wiki:ShmDocMenuWorkSpectrum
Last modified 14 years ago Last modified on 02/05/2009 11:55:50 AM

Menu entry Spectrum

Computes spectrum of selected trace part, i.e. the trace within the drag window selected by the right mouse button, and displays it in a separate log-log scaled window. Repeated call of this menu entry adds traces to the spectrum display in changing colours. To clear the spectrum window from traces use the Quit menu entry of the spectrum window. This closes the window. A new call to Spectrum reopens the window with only one trace in it. The first trace in the spectrum display determines the frequency and amplitude borders. The borders may be changed using the Display menu entries of the spectrum window. A click of the middle mouse button toggles a crosshair cursor.

There are three computation algorithms available: a PSD estimator (spctrm routine from Numerical Recipes), an FFT and a squared FFT algorithm (based on the realft routine of Numerical Recipes). For the PSD estimator the selected time window is divided in to n subwindows with or without time overlap. Each subwindow is processed with one of the above algorithms and the results are averaged and displayed in the spectrum window. The FFT algorithms take the whole time window and therefore do not compute averages.

If you intend to display displacement spectra please make sure that the input seismograms are displacement traces.

Spectrum:

  • Insert: insert current spectrum to the display window.
  • Setup: parameter setup for the spectrum algorithms. Grid: switch on/off the log-log grid in the display window. End of data: the algorithms need time series with a length of a power of two. 'pad zeros' add zeros to the end of the selected time window to read the next power of two. 'truncate' cuts the trace down to the closest smaller power of two. Method: choose the computation algorithm. Overlap: use overlapping/not overlapping time windows in the PSD estimator algorithm. No. of windows: number of data windows for averaging used in the PSD algorithm.
  • Fit Line: fit a line to the spectrum displayed. After selecting this entry choose two frequencies by clicking the left mouse button. All spectrum values between these two frequencies are used for a straight line fit. This line may be used for a corner period estimation.
  • Screendump: make screendump of a graphic window (like screendump in the main window).
  • Quit: close spectrum window and delete all spectra computed so far.

Display:

  • Automatic: choose default borders for frequencies and amplitudes in the spectrum window.
  • Inc Bottom Freq: increase bottom frequency by one magnitude.
  • Dec Bottom Freq: decrease bottom frequency by one magnitude.
  • Inc Top Freq: increase top frequency by one magnitude.
  • Dec Top Freq: decrease top frequency by one magnitude.
  • Inc Bottom Ampl: increase bottom amplitude by one magnitude.
  • Dec Bottom Ampl: decrease bottom amplitude by one magnitude.
  • Inc Top Ampl: increase top amplitude by one magnitude.
  • Dec Top Ampl: decrease top amplitude by one magnitude.

Method:

  • Power Spec: recompute all spectra with the PSD estimator routine.
  • FFT: recompute all spectra with the FFT routine.
  • FFT squared: recompute all spectra with the squared FFT routine.

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