GEPH316 Field Data: Velocity analysis

GEPH316 Seismic Data Processing

Field Data Workshop 2000

16 Velocity analysis


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Fold of gathers

A gather is a collection of seismic traces made according to some trace header parameter.
To perform velocity analyses, the data should be gathered according to cdp point number and sorted according to increasing offset distance.
The seismic line was cdp gathered and sorted in the previous section.
Now inspect the cdp and offset header word values using the seismic unix sugethw command, which shows all the numbers in the headers of the line. The information is piped through the more command to throttle back the output.

sugethw < line.sorted key=tracl,sx,gx,cdp,offset | more

Note the number of traces in each gather, which will give the fold of the gather, as shown in the CDP gather diagram included in the practical handout.
- The CDP point at cdp=07m, halfway between sx=00 and gx=14, has only 1 trace, so the data is single-fold.
- The CDP point at cdp=08m has 2 traces, so the data is 2-fold. One trace comes from sx=00 and gx=16 and one from sx=01 and gx=15.
- Eventually, CDP point at cdp=18m has 12 traces and has reached the maximum of 12-fold.
- The 12-fold data continues through the central region of the line from cdp=18m as far as cdp=31m, halfway between sx=24 and gx= 38.
- Finally, the right hand end of the line decays back down to single-fold at CDP point cdp=42m, halfway between sx=24 and gx= 60.

Velocity spectra for synthetic data

Get a copy of the velocity analysis shell script called velan. Lancaster Unix users can use the command line below. Non-Lancaster web-readers can get velan file from here.
Get a copy of the synthetic seismogram shell script called synthesise and the model file model00deg. Lancaster Unix users can use the command line below. Non-Lancaster web-readers can get synthesise and model00degfiles from here.

cp /home/ufs1/51/robinsob/geophysics/seismic/geometrics/refl000303/velan .
cp /home/ufs1/51/robinsob/geophysics/seismic/geometrics/refl000303/synthesise .
cp /home/ufs1/51/robinsob/geophysics/seismic/geometrics/refl000303/model00deg .

Run the synthetic seismogram shell script called synthesiseusing the following commands. It should produce a model velocity section and a synthetic seismogram section, showing 5 reflecting horizons with velocity increasing with depth.

synthesise

In the following command, the parameters for the velocity analysis are:
- use the cdp gathered data in file line.sorted
- start at cdp 1
- finish at cdp 1
- step every 1 cdp points
- each gather is 20 fold

velan synth.seg 1 1 1 20

Enlarge the synthetic seismogram window and put it exactly alongside the velocity spectrum window.
At each stage in the velocity analysis, the shell command will perform as follows:
- Display a coloured velocity spectrum contour plot for the particular CDP gather point, with arrival time in seconds down the vertical axis and test stacking velocities in ms^-1 along the horizontal axis. The contured parameter is the coherent strength of NMO stacked arrivals for the particular test velocity and arrival time.
- The velocity spectrum should show a line of isolated peaks stretching downwards from near the top left corner. The peaks will only be strong if there are corresponding moveout hyperbolae at those particular depths and velocities. In the case of this data, the V₁ velocity is about 500 ms^-1, so there should be a peak with this velocity near the top of the spectrum, opposite the first reflection on the synthetic window.
- Starting from the top, the operator should put the mouse pointer over a peak in the spectrum and then press lower case letter s to digitise the point.
- Select the 5 peaks moving down through the spectrum.
- The operator should press upper case letter Q to quit picking the velocities for the particular CDP point.
- Display a graph of the picked stacking velocities against arrival time.
- Check that the operator is happy with the picked velocities. Enter y (Yes) to continue or n (No) to reject the picked velocities for that CDP point and try again.
The results of the velocity analysis are stored in the file stkvel.nmo.

Velocity spectra for field data

It is best to use the full 12-fold gathers from CDP point 18m onwards for velocity analyses and the last point with 12-fold data is CDP point 31m.
To cover the line fairly quickly, perform the velocity analysis at CDP points from 20m to 30m in steps of 5m to give 3 velocity analyses.
In the following command, the parameters for the velocity analysis are:
- use the cdp gathered data in file line.sorted
- start at cdp 20
- finish at cdp 30
- step every 5 cdp points
- each gather is 12 fold

velan line.sorted 20 30 5 12

At each stage in the velocity analysis, the shell command will perform as follows:
- Display a coloured velocity spectrum contour plot for the particular CDP gather point, with arrival time in seconds down the vertical axis and test stacking velocities in ms^-1 along the horizontal axis. The contured parameter is the coherent strength of NMO stacked arrivals for the particular test velocity and arrival time.
- The velocity spectrum should show a line of isolated peaks stretching downwards from near the top left corner. The peaks will only be strong if there are corresponding moveout hyperbolae at those particular depths and velocities. In the case of this data, the V₁ velocity is about 350 ms^-1, so there should be a peak with this velocity near the top of the spectrum.
- Starting from the top, the operator should put the mouse pointer over a peak in the spectrum and then press lower case letter s to digitise the point.
- Select about 4 peaks moving down through the spectrum. For this velocity spectrum, the stacking velocity does not seem to rise much above 350 to 1000 ms^-1, although the second layer should have an interval velocity of about 2000 ms^-1,
- The operator should press upper case letter Q to quit picking the velocities for the particular CDP point.
- Display a graph of the picked stacking velocities against arrival time.
- Check that the operator is happy with the picked velocities. Enter y (Yes) to continue or n (No) to reject the picked velocities for that CDP point and try again.
- Continue by displaying the velocity spectrum for the next CDP point until all the chosen distances have been treated.
The results of the velocity analysis are stored in the file stkvel.nmo.

Editing the velocity file

Inspect the contents of the velocity pick file using:

more stkvel.nmo

This shows for each cdp point in turn, the array of picked NMO time and velocity points.
Most of them appear to be in the range of 350 to 1000 m^-1
Use the ue Microemacs text editor to clean up the file, so all the cdp points are on one line.

ue stkvel.nmo

When you have finished, the file should look similar to the following.
Use Escape + z to save the file and exit from the editor.

cdp=20,25,30
tnmo=0.001467,0.01907,0.05035,0.1031
vnmo=408.1,931.4,847.7,1287
tnmo=0.0009777,0.03324,0.04009,0.07137,0.1501
vnmo=418.6,1005,983.7,1224,1538
tnmo=0.0009777,0.03275,0.06893,0.1383
vnmo=408.1,962.8,1015,1528

This file format is ready to be used in the NMO correction of the reflection data, in the next section.


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