GEPH316 Seismic Data Processing Field Data Workshop 2000 6 Reducing quantity of data
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### Resampling the traces

Seismic data files are large and require a lot of computer processing time. It is beneficial to reduce the quantity of data as early in the processing sequence as possible, provided that no useful information is lost. Often the ends of the traces will not contain any useful reflections and so may be discarded. Sometimes the data is sampled more frequently than is necessary, so the extra interleaved data values may be removed. In general, it is sufficient to have about 8 points per cycle for the highest useful frequency.

### Resampling the traces

• The suresamp command can be used to resample the trace time series. The command is very flexible in specifying the type of resampling. It can discard unwanted points, from the start, end or middle of the trace, and interpolate extra points if required. Try to understand the resampling possibilities by typing the name of the command.
```suresamp

```
• The easiest form of resampling is to specify the new sampling interval dt in seconds and number of time points nt that are required. Use the following command to resample shot 10, so that a better spectrum may be produced directly.
• The number of points in the trace is being halved from dt=0.00025s to dt=0.0005s, so the number of points required in the trace will drop from nt=1000 to nt=500. This reduces the sampling frequency fs from 4kHz to 2kHz, and the Nyquist frequency fN from 2kHz to 1kHz.
• Also, the last 50ms of the 250ms long data contains no useful information, so reduce nt=400.
• This resampling will remove 60% of the data points, the file size and the processing time. Test this on the data for shot s05.
```suresamp <s05 >s05.resamp dt=0.0005 nt=400

```
• Plot the old and new time series and observe the slightly more jagged shape to the wavelets, due to the fewer points.
```suxwigb <s05 &
suxwigb <s05.resamp &
```
• Resample the whole line and keep the old file.
```suresamp <line >line.resamp dt=0.0005 nt=400
```

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Comments on this information may be mailed to B.Robinson@lancaster.ac.uk