f w schroeder 04 l 9 – seismic resolutioncourtesy of exxonmobil lecture 8 layer thickness 11020 20...

F W Schroeder ‘ 04

L 9 – Seismic ResolutionCourtesy of ExxonMobil

Lecture 8Lecture 8

Layer ThicknessLayer Thickness11 1010 2020

20 m

s20

ms

3030

basebase

toptop toptop

**



Vertical ResolutionVertical Resolution

• Resolution vs Detection

• Thin Bed Response and Tuning

Lateral ResolutionLateral Resolution

• Fresnel Zone

• Migration and Lateral Resolution

Outline



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!



Detection: Detection: Ability to identify that some feature Ability to identify that some feature existsexists

Detection: Detection: Ability to identify that some feature Ability to identify that some feature existsexists

• Detection limit is always smaller than the resolution limit

• Detection limit depends upon Signal-to-Noise

Resolution vs. Detection

Resolution: Ability to distinguish two features Resolution: Ability to distinguish two features from one anotherfrom one another

Resolution: Ability to distinguish two features Resolution: Ability to distinguish two features from one anotherfrom one another



What is the minimum vertical What is the minimum vertical distance between two distance between two subsurface features such subsurface features such that we can tell them apart that we can tell them apart seismically?seismically?

What is the minimum vertical What is the minimum vertical distance between two distance between two subsurface features such subsurface features such that we can tell them apart that we can tell them apart seismically?seismically?

Vertical Resolution

Sh

ale

San

dS

d

Gamma Ray

ShaleBaseline

For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands?



Thick Bed Response

Question: What is a thick bed?Impedance CompositeWavelet

1R. C. Wavelet

2

NOInterference

Dp

Wavelet 1 ends before Wavelet 2 begins

Top of BedResponse

Base of BedResponse

Answer: A thick bed is one that has a TWT > Dp

B

A

C



Partial Interference

TWT thickness = 0.9 * Dp

Impedance CompositeWavelet1

R. C. Wavelet2

SomeInterference

Wavelet 2 starts before Wavelet 1 ends

Top of BedResponse

Base of BedResponse

2nd half-cycle from Wavelet 1and 1st half-cycle from Wavelet 2

form a trough doublet

B

A

C

Dp



Maximum Interference - Tuning

TWT thickness = ½ Dp

Impedance CompositeWavelet1

R. C. Wavelet2

MaximumInterference

Wavelet 2 starts before Wavelet 1 ends

Top of BedResponse

Base of BedResponse

2nd half-cycle from Wavelet 1and 1st half-cycle from Wavelet 2

are completely in phaseresulting in 2x amplitude

B

A

C

Dp



Determining Vertical Resolution

Per

iod

(ms)

wavelength = period X velocity

PulseInput Parameters:

Velocity at the zone of interest

Peak Frequency of the pulse at the zone of interestComputations:

Period = 1/Peak Frequency

Wavelength = Period * Velocity

Limit of Vertical Resolution = Wavelength/4



A Simple Exercise - 2 Zones

Calculating Vertical Resolution



Typical Vertical Resolution

Shallow EventShallow EventVelocity = 2000 Meters / secVelocity = 2000 Meters / sec

Pulse:Pulse:

Center Frequency = 50 HzCenter Frequency = 50 Hz

Period = 1 / 50 = .020 secPeriod = 1 / 50 = .020 sec

Wavelength = .020 x 2000 = 40 Wavelength = .020 x 2000 = 40 MetersMeters

Limit of resolution = 40 /4 = 10 Limit of resolution = 40 /4 = 10 MetersMeters Deep Event Deep Event

Velocity = 3000 Meters / secVelocity = 3000 Meters / sec

Pulse:Pulse:

Center Frequency = 20 HzCenter Frequency = 20 Hz

Period = 1 / 20 = .050 secPeriod = 1 / 20 = .050 sec

Wavelength = .050 x 3000 = 150 Wavelength = .050 x 3000 = 150 MetersMeters

Limit of resolution = 150 / 4 = 37.5 Limit of resolution = 150 / 4 = 37.5 MetersMeters



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



What Is Lateral Resolution?

Would we image the narrow horst?

Would we image all three channel sands?



What is the minimum horizontal distance What is the minimum horizontal distance between two subsurface features such that we between two subsurface features such that we can tell them apart seismically?can tell them apart seismically?

Lateral Resolution

Neidell & Poggiaglioimi, 1977

AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.



• 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

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



800 m

Migration Reduces Lateral Smearing

Ideal / ModelResponse

Image AfterMigration

StackNo Migration



Standard Migration High-end Migration

Good Migration Enhances Resolution



Fresnel Zone Equations

Pre-Migration Post-Migration

where:

Fd = Fresnel Diameter

Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength

Fd = λ /4 = Vavg /4 FFd = Vavg T/F



Another Simple Exercise - 2 Zones

Calculating Fresnel Zone Diameters



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– 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



Graphical Answers

Fresnel Zone Circles

Shallow Window282 m pre-migration10 m post-migration

Deep Window1900 m pre-migration47.5 m post-migration

1 km1 km



• 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

Summary: Lateral Resolution

f w schroeder 04 l 9 – seismic resolutioncourtesy of exxonmobil lecture 8 layer thickness 11020 20...

Documents

f w schroeder

vertical resolution

detection resolution

migration slide

c slide

resolution bandwidth

noise resolution

resolution analogy