# Seismic Resolution Lecture 8 * Layer Thickness top 20 ms base

## Presentation on theme: "Seismic Resolution Lecture 8 * Layer Thickness top 20 ms base"— Presentation transcript:

Seismic Resolution Lecture 8 * Layer Thickness top 20 ms base 1 10 20
30 base top * SLIDE 1 Slide introduces topic: Seismic Resolution This shows a simple sediment wedge model and its seismic expression – we’ll talk about it in this lecture Courtesy of ExxonMobil L 9 – Seismic Resolution

Vertical Resolution Lateral Resolution Outline Resolution vs Detection
Thin Bed Response and Tuning Lateral Resolution Fresnel Zone Migration and Lateral Resolution SLIDE 2 We need to discuss two components of seismic resolution: Vertical resolution Lateral resolution Courtesy of ExxonMobil 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! SLIDE 3 Here is an analogy that we can all relate to: You are driving at night You spot a light in the distance coming towards you You wonder, I seem to see only 1 light; is it a car or a motorcycle As the vehicle gets never, we realize it is not a single light but two headlamps – so it is a car You first detected some light and know there was a vehicle It was not until the vehicle was closer that we were able to resolve two headlights and realize it was a car This analogy helps explain the difference between Detecting something with seismic data, and Resolving two closely-spaced objects Courtesy of ExxonMobil 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 SLIDE 4 Detection is the ability to identify that some feature exists Resolution is the ability to distinguish two features from one another Courtesy of ExxonMobil L 9 – Seismic Resolution

Vertical Resolution Gamma Ray What is the minimum vertical distance between two subsurface features such that we can tell them apart seismically? Shale Baseline Shale SLIDE 5 As an example of vertical resolution, consider the geology indicated by the gamma ray log At a gross scale, there is a thick shale unit on top of a thick sand unit But the sand unit has a thin shale layer interfingered with it near the top Low resolution seismic data would detect a shaley unit sitting on top of a sandy unit - one interface Seismic data with high resolution would resolve 3 interfaces, identifying the thin shale unit within the predominantly sandy unit Sd For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands? Sand Courtesy of ExxonMobil L 9 – Seismic Resolution

Thick Bed Response A Dp B C Question: What is a thick bed?
Wavelet 1 ends before Wavelet 2 begins Question: What is a thick bed? Impedance R. C. Wavelet 1 Wavelet 2 Composite A Top of Bed Response Dp NO Interference B SLIDE 6 To further explain vertical resolution, let’s begin by considering a thick sand (unit B) sandwiched between shales (units A and C) The RC at the top and base of the sand are shown along with the individual wavelets Note the pulse duration is less than the thickness of the sand unit The wavelet associated with the upper RC is fully represented (going down) before the wavelet associated with the lower RC starts There is no wavelet interference A thick bed is one in which the bed thickness in units of two-way time is greater than the pulse duration C Base of Bed Response Answer: A thick bed is one that has a TWT > Dp Courtesy of ExxonMobil L 9 – Seismic Resolution

2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2
Partial Interference TWT thickness = 0.9 * Dp Wavelet 2 starts before Wavelet 1 ends Impedance R. C. Wavelet 1 Wavelet 2 Composite A Top of Bed Response B Dp Some Interference C SLIDE 7 Here the thickness of unit B has been decreased to 0.9 times the pulse duration The wavelet associated with the upper RC does not complete (going down) before the wavelet associated with the lower RC starts There is some wavelet interference – the end of the “upper” wavelet overlaps the top of the “lower” wavelet An interpreter still would be able to distinguish two RCs, but the trough is a “doublet” Base of Bed Response 2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 form a trough doublet Courtesy of ExxonMobil L 9 – Seismic Resolution

Maximum Interference - Tuning
TWT thickness = ½ Dp Wavelet 2 starts before Wavelet 1 ends Impedance R. C. Wavelet 1 Wavelet 2 Composite A Top of Bed Response B Dp Maximum Interference C Base of Bed Response SLIDE 8 On this slide, the thickness of unit B has been decreased to 1/2 the pulse duration The second part of the wavelet associated with the upper RC overlaps with the first half of the wavelet associated with the lower RC Wavelet interference is at a maximum The trough is larger by about a factor of two than if there was only one RC It is more difficult for an interpreter to distinguish two RCs 2nd half-cycle from Wavelet 1 and 1st half-cycle from Wavelet 2 are completely in phase resulting in 2x amplitude Courtesy of ExxonMobil L 9 – Seismic Resolution

Determining Vertical Resolution
Input Parameters: Velocity at the zone of interest Peak Frequency of the pulse at the zone of interest Period (ms) wavelength = period X velocity Pulse Computations: Period = 1/Peak Frequency Wavelength = Period * Velocity Limit of Vertical Resolution = Wavelength/4 SLIDE 9 To determine seismic resolution, there are two parameters we need to know or estimate The velocity in the zone we are interested in The peak frequency of the pulse in the zone of interest We need to calculate the wavelength of the data Vertical resolution is ¼ the wavelength The calculation is shown in the center of the slide We get the period from 1/peak frequency We then get the wavelength by multiplying the period by the velocity If you prefer, wavelength = velocity / peak frequency (simple substitution) Next we divide the calculated wavelength by 4 to get the vertical resolution Courtesy of ExxonMobil L 9 – Seismic Resolution

A Simple Exercise - 2 Zones
SLIDE 10 Time for an exercise You will calculate the vertical frequency for: A shallow zone A deep zone The next slide has the ANSWER Have the students do the exercise before proceeding Calculating Vertical Resolution Courtesy of ExxonMobil L 9 – Seismic Resolution

Typical Vertical Resolution
Shallow Event Velocity = 2000 Meters / sec Pulse: Center Frequency = 50 Hz Period = 1 / 50 = sec Wavelength = .020 x 2000 = 40 Meters Limit of resolution = 40 /4 = 10 Meters Deep Event Velocity = Meters / sec Pulse: Center Frequency = 20 Hz Period = 1 / 20 = sec Wavelength = x = 150 Meters Limit of resolution = 150 / 4 = Meters SLIDE 11 ANSWER The shallow zone of interest has a wavelength of 40 meters; a vertical resolution of 10 meters The deep zone of interest has a wavelength of 150 meters; a vertical resolution of 37 meters Courtesy of ExxonMobil 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 SLIDE 12 To summarize our discussion of vertical resolution: Resolution is the ability…… Thin bed response …… Short-duration ……. To improve …… Courtesy of ExxonMobil L 9 – Seismic Resolution

What Is Lateral Resolution?
Would we image the narrow horst? Would we image all three channel sands? SLIDE 13 What do we mean by lateral resolution? It means how wide does a feature have to be for us to correctly resolve it For example, in the upper diagram, there is a narrow horst block in the center If this horst is only 10 meters wide, we probably would not resolve the two edges. If it was 2 km wide, we would not have any problem resolving the horst What is the minimum width for which we could resolve both edges? This is why we want to know the lateral resolution of the seismic data In the lower diagram, we have three channel deposits of different widths Would we resolve all three; or only the widest one Again, this is why we want to know the lateral resolution of the seismic data Courtesy of ExxonMobil L 9 – Seismic Resolution

Lateral Resolution What is the minimum horizontal distance between two subsurface features such that we can tell them apart seismically? Neidell & Poggiaglioimi, 1977 SLIDE 14 Here is a ‘classic’ seismic model presented by Neidell & Poggiaglioimi, 1977 In the model there is a reflector (upper black line) that has gaps in it of varying width On the next slide, we will explain what a Fresnel zone (FZ) is; for now Accept that the first gap = 2x the FZ The second gap = 1x the FZ Etc. The lower part of the figure shows the modeled seismic response (unmigrated) Looking at the modeled seismic, we would: Recognize the first gap Probably recognize the second gap Would wonder if the third gap is a break in the reflector And probably not recognize any break for the fourth gap Remember, the model is ‘noise-free’ AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use. Courtesy of ExxonMobil 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 SLIDE 15 As promised, we will now explain what a Fresnel zone (FZ) is The seismic waves “illuminate” an area of a subsurface boundary – like the cone of light from a flashlight shining on a carpet All the information within this “illuminated” area is “lumped together” or averaged The size of this “illumination” circle equals the area in which the seismic wave is ¼ the wavelength of the pulse The diameter of this circle is called the FZ Shallow in the data the FZ is narrow; it gets progressively broader as we go deeper Using our flashlight analogy: If our flash light is close to the carpet, the circle of light is small If our flash light is far from the carpet, the circle of light is large 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 L 9 – Seismic Resolution

Migration Reduces Lateral Smearing
Ideal / Model Response 800 m Stack No Migration SLIDE 16 Fortunately for us, the data processing step called migration: Not only better positions the reflections in 3D space, but Also greatly improves lateral resolution This slide shows a reflection indicating a strong decrease in impedance (zero phase central trough) on the left and a abrupt change to a moderate increase in impedance (zero phase central peak) on the right The ideal response is in the upper figure The real-world response is shown in the central figure – a stacked section without migration The bottom shows what happens when seismic migration is applied to the data in the central figure Note how the abrupt change in the center is “smeared” in the central figure The FZ for this example is on the order of 800 m (red arrow) Also note how the migration process has “cleaned up” the image and the abrupt change is much better imaged Image After Migration Courtesy of ExxonMobil L 9 – Seismic Resolution

Good Migration Enhances Resolution
SLIDE 17 Here is a seismic line with two types of migration: On the left a standard (fast,cheap) migration algorithm was used On the right, a more sophisticated (more time, money, people-hours) algorithm was used Note the fault on each image The termination of reflections are much sharper on the right; the fault can be more precisely drawn On the left the reflection terminations are more “smeared” since the lateral resolution is much lower Standard Migration High-end Migration Courtesy of ExxonMobil 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 SLIDE 18 Here are the equations that we use to calculate the Fresnel diameter The equation on the left is for data that have not been migrated The parameters are the average velocity down to the zone of interest, the time down to the zone of interest, and the frequency at the zone of interest The equation on the right is for data that has had a seismic migration process applied to it The parameters are the wavelength of the pulse at the zone of interest; or by substitution the average velocity and the frequency Courtesy of ExxonMobil L 9 – Seismic Resolution

Another Simple Exercise - 2 Zones
SLIDE 19 Let’s do another exercise You will be given the necessary parameters for: A shallow zone A deep zone The ANSWER is on the next slide Give the students some time to work the exercise Calculating Fresnel Zone Diameters Courtesy of ExxonMobil 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 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 SLIDE 20 ANSWER to the exercise For the shallow zone – pre-migration, the FD is 282 m; after migration it is reduced to 10 meters – what an improvement For the deep zone – pre-migration, the FD is 1900 m – almost 2 km; after migration it is reduced to 48 meters – another substantial improvement Courtesy of ExxonMobil L 9 – Seismic Resolution

Graphical Answers Fresnel Zone Circles Shallow Window Deep Window
282 m pre-migration 10 m post-migration SLIDE 21 This shows the area over which the seismic “smears” the geologic information from our last exercise Note the 1 km scale bar The small green circle in the upper left is the FD for the shallow zone before migration There is a white circle in the center which is the FD after migration The large circle on the right is the FD for the deep zone The white circle in the center is the FD after migration Even if the seismic reflections are fairly flat lying (horizontal), this shows the benefit of migrating the data – even though the reflctions are not repositioned very much since dips are very low Deep Window 1900 m pre-migration 47.5 m post-migration 1 km Courtesy of ExxonMobil 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 SLIDE 22 In summary for lateral resolution: Migration …… Large aperture …. Fine ….. Prestack …… Depth migration ….. Courtesy of ExxonMobil L 9 – Seismic Resolution

Download ppt "Seismic Resolution Lecture 8 * Layer Thickness top 20 ms base"

Similar presentations