Introduction
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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.
Illustration of a marine seismic survey using a towed streamer
In this section we will visualize seismic data, import seismic and interpret seismic.
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Display a seismic line- In the folder 3. Sediment Data / Seismic Data / ATL-99 right click the seismic line ATL-99-1 and click Seismic Display
- In the folder 3. Sediment Data / Seismic Data / ATL-LOS-00 right click the seismic line ATL-LOS-00-1 and click Seismic Display
- Investigate the seismic lines. Can you identify the Seabed and Sediment Base?
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Display the interpretation- In the folder 3. Sediment Data / Seismic Interpretations right click the Ihorizon Seabed and click IHorizon Display.
- In the folder 3. Sediment Data / Seismic Interpretations right click the Ihorizon Sediment Base and click IHorizon Display.
- Investigate the interpreted horizons. Notice that there is only interpretation for the first line.
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Interpret the Seabed- Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap
- Select Interpetation folder: Click the Browse button,select the folder Seismic Interpretation Interpretations and click OK
- Select Ihorizon: Click the Browse button, select Seabed and click OK
- 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)
- Keep the rest of the settings as default and click the button.
- 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.
- When you want to stop, click the button. This will save the interpretation. You can continue to interpret the seabed by clicking the button again.
- When you are done with the seabed, click the button.
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Interpret the Sediment Base- Change Ihorizon: Click the Browse button, select Sediment Base and click OK
- Keep the rest of the settings and click the button.
- 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.
- When you want to stop, click the button. This will save the interpretation. You can continue to interpret the sediment base by clicking the button again.
- When you are done with the sediment base, click the button.
- Click Close on the Seismic Interpretation menu.
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Import SEG-Y files- 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.
- Click the Defaults button in order to reset the menu to the default options.
- Click the 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.
- Look at the information in the EBCDIC header and see if the information matches the import settings.
- Click Scan. Geocap willinvestigate the SEG-Y file.
- Click the Scan results tab. Investigate the range of the file and that the coordinates, shot point range etc. looks correct.
- Click Execute
- 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
- Click OK again
- Click OK in order to close the SEG-Y import command.
- In the folder 3. Sediment Data / Seismic Data / ATL-LOS-00 right click the seismic line ATL-LOS-00-2 and click Seismic Display
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Auto track the Seabed- Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap.
- Select Interpetation folder: Click the Browse button,select the folder Seismic Interpretation and click OK.
- Select Ihorizon: Click the Browse button, select Seabed and click OK
- Select the Seismic: Click the button and click on the seismic line ATL-LOS-00-2 in the display window (with your left mouse button)
- Change the Snap mode to Min
- Change the Track mode to Auto
- Keep the rest of the settings and click the button.
- 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.
- Zoom out and see if the whole seabed was interpreted. If not, continue to press the <space> key where it is missing interpretation.
- When the whole seabed has been interpreted, click the button.
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Complete the Sediment Base interpretation- Change Ihorizon: Click the Browse button, select Sediment Base and click OK
- Change the Snap mode to Off
- Click the button.
- Complete the sediment base interpretation and click
- Close the seismic interpretation menu when you are done.
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