L. Seismic 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 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 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:

1. Click Browse button to select the interpretation folder
2. Select the seismic interpretation folder 3.Sediment Data / Seismic Interpretations

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

1. Click the 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.

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:

• Click the button and click on the seismic line 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

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.

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 button

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.

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.