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Introduction

In order to use the sediment formulae criterion in UNCLOS Article 76, we need a dataset containing the sediment thickness. This dataset can be based on seismic data. This tutorial has some exercises which cover the basics for working with seismic dataaccurately 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.

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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

 

Import SEG-Y files

This exercise will guide you through import of a seismic line. 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. In the example we will use the SEG-Y file which is stored with the Atlantis project.

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
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Display a seismic line

  1. In the folder 3. Sediment Data / Seismic Data
In this exercise we will also generate a sub-folder in order to organize our data.
  1. Locate / ATL-99 right click the seismic line ATL-99-1 and click Seismic Display
  2. In the folder 3. Sediment Data / Seismic Data in your project
  3. Right click the folder and select New > Folder
  4. 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)
  5. Right click the folder and select Import > SEG-Y 2D. The SEG-Y import menu will appear.
  6. Click the Defaults button in order to reset the menu to the default options.
  7. Click the file browse button and select the right click the seismic line ATL-LOS-00-1.segy-file located in the Atlantis folder structure on disk (Projects/Atlantis_2_1/Data/Sediment data/Atl-los-1 -segy).
  8. Click Open. The file follows the SEG-Y standard and we do not have to change the default settings.
  9. It might be a good idea to check that the Storage: in the lower right corner is set to 8 bit.
  10. Click Execute
  11. You will be prompted with a dialog asking you for datum and projection for the seismic line. Select WGS 84 and Mercator and click OK
  12. Click OK again
  13. Click OK in order to close the SEG-Y import command.

In order to calculate the sediment thickness based on a seismic line we have to interpret the seabed surface and the base of sediments. This is done by using the Seismic Interpretation (Xi) functionality.

  1. and click Seismic Display
  2. Investigate the seismic lines. Can you identify the Seabed and Sediment Base?
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Interpret a seismic line

In this exercise we are going to interpret the seabed and basement of the seismic line. First we need to display the seismic line we want to interpret.

  1. Right click the seismic line and select Seismic display.
  2. Right click the seismic line and select Zoom to Data

Next we open the Seismic Interpretation (Xi) dialog:

  • Locate Seismic Interpretation under Tools in the main menu.

First we need to select where our interpretations should be stored:

  • Click Browse button to select the interpretation folder
    • Select the seismic interpretation

    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

    Next we need to specify the horizons we want to interpret:

    1. Click the Image Removed button in order to create a new IHorizon. The IHorizon dialog will appear
    2. Type in Seafloor as the name of the new horizon, and click OK
    3. Click the Graphical Settings tab
    4. Set Line Width to 5
    5. Check the User Defined Color and pick a color for the horizon.
    6. Click OK
    7. Click the same new button again, and call the next horizon Sediment Base. right click the Ihorizon Sediment Base and click IHorizon Display.
    8. 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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    Before we can start interpreting we need to specify which horizon it is that we want to interpret.

    • Click the Browse button in the IHorizon row. Select Seabed and click OK

    Next we need to select which seismic line we want to interpret:

    Interpret 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 Interpretations and click OK
    3. Select Ihorizon: Click the 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)

    Finally, since we're starting off with manual interpreting, we turn off the snap.

    Set Snap mode to Image Removed
    2. Keep the rest of the settings as default and click the Image Added button.
    3. 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.
    4. When you want to stop, click the Image Added button. This will save the interpretation. You can continue to interpret the seabed by clicking the Image Added button again.
    5. When you are done with the seabed, click the Image Added 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 the 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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    We have now set up everything we need and are ready to start interpreting. Interpreting manually:

    Click

    Interpret the Sediment Base

    1. Change Ihorizon: Click the browse button, select Sediment Base and click OK
    2. Keep the rest of the settings and click the Image Modified button.
    3. Pick 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 seismic line sediment base and pressing the p or <space> key on the keyboard. If you want Press d to undo a pick, you can press the d key.
    4. When you are finished interpreting want to stop, click the Image Modified 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 timeThis will save the interpretation. You can continue to interpret the sediment base by clicking the Image Added button again.
    3. When you are finished interpreting done with the sediment base, click the Image Modified 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.
    1. Click Close on the Seismic Interpretation menu.

    Your interpretation should look something like this when you are finished:
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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. If you have problems importing a SEG-Y file at a later stage, please contact support@geocap.no

     

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    Interpreting with auto-tracking:

  • 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.
  • Select Snap mode accordingly.
  • Select Track mode to be Auto.
  • 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.
  • Pick the horizon to autotrack by pressing the <space> or p key on the keyboard
  • The horizon is automatically interpreted
  • If the interpretation stopped at one point press <space> or p again to continue where it stopped.
    • When you are happy with the interpretation click the Image Removed button

    Import SEG-Y files

    1. In the folder 3. Sediment Data/Seismic Data right click the folder ATL-LOS-00 and click Import > SEG-Y 2D. The SEG-Y import menu will appear.
    2. Click the Defaults button in order to reset the menu to the default options.
    3. Click the browse button and select the ATL-LOS-00-2.sgy-file located in the Atlantis folder structure on disk (Data/Atlantis/Sediment Data/ATL-LOS-00-2.sgy). Click Open.
    4. Look at the information in the EBCDIC header and see if the information matches the import settings.
    5. Click Scan. Geocap willinvestigate the SEG-Y file.
    6. Click the Scan results tab. Investigate the range of the file and that the coordinates, shot point range etc. looks correct.
    7. Click Execute
    8. 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
    9. Click OK again
    10. Click OK in order to close the SEG-Y import command.
    11. 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 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.

    Interpreting with insertion (semi-auto):

  • Keep the Snap mode to be same as you used before.
  • Select Track mode to be Insert
  • Set Interval to 5. This will interpret a point at every fifth trace.
  • Click the Image Removed button.
  • Pick a point on a strong reflection (for example the seabed).
  • Pick a new point a bit to the side, on the same reflection. Observe that auto tracker automatically interprets the line.
  • When you are finished interpreting click the Image Removed button.
    • Observe that the old interpretation is updated with the new interpretation
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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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    The auto tracker can be very useful on strong reflectors. However when interpreting the base of sediments, the manual approach is probably better.

  • Use a combination of the editing methods above to do an interpretation of the seabed
  • Before starting on the base of sediments, we need to tell the interpreter that we are starting on a new horizon.
  • Click Browse on the IHorizon row, and select Sediment Base
  • Interpret the sediment base using the same methods as for the seafloor.
    • When you are finished with both horizons, click the Close button.

    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