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 http://www.ig.utexas.edu/people/staff/mrinal/Results/carlos_nn1_files/image006.jpg Subsurface ‘Image’ Courtesy of ExxonMobil L 3 - Types of Data

Seismic Processing

Deconvolution – removing unwanted parts of the signal “spiking”

Statics Correction

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

      Time for an Exercise 90º Where would the reflection lie? 1 2 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

      Time for an Exercise Compass 1 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

      Time for an Exercise Where would the reflection lie? 1 2 3 4 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

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

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

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

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

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

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