GEPH316 Seismic Data Processing
Field Data Workshop 2000
5 Viewing seismic data
Inspecting the data
It is important to spend time on a detailed examination of the raw data to identify obvious reflections, other arrivals, specific problems and coherent noise. Wrong identification of events can lead to the wrong choice of processing steps and an incorrect final seismic section. This inspection requires a graphical display of the data.
The viewing of data usually is performed after the data is reduced and the line geometry is defined. Here it is used earlier in order to give a good introduction to the SU package and to the field data.
Viewing all the line
- It is possible to view the complete seismic line of 25 shots in one picture using the CWP/SU package.
- The SU package has plotting facilities built on top of the normal X window plotting facilities, such as the xwigb wiggle trace plot, the ximage and the xmovie image plot commands.
- The suximage command plots a shaded image of all the data, similar to a remote sensing image.
- Use the following suximage command to get a quick look at all the data.
suximage <line &
- This plot is rather dense but it shows all the data and it gives an impression of any variations along the line.
- You have a print out of the suximage in the practical handout, so you can refer to it throughout the workshop.
- Most of the CWP/SU package is 'self-documenting'. Type the name of the suximage command to find out more information about how it works.
- The plotting commands have options to label the picture axes and provide the title for the plot.
- Use the following suximage command to get a labelled plot of all the data.
suximage <line title='Line in raw form' label1='Time (s)' label2='Trace number' &
- Compare the suximage output with the following suxmovie command picture.
suxmovie <line title='Line in raw form' label1='Time (s)' label2='Trace number' &
Viewing part of a window
- Once a plot has been displayed, it is often desirable to view part of the picture in enlarged detail by zooming in. This can be performed locally at the X terminal using the mouse facilities.
- Point the mouse cursor at the top left corner of the suximage plot.
- Hold the left mouse button down.
- Wipe across to halfway down the right hand side of the plot.
- Release the mouse button.
- The terminal should automatically redisplay the selected area of the plot. This is very useful to inspect the early direct waves and refracted waves without using any further SU commands.
- To redisplay the complete picture afterwards, just single-click in the middle of the picture.
- Bring the suximage plot of the whole line to the front and then use this redisplay facility to look at just shots 5 and 6 = traces 61 to 84 at the left side of line.
Printing all the line
- The SU package has postscript ps printing facilities which use very similiar commands to the normal X window plotting facilities.
- The suxwigb wiggle trace plot becomes the fast crude supswigb or the slower but neater supswigp.
- The suximage plot becomes supsimage.
- The suxmovie plot becomes supsmovie.
- If you want a printed copy of the raw line for your report, use the following supsimage command sequence to print the whole line.
- The lp command sends the file to the printer. The -d mono option specifies the mono black/white postscript printer.
supsimage <line title='Line in raw form' label1='Time (s)' label2='Trace number' > line.ps
lp -d mono line.ps
Extracting one shot
- It is useful to be able to extract single seismic shots from the complete seismic line of 25 shots using the suwind windowing command.
- Each seismic trace has various parameters in the header of the trace, so we may use the shot x position parameter sx to select the desired data. The values of the desired sx value lie between the identical minimum and maximum values of 05.
- Run the following command to make a working file s05 containing just the traces for the hammer shot at 05m.
suwind <line >s05 key=sx min=05 max=05
- Run the following command to make a working file s06 containing just the traces for the shotgun shot at 06m.
suwind <line >s06 key=sx min=06 max=06
Viewing one shot
- For plotting data, the suxwigb command is the most generally useful plotting command to produce a variable area wiggle trace plot of the data.
- Use the following command to plot just the 12 traces of data from the shot at 05m.
suxwigb <s05 title='Normal hammer shot at 05m' label1='Time (s)' label2='Trace number' &
- You have a print out of this suxwigb picture of shot 05m in the practical handout, so you can refer to it throughout the workshop. If you want another print out you can use the following:
supswigp <s05 title='Normal hammer shot at 05m' label1='Time (s)' label2='Trace number' > s05.ps
lp -d mono s05.ps
- Use the following command to plot just the 12 traces of data from the shot at 06m.
suxwigb <s06 title='Buffalo gun shot at 06m' label1='Time (s)' label2='Trace number' &
Inspecting the shots
- It is important to inspect the raw seismic data to identify various arrivals before doing any data processing.
- Arrange the S05 and S06 seismic sections side by side, to occupy half of the screen each.
- Carefully inspect all the traces on both sections to see the following features and answer the questions:
- What is the first arrival? Note that it does not go through the origin. Why is this? Estimate the apparent velocity by expanding the useful part of the section so that you can read off the time scales for trace 1 and trace 12 which are 22m apart.
- Compare this information with the plus-minus interpretation spreadsheet information.
- What are the broad rolling waves in the lower half of each section? Estimate the apparent velocity. Why are these waves smaller on section S06 than the first arrivals? What was different about the shot?
- What is the very narrow, sharp arrival running across the middle of the section? Estimate the apparent velocity. Why are these waves smaller on section S06?
- Why does trace number 6 appear different on both sections? This was the 100 Hz geophone. What effects has it had on the traces? Is it passing any frequencies below 100 Hz? Note that some of the first arrivals appear to be earlier on this trace. This may be more obvious in the ximage of the whole line.
- Can you identify any hyperbolic reflected arrivals running through the top of the section, after the first arrivals? How many reflection events can you identify?
- Look at trace 12 on both sections. Can you identify the shape of a single seismic wavelet by viewing the leading first arrivals? Note how the following reflection event interferes with the tail of the leading wavelet.
Removing unwanted graphics windows
- To get rid of unwanted graphical X windows you may click the close box in the top left corner of the window that you want to discard.
- The SU package has a simple command to get rid of multiple unwanted graphical X windows of particular types. Simply zap the type of window that you want to discard.
- Use the following commands to get rid of any remaining xwigb and xmovie windows. Then check which files are left in your directory.
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Copyright © Brian Robinson 2000. All rights reserved.
Last updated on 7/03/2000 by BR.