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F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Lecture 8 Layer Thickness 11020 20 ms 30 base toptop *

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Presentation on theme: "F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Lecture 8 Layer Thickness 11020 20 ms 30 base toptop *"— Presentation transcript:

1 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Lecture 8 Layer Thickness ms 30 base toptop *

2 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Vertical Resolution Resolution vs Detection Thin Bed Response and Tuning Lateral Resolution Fresnel Zone Migration and Lateral Resolution Outline

3 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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!

4 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Detection: Ability to identify that some feature exists 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 from one another

5 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil What is the minimum vertical distance between two subsurface features such that we can tell them apart seismically? Vertical Resolution Shale Sand Sd Gamma Ray Shale Baseline For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands?

6 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Thick Bed Response Question: What is a thick bed? ImpedanceCompositeWavelet 1 R. C.Wavelet 2 NO Interference DpDp Wavelet 1 ends before Wavelet 2 begins Top of Bed Response Base of Bed Response Answer: A thick bed is one that has a TWT > D p B A C

7 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Partial Interference TWT thickness = 0.9 * D p ImpedanceCompositeWavelet 1 R. C.Wavelet 2 Some Interference Wavelet 2 starts before Wavelet 1 ends Top of Bed Response Base of Bed Response 2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 form a trough doublet B A C DpDp

8 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Maximum Interference - Tuning TWT thickness = ½ D p ImpedanceCompositeWavelet 1 R. C.Wavelet 2 Maximum Interference Wavelet 2 starts before Wavelet 1 ends Top of Bed Response Base of Bed Response 2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 are completely in phase resulting in 2x amplitude B A C DpDp

9 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Determining Vertical Resolution Period (ms) wavelength = period X velocity Pulse Input Parameters: Velocity at the zone of interest Peak Frequency of the pulse at the zone of interest Computations: Period = 1/Peak Frequency Wavelength = Period * Velocity Limit of Vertical Resolution = Wavelength/4

10 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil A Simple Exercise - 2 Zones Calculating Vertical Resolution

11 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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

12 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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

13 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil What Is Lateral Resolution? Would we image the narrow horst? Would we image all three channel sands?

14 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil What is the minimum horizontal distance between two subsurface features such that we 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.

15 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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

16 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 800 m Migration Reduces Lateral Smearing Ideal / Model Response Image After Migration Stack No Migration

17 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Standard MigrationHigh-end Migration Good Migration Enhances Resolution

18 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Fresnel Zone Equations Pre-MigrationPost-Migration where: F d = Fresnel Diameter Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength F d = λ /4 = V avg /4 FF d = V avg T/F

19 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Another Simple Exercise - 2 Zones Calculating Fresnel Zone Diameters

20 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil Typical Lateral Resolution Shallow Event –Time = 1.0 s –V int = V avg = 2000 m/s –Pulse = 50 Hz –PreMig Fresnel Diameter = 282 m –PostMig Fresnel Diameter = 10 m Deep Event –Time = 5.0 s –V int = 4600 m/s –V avg = 3800 m/s –Pulse = 20 Hz –PreMig Fresnel Diameter = 1900 m –PostMig Fresnel Diameter = 47.5 m

21 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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

22 F W Schroeder 04 L 9 – Seismic ResolutionCourtesy of ExxonMobil 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


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