h4. 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.
# Right click the seismic line and select *Seismic display*.
# Right click the seismic line and select *Zoom to Data*
Next we open the Xi dialog:
* Locate *Xi 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 folder *3.Sediment Data / Seismic Interpretations*
Next we need to specify the horizons we want to interpret:
# Click the new !att:_img_icon^new_horizon_btn.png! button in order to create a new IHorizon. The IHorizon dialog will appear
# Type in *Seafloor* as the name of the new horizon, and click *OK*
# 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 button row. Select Seabed and click OK
Next we need to select which seismic line we want to interpret:
* Click the select 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 off:
We have now set up everything we need and are ready to start interpreting. Interpreting manually:
# Click the Start button: ,
# Pick points on the seismic line by pointing the mouse cursor on the seismic line and clicking the *P* key on the keyboard. If you want to undo a pick, you can click the *D* key.
# When you are finished interpreting click the stop button.
Editing an interpretation:
# Click the Start button,
# Re-pick points in an area where you have picked points before. You pick points the same way as you did the first time.
# When you are finished interpreting click the stop button.
If you interpret a horizon in an area where there is an earlier interpretation, your new interpretation will replace the old one. Deleting parts of an interpretation:
# Click the Erase button,
# Pick two points: the start- and the end point of the part of the interpretation that you want to delete
# The part of the interpretation between the two points is automatically deleted
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:
{indent}
* current active seismic color table -- look in the lower right corner of Geocap
* the color table specified in *Seismic display*, if any
* the color table dragged and dropped on top of the seismic line, if any
{indent}
# 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 clicking the *P* key on the keyboard
# The horizon is automatically interpreted
Interpreting with insertion (semi-auto):
# Select Snap mode to be Max
# Select Track mode to be Insert
# Set Interval to 5. This will interpret a point at every fifth trace.
# Click the start 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 stop button.
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 button 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.
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