Geopsy: Filter

Filter is a Basic signal processing tool in the Waveform menu and a tab in the advanced signal processing tools. In the first case, it applies a filter to the signals in the active signal viewer. In the second case, it applies a filter to the active signals to select appropriate windows for the tool computation.

In Waveform menu, by clicking on Filter, a pop-up window appears to customize the filter. In that case "Filter" modifies the temporary signals in the active viewer. In an advanced tools like H/V tool, checking the "Anti-triggering on filtered signals" checkbox (Tab "Time" sub-tab "General") let the user select windows only based on the chosen frequency band. In that case the filter is not applied for the computation but only for the window selection.

Filter parameters

The basic parameters for the filter is the type of filter:

• "Low pass" filters the frequencies above a given value.
• "High pass" filters the frequencies below a given value.
• "Band pass" filters the frequencies out of a given frequency range.
• "Band reject" filters the frequencies inside a given frequency range.

The cut-off frequency(ies) of the filter should be provided in the neighboring text box(es). All the values are in Hertz.

Filter method

Two filters are currently implemented in Geopsy:

• "Butterworth filter" is an outphasing filter... It does not conserve the phase of the signal. The "Causal" check box ensures the filter will be causal (a point of the filtered signal is only computed using previous data points in time, so that no artificial effect can be created before the arrival of a propagating wave, for example). The "Order" of the filter defines the number of data points used for the computation of a single filtered point. Low order affect less the signal but make the filter less efficient. High orders make the filter more efficient but may affect more the signal. The default value is 2.

  \li abs(\a nfil) = order of Butterworth filtre
  \li if \a nfil is negative, the filter is done in both directions, and it is non-causal but non-outphasing
  \li if \a nfil is positive, ,the filter is done in only one direction, and it is causal but outphasing

• "Cosine Taper" is a FIR filter. The "Width" parameter gives the width, in %