[Users] Creating a 2D-only survey

[Bert Bril]

Hello,

I get a lot of support e-mail with users confused about how to set up a 
2D-only survey. So here goes.


** Setting up the survey

The important thing to remember is that in OpendTect, everything is tied 
to a 3D (X,Y,Z) with Inline/Crossline coordinate system. This is 
regardless of whether you have 3D data at all. Thus, for 2D only you 
will have to fake a 3D inline/crossline system. If you want to use both 
3D and 2D data, set up the survey according to the 3D cube(s).

Thus, after pressing the 'New' button, you should use Survey Ranges : 
and select 'Set for 2D only', and press 'Go ...'. There, you fill in 
approximate values for your X and Y ranges, and an estimate of the 
average trace distance. In fact, rather than the average trace distance, 
you may enter the size of the grid cells you would want when creating a 
3D horizon from your trace picks.

If you can't read these facts off a base map or some kind of report, you 
can still use the 'Scan SEG-Y file' to get an idea of these ranges. But 
don't be surprised if OpendTect says it cannot set up the survey from 
the SEG-Y file! This is expected behavior. A 2D line has trace numbers 
and coordinates, no inlines and crosslines.

After the 'Set for 2D only', you have to fill in the Z range (again, no 
precision needed; 2D lines can stick out of the display box in all 
directions). And that should be it. Press OK and select the new survey.


** Loading the seismic data

The format can be SEG-Y, Simple file or something from a plugin. The 
important thing here is that 2D seismics require a good trace number. 
Whether this is a shot point, CDP or just sequence number is entirely 
your business. But beware: OpendTect orders the line according to this 
number, so it must be 'well-behaved'.

Then there is the issue of X and Y coordinates. These are mandatory for 
all input formats. So, you cannot load SEG-Y with no coordinates and 
afterwards mix in the coordinates from for example a UKOOA file. The 
coordinates need to be there from the start, and cannot be changed 
inside OpendTect. Maybe someone will make such a utility at some point 
in time, but it's not there at the moment.

The import dialog will ask you for the name of a line set. If you have 
only one 2D line this may seem like overkill, but OpendTect has a 
grouping for 2D lines in 'Line Set's. These can equivalent to, but don't 
need to be made out of, "2D surveys". An OpendTect survey can hold 
multiple line sets.

Within the line set, each line has a name, whcih you will need to 
specify. You can also specify the 'Attribute' for the data you are 
importing. This is similar to the 'Line class' in the big workstations. 
You can set this in the line set selection sub-dialog (push the 'Select' 
button). It goes without saying that, as one line can have multiple 
attributes, these attribute import data files need to have the same X 
and Y values in the headers. Usually additional attributes are only 
generated inside OpendTect, not imported.


** Displaying the data

Right-click on '2D Seismics' and click on 'Add'. Select the line set, 
then select one or more lines. Then the lines will be displayed - that 
is - their geometry. If you want to see seismic data, you have to set 
the attribute for this line by right-clicking on the attribute field in 
the tree.

If you display many lines, you can set the attribute for all these lines 
by right-clicking on the line set name, 'Edit Attribute'.


** Troubleshooting

- Often, people do not find a good trace number in the SEG-Y headers. 
Without that, weird things can happen. Use the 'Examine' button in the 
SEG-Y setup dialog to figure out what header value may be usable as 
trace number. Or better: find the number that you recognize the data by.

- If X and Y in the SEG-Y file are not correct, the data may be a long 
way outside the survey box. Then, nothing may be there after loading a 
line, or even the entire survey box disappears.

- If you have trouble with your graphics card, the lines may remain 
white after loading the seismic data. But if your mouse goes over the 
line, you can see coordinates, trace numbers and amplitudes. Then try:
   . installing the latest driver
   . make sure you have a good card (hopefully nVidia-based)
   . try to disable shading in Utilities-Settings-Look and feel.

- If everything else fails, you may want to contact support at 
opendtect.org.


Bert Bril

-- 
-- dGB Earth Sciences B.V.
-- Nijverheidstraat 11-2, 7511 JM Enschede, The Netherlands
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