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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.

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

Exercises

Display Seismic Lines and Interpretation

 

Exercise

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?
Exercise

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. 

Exercise

Interpret the Seabed

  1. Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap
  2. Select the Interpetation folder: Click the Browse button and select the folder Seismic Interpretation
  3. Select the Ihorizon: Click the Browse button, select Seabed and click OK
  4. Select the Seismic: Click the 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 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 button. This will save the interpretation. You can continue to interpret the seabed by click the button again.
  8. When you are done with the seabed, click the button.

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 .

 

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.

Exercise

Interpret the Sediment Base

  1. Select the Ihorizon: Click the Browse button, select Sediment Base and click OK
  2. Keep the rest of the settings and click the 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 button. This will save the interpretation. You can continue to interpret the seabed by click the button again.
  5. When you are done with the sediment base, click the button.

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

 

 

Exercise Continued

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.
Exercise Continued

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 buttono
Exercise Continued

We have now set up everything we need and are ready to start interpreting. Interpreting manually:

  1. Click the 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 button.

Editing an interpretation:

  1. Click the 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 button.
  4. Observe that the old interpretation is updated with the new interpretation.

Deleting parts of an interpretation:

  1. Click the 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.
Exercise Continued

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 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 button.
  8. Observe that the old interpretation is updated with the new interpretation.

 

Import Seismic Lines

 

Exercise

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 3. Sediment Data/Seismic Data In this exercise we will also generate a sub-folder in order to organize our data.

  1. Locate the folder 3. Sediment Data/Seismic Data in your project
  2. Right click the folder and select New > Folder
  3. 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)
  4. Right click the folder and select Import > SEG-Y 2D. The SEG-Y import menu will appear.
  5. Click the Defaults button in order to reset the menu to the default options.
  6. Click the file browse button and select the 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).
  7. Click Open. The file follows the SEG-Y standard and we do not have to change the default settings.
  8. It might be a good idea to check that the Storage: in the lower right corner is set to 8 bit.
  9. Click Execute
  10. 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
  11. Click OK again
  12. Click OK in order to close the SEG-Y import command.

Interpret Seismic Continued

 

 

 

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