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 American Association of Petroleum Geologists

Slides and talking points are provided courtesy of AAPG Visiting Geoscientist Fred W. Schroeder.

The notes for each slide are printed next to each thumbnail. Below each thumbnail are download links for the individual slide. Right-click on a link to save the file to your hard drive. To preview the full-size slide image, click on the thumbnail.

# Seismic Resolution

• Printing Instructions:
• 8a-“Calculating Vertical Resolution”
• one document, 1 page, letter size, B&W
• 8b-"Calculating Lateral Resolution"
• one document, 1 page, letter size, B&W
• Supplies:
• 8a and 8b - Pencils or pen, calculator (cell phone OK)
• Solutions for these exercises are provided within the Lecture Slides

#### 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

#### Slide 2

• We need to discuss two components of seismic resolution:
• Vertical resolution
• Lateral resolution

#### 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

#### Slide 4

• Detection is the ability to identify that some feature exists
• Resolution is the ability to distinguish two features from one another

#### 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

#### 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

#### 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”

#### 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

#### 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

#### 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

#### Slide 11

• 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

#### Slide 12

• To summarize our discussion of vertical resolution:
• Resolution is the ability...
• Thin bed response...
• Short-duration...
• To improve...

#### 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

#### 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'

#### 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

#### 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

#### 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

#### 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

#### 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

#### Slide 20

• 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

#### 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

#### Slide 22

• In summary for lateral resolution:
• Migration...
• Large aperture...
• Fine...
• Prestack...
• Depth migration...

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