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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Interpret the Seabed- Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap
- Select the Interpetation folder: Click the Browse buttonandselect the folder Seismic Interpretation
- Select the Ihorizon: Click the Browse button, select Seabed and click OK
- 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)
- Keep the rest of the settings as default and click the Image Modified 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 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 the Image Modified button again.
- When you are done with the seabed, click the Image Modified button.
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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 Modified. |
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- Change the Ihorizon: Click the Browse button, select Sediment Base and click OK
- Keep the rest of the settings and click the Image Modified 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 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 by click the Image Modified button again.
- When you are done with the sediment base, click the Image Modified button.
- Click Close on the Seismic Interpretation menu.
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Auto track the Seabed- Go to Tools > Seismic Interpretation. This is the interpretation system in Geocap
- Select the Interpetation folder: Click the Browse buttonandselect the folder Seismic Interpretation
- Select the Ihorizon: Click the Browse button, select Seabed and click OK
- Select the Seismic: Click the Image Modified 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 Image Modified 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 Image Modified button.
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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- Change the Ihorizon: Click the Browse button, select Sediment Base and click OK
- Change the Snap mode to Off
- Click the Image Modified button.
- Complete the sediment base interpretation
- Close the seismic interpretation menu when you are done.
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