Using yulewalker.m

The Octave program, yulewalker.m can be used to read a single seismic trace from a BSEGY file and compute an all pole spectrum. The order of the process is set by the number of terms to include in the computation of an autocorrelation. Figure 50 shows the Geologan down-hole data.

Figure 50: (C). Geologan down-hole data. Octave program yulewalker.m is used to select trace 30. (A) Picking a length of the autocorrelation (nlag=116), (B) Downhole data, (C) Selected signal trace 30, (D) Yule Walker all pole spectral estimate.

To run yulewalker.m, you need to have the following files in the same directory as the BSEGY data (in this case, a file named twav.seg). Start Octave from within the directory with these files:

• segyinfo.m
• bsegin.m
• yulewalker.m
• twave.seg (or whatever *.seg file you want to analyze)

From within the Octave text window, issue the command,

yulewalker;

The program will prompt you for the file name in the Octave text window. Here, we enter twav.seg. Then a GUI dialog panel will pop up and ask for the following input:

• Input? Data or Autocorrelation Is the file a time signal or an autocorrelation?
  A GUI will pop up showing the number of traces, maximum recorded time, and sample interval. Click OK
• trace number (the signal we wish to compute a spectrum for)
• Max Time and Frequency Entry boxes for calculation and display.
• Remove DC? Remove or not to remove DC (zero frequency) content.
• Pick Order of Spectrum Click with mouse on autocorrelation.
  Yule Walker spectrum both in amplitude and Decibel displays will pop up.

The program then generates the plots as shown in Figure 50 A, B, C, and D. The Decibel version of the spectrum is not shown here.