1. From Raw Data to an Image
Field Record
(marine)
Data Processing
Stream
Offset
Time
SLIDE 17
This first image shows what the raw seismic data for one “explosion” recorded at about 50 “listening” devices looks like
The raw seismic data is sent to a data processing center
Here specialists in signal analysis, data analysis, imaging, math, computer science, etc. transform the raw data into an image of the subsurface
Again at large companies, there are many experts in data processing that can spend an entire career processing seismic data
Seismic imaging is an area that remains a focus of R&D – obtain better images faster and cheaper
We cover some basics of data processing in Unit 5
Once the data are processed and we have an image of the subsurface, the next step is to interpret the data – turn images such as the one in the lower right into a model of the subsurface rocks and fluids
Several Units focus on sesimic interpretation and data anlysis
Subsurface
‘Image’
Courtesy of ExxonMobil
L 3 - Types of Data

2. Seismic Processing

3. Deconvolution – removing unwanted parts of the signal “spiking”

4. Statics Correction

5. The reflection is displayed beneath the source-receiver midpoint
Positioning Problems
Energy
Source  
0.2 s up
0.2 s down
SLIDE 13
Thus far we have kept everything quite simple by assuming the boundaries that generate reflections are horizontal (flat)
Problems develop if the boundaries are not flat – they have some dip
When the boundary is dipping, what we record is the energy that travels from the source and hits the boundary at 90 degrees
In the figure on the left, we record the energy that follows the dotted black line
In this case, the energy travels 0.2 seconds down to the bounce point and 0.2 seconds up to the receiver – a total of 0.4 seconds
When this is displayed, we would plot the reflection vertically below the shot at 0.4 seconds
So we capture the reflected energy, but do not place it in the correct position
This is another thing we have to correct – but, don’t worry, we have methods to apply this correction!
0.4 s -
Bounce
Point
The reflection is displayed beneath the source-receiver midpoint
The seismic ray hits an inclined surface at 90º and reflects back
Courtesy of ExxonMobil
L 5 – Seismic Method

6.       Time for an Exercise 90º Where would the reflection lie? 12 3 4 5 6      
SLIDE 14
Time for an exercise!
Here we have a dipping seafloor
We will consider 6 shot locations and only the direct down and direct up ray path
That direct down – direct up raypath hits the seafloor where it is 90 degrees – as shown for shot #1
We want to figure out where the reflected energy would be displayed (without corrections)
90º
Where would the reflection lie?
Courtesy of ExxonMobil
L 5 – Seismic Method

7.       Time for an Exercise Compass1 2 3 4 5 6      
SLIDE 15
To figure out where the reflected energy would be displayed, we can use a compass
Place the point on the shot point (here shot point #1)
Place the pencil point at the place where the raypath hits the seafloor at 90 degrees – as shown for shot #1
Compass
Where would the reflection lie?
Courtesy of ExxonMobil
L 5 – Seismic Method

8.       Time for an Exercise Where would the reflection lie? 1 2 34 5 6      
SLIDE 16
Next we swing an arc so that the pencil point is directly below the shot point
Thus on the previous slide we captured the distance (for a seismic section that would be related to the time)
And on this slide we have determined where it would be displayed – directly beneath the associated shot
NOW THE STUDENTS SHOULD SWING ARCS FOR THE OTHER SHOTS
Where is the “bounce” point – where the ray path is at 90 degrees
Swing an arc to locate where the recorded reflection would be plotted/displayed
This should take about 5 minutes
Where would the reflection lie?
Courtesy of ExxonMobil
L 5 – Seismic Method

9. The reflection is downdip and its dip is less than the interface
Exercise Answer 1 2 3 4 5 6      
SLIDE 17
ANSWER – This is what you should have found
The seafloor (reflection surface) is the black line
BUT the seismic reflection is displayed where the red line is located
We considered only 6 points – if the shots were more closely spaced, we would get a continuous reflection
NOTE: a continuous surface in the subsurface will result in a continuous reflection on the seismic section
The reflection is downdip and its
dip is less than the interface
Courtesy of ExxonMobil
L 5 – Seismic Method

10. Wavefront Migration – Correcting for Location
Sweep Ellipse S R
Unmigrated energy on single trace... S R
Sweep Ellipse OR
SLIDE 18
There is a data processing procedure that we call seismic migration that corrects this mis-positioning problem
For each trace, the computer swings arcs (based on the velocities) to find all the possible locations from which a reflection could have originated
To illustrate, consider the black peak circled by a green oval in the upper left – we will call this the “green” peak
In the lower left we have swung arcs to define all the possible reflections points
The dotted green arc shows the possible reflections points for the “green” peak
On the right, we show 3 possible shot-receiver pairs that could be the reason for the “green” peak
But which one is correct – of these 3 or some other case?
Seismic migration will answer this for us
...spread to all possible locations of origin S R
Sweep Ellipse OR
Courtesy of ExxonMobil
L 5 – Seismic Method

11. Migration – Power of Correlation
Two reflections on unmigrated data
After spreading to all possible locations
SLIDE 19
Since we are dealing with waves where we have some positive and negative numbers and closely-spaced traces, as we migrate all the traces something wonderful happens
For the small piece of the arcs from the true position (the right answer mentioned on the previous slide) there is constructive interference and the wave shape is preserved and enhanced
For all other places along the arcs, there is destructive interference – positive numbers are canceled by negative numbers
So in simple terms, all the incorrect places along each arc are wiped out, but the correct location is preserved
On the left side of this slide we show 2 seismic reflections
They are dipping, so they are not is the correct position
Since the correction has NOT been applied, we call this unmigrated data
On the right side of this slide we show the 2 seismic reflections after they have been migrated
Both reflections have been moved
The red dotted lines show where the unmigrated reflections (left figure) are for reference
Migration moves the reflections deeper and further updip
Low dips -> slight corrections moving events deeper and updip
High dips -> larger corrections moving events deeper and updip
The cancellation is not perfect, so you see some ‘noise’ away from the reflections
Reflections are not positioned in the subsurface correctly since they have dip
Constructive interference occurs where the reflections are properly positioned
Destructive interference dominates where the reflections are NOT properly positioned
Courtesy of ExxonMobil
L 5 – Seismic Method

12. Other methods of migration
Diffraction Stack – Sum arrivals along Diffraction Hyperbola

13. Seismic Migration Positioning Problems ‘Blur’ the Image
Unmigrated Image
Positioning Problems ‘Blur’ the Image
SLIDE 20
Here is a comparison of the same seismic line before (upper) and after (lower) seismic migration
Note there is poor imaging – lots of smearing – of the structure
Performing even a simple migration has improved the image of the structure – in this case there is a thrust fault
Migrated Image
Migration Reduces
Positioning Problems, which Improves the Image
Courtesy of ExxonMobil
L 5 – Seismic Method

14. Migration Example – North Sea Salt Dome
Velocity Model
Kirchoff Migration
Reverse Time Migration
CGGVeritas