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Introduction

In order to accurately calculate the sediment thickness we have to interpret the seabed surface and the base of sediments on seismic lines. In Geocap these two layers are called horizons. The distance between the seabed and basement horizons is the sediment thickness. 

When collecting seismic data a seismic wave travels from its source to a given reflector and returns to a receiver at the sea surface. The elapsed time is called two way travel time. In most cases this will be the depth unit (in milliseconds) of your seismic. Since the seismic is in time (milliseconds) and we need to calculate the sediment thickness in meters we need to do a conversion. This is called a depth conversion. The formula itself is simple: Distance = Speed * Time. The difficult part of the formula is finding the correct speed/velocity model, since the velocity changes through the different layers in the subsurface. In Geocap the interpretation is normally done in the time domain. This gives us the flexibility to change the velocity model and just update the sediment thickness calculation accordingly. We will have a look at different velocity models in the next section.

Image Added
Illustration of a marine seismic survey using a towed streamer

In this section we will visualize seismic data, import seismic and interpret seismic.

Exercises

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Display Seismic Lines and Interpretation

 

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Display a seismic line

  1. In the folder 3. Sediment Data / Seismic Data / ATL-99 right click the seismic line ATL-99-1 and click Seismic Display
  2. In the folder 3. Sediment Data / Seismic Data / ATL-LOS-00 right click the seismic line ATL-LOS-00-1 and click Seismic Display
  3. Investigate the seismic lines. Can you identify the Seabed and Sediment Base?
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Display the interpretation

  1. In the folder 3. Sediment Data / Seismic Interpretations right click the Ihorizon Seabed and click IHorizon Display.
  2. In the folder 3. Sediment Data / Seismic Interpretations right click the Ihorizon Sediment Base and click IHorizon Display.
  3. Investigate the interpreted horizons. Notice that there is only interpretation for the first line.

Interpret Seismic Lines

In the next exercises we are going to complete the interpretation of the Seabed and Sediment Base for the seismic line ATL-LOS-00-1. 

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Interpret the Seabed

  1. Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap
  2. Select the Interpetation folder: Click the Browse browse buttonand,select the folder Seismic InterpretationInterpretations and click OK
  3. Select the Ihorizon: Click the Browse browse button, select Seabed and click OK
  4. Select the Seismic: Click the Image Modified button and click on the seismic line ATL-LOS-00-1 in the display window (with your left mouse button)
  5. Keep the rest of the settings as default and click the Image Modified button.
  6. Zoom to where the yellow line ends on the previous line and start to pick points on the seismic line by pointing the mouse cursor on the seabed and pressing the <space> key on the keyboard. Press d to undo a pick.
  7. When you have finished to interpret the seabed or just want to stop, click the Image Modified button. This will save the interpretation. You can continue to interpret the seabed by click clicking the Image Modified button again.
  8. When you are done with the seabed, click the Image Modified button.
Tip

You do not have to delete parts of the interpretation if it is wrong. Just re-interpret the part that is wrong and Geocap will automatically update the Ihorizon when you click Image Removedthe stop button.

 

We are now going to interpret the Sediment Base but before we can do this we need to change the Ihorizon we are interpreting on.

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Interpret the Sediment Base

  1. Select the Change Ihorizon: Click the Browse browse button, select Sediment Base and click OK
  2. Keep the rest of the settings and click the Image Modified button.
  3. Zoom to where the blue line ends on the previous line and start to pick points on the seismic line by pointing the mouse cursor on the sediment base and pressing the <space> key on the keyboard. Press d to undo a pick.
  4. When you have finished to interpret the sediment base or just want to stop, click the Image Modified button. This will save the interpretation. You can continue to interpret the seabed sediment base by click clicking the Image Modified button again.
  5. When you are done with the sediment base, click the Image Modified button.
  6. Click Close on the Seismic Interpretation menu.

Your interpretation should look something like this when you are finished:
Image Modified 

 

 

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The auto tracker can be very useful on strong reflectors. However when interpreting the base of sediments, the manual approach is probably better.

  1. Use a combination of the editing methods above to do an interpretation of the seabed

  2. Before starting on the base of sediments, we need to tell the interpreter that we are starting on a new horizon.

     

  3. Click Browse on the IHorizon row, and select Sediment Base
  4. Interpret the sediment base using the same methods as for the seafloor.
  5. When you are finished with both horizons, click the Close button.
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Interpreting with auto-tracking:

  1. To select Snap mode look at the seismic display and identify the reflector representing the seabed. Compare the color of the seabed reflector with the seismic color table. The color on the left side of the color table represents minima; the color on the right represents maxima. The seismic color table used is one of three:

    1. current active seismic color table – look in the lower right corner of Geocap.
    2. the color table specified in Seismic display, if any.
    3. the color table dragged and dropped on top of the seismic line, if any.
  2. Select Snap mode accordingly.
  3. Select Track mode to be Auto.
  4. Set Delta Z. Delta Z is the maximum allowable vertical distance between two auto-tracked points. Z is usually time in ms. A good rule of thumb is that Delta Z should be the double of the sample interval in the seismic. In our case that is 8.
  5. Pick the horizon to autotrack by pressing the <space> or p key on the keyboard
  6. The horizon is automatically interpreted
  7. If the interpretation stopped at one point press <space> or p again to continue where it stopped.
  8. When you are happy with the interpretation click the Image Removed buttono Image Removed
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We have now set up everything we need and are ready to start interpreting. Interpreting manually:

  1. Click the Image Removed button.
  2. Pick points on the seismic line by pointing the mouse cursor on the seismic line and pressing the p or <space> key on the keyboard. If you want to undo a pick, you can press the d key.
  3. When you are finished interpreting click the Image Removed button.

Editing an interpretation:

  1. Click the Image Removed button.
  2. Re-pick points in an area where you have picked points before. You pick points the same way as you did the first time.
  3. When you are finished interpreting click the Image Removed button.
  4. Observe that the old interpretation is updated with the new interpretation.

Deleting parts of an interpretation:

  1. Click the Image Removed button.
  2. Pick two points: the start- and the end point of the part of the interpretation that you want to delete.
  3. The part of the interpretation between the two points is automatically deleted.
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Interpreting with insertion (semi-auto):

  1. Keep the Snap mode to be same as you used before.
  2. Select Track mode to be Insert
  3. Set Interval to 5. This will interpret a point at every fifth trace.
  4. Click the Image Removed button.
  5. Pick a point on a strong reflection (for example the seabed).
  6. Pick a new point a bit to the side, on the same reflection. Observe that auto tracker automatically interprets the line.
  7. When you are finished interpreting click the Image Removed button.
  8. Observe that the old interpretation is updated with the new interpretation.

 

Import Seismic Lines

 

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Import SEG-Y files

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Seabed and Sediment Base interpretations

Import Seismic Lines

There is one more seismic line in this area but it has not been imported yet. In the next exercise we will import this line and then continue to interpret it.

The format for seismic lines is SEG-Y. This format was developed by the Society of Exploration Geophysicists (SEG) in 1973. Since then format has evolved in to many different "flavors", which sometimes makes it difficult to import. When importing a SEG-Y file it is important to set the byte positions correctly. If you are lucky there might be some information about the byte positions in the EBCDIC header, and if you are even luckier the default settings in Geocap will be correct.  

This exercise will not go into details on seismic/SEG-Y import. It will cover the simple case for a plain SEG-Y file which follows the SEG-Y standard and has navigation stored in the trace header.

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 If you have problems importing a SEG-Y file

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A SEG-Y file can only be imported into a seismic data folder. If you are using Geocap's standard UNCLOS folder structure then such a folder is located in 3. Sediment Data/Seismic Data In this exercise we will also generate a sub-folder in order to organize our data.

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at a later stage, please contact support@geocap.no

 

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Import SEG-Y files

  1. In the folder 3. Sediment Data/Seismic Datain your project
  2. Right click the folder and select New > Folder
  3. Right right click the folder and select Rename. Give the folder the name of the survey. In this case ATL-LOS-00 (or give it another name if you are importing into the Atlantis project and the folder already exists)Right click the folder and select Import > SEG-Y 2D. The SEG-Y import menu will appear.
  4. Click the Defaults button in order to reset the menu to the default options.
  5. Click the file browse button and select the ATL-LOS-00-12.segysgy-file located in the Atlantis folder structure on disk (ProjectsData/Atlantis_2_1/Sediment Data/Sediment data/Atl-los-1-segy).
  6. Click Open. The file follows the SEG-Y standard and we do not have to change the default settings.
  7. It might be a good idea to check that the Storage: in the lower right corner is set to 8 bitATL-LOS-00-2.sgy). Click Open.
  8. Look at the information in the EBCDIC header and see if the information matches the import settings.
  9. Click Scan. Geocap willinvestigate the SEG-Y file.
  10. Click the Scan results tab. Investigate the range of the file and that the coordinates, shot point range etc. looks correct.
  11. Click Execute
  12. You will be prompted with a dialog asking you for datum and projection for the seismic line. Select WGS 84 and Mercator 0 and click OK
  13. Click OK again
  14. Click OK in order to close the SEG-Y import command.
  15. In the folder 3. Sediment Data / Seismic Data / ATL-LOS-00 right click the seismic line ATL-LOS-00-2 and click Seismic Display

 

Interpret

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Seismic Lines continued

We are now going to complete the interpretation of the Seabed and Sediment Base on the seismic line ATL-LOS-00-2. This seabed on this seismic line is very flat and has a strong continuous reflector. This lets us use the auto tracker to automatically interpret the seabed.

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Auto track the Seabed

  1. Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap.
  2. Select Interpetation folder: Click the browse button,select the folder Seismic Interpretation and click OK.
  3. Select Ihorizon: Click the browse button, select Seabed and click OK
  4. Select the Seismic: Click the Image Added button and click on the seismic line ATL-LOS-00-2 in the display window (with your left mouse button)
  5. Change the Snap mode to Min
  6. Change the Track mode to Auto
  7. Keep the rest of the settings and click the Image Added button.
  8. Zoom in close to the seabed, point the mouse cursor anywhere on the seabed and press the <space> key. This will automatically interpret the seabed.
  9. Zoom out and see if the whole seabed was interpreted. If not, continue to press the <space> key where it is missing interpretation.
  10. When the whole seabed has been interpreted, click the Image Added button.

 

We are now going to interpret the Sediment Base on the seismic line ATL-LOS-00-2.

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Complete the Sediment Base interpretation

  1. Change Ihorizon: Click the browse button, select Sediment Base and click OK
  2. Change the Snap mode to Off
  3. Click the Image Added button.
  4. Complete the sediment base interpretation and click Image Added
  5. Close the seismic interpretation menu when you are done.

The seabed and sediement base interpretation for all lines should look something like this:
Image Added
Seabed and Sediment Base interpretations