre-migration 10 m post-migration
Deep Window
1900 m pre-migration 47.5 m post-migration
1 km
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
Summary: Lateral Resolution • Migration enhances lateral resolution • Large aperture (receiver cable length) is needed for high lateral resolution • Fine spatial sampling is needed for high lateral resolution
• Prestack migration provides better lateral resolution than poststack migration • Depth migration provides better resolution than time migration
Courtesy of ExxonMobil
F W Schroeder ‘ 04
L 9 – Seismic Resolution
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