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Filter

At present Easy HVSR implements a single method for filtering the windows, essentially based on the evaluation of ratios between averages, useful to detect the possible presence of transients that may have disturbed the recording and that should therefore be discarded. Note that applying the filter to the windows is not required for the computation. Although it would be inadvisable, the user can proceed with the analysis without applying any filter. This is because on some occasions the filter may be unsuitable, and a more careful observation of the traces by the user can lead to decidedly better results.

The filter assumes that windows have already been identified, and works as follows:

  1. One window at a time is considered among those identified (its duration in seconds is denoted as LP)

  2. The window under consideration obviously has a counterpart on every trace: one trace at a time is therefore considered

  3. The average of the values taken by the trace within the time interval of the window LP is computed

  4. The window under consideration is divided into further time sub-windows of size SP (in Easy HVSR this value is entered in the field named Control window size within the section named Filter in the Tab named Selection)

  5. The average of the values taken by the trace within each sub-window of size SP is computed

  6. The average of each sub-window is compared with the average of the window of size LP by computing their ratio

  7. If even one of these ratios falls outside the acceptability range, the window of size LP is discarded on all traces

  8. The operation is repeated for all identified windows

An option of this technique consists in considering the residual windows: the size of the sub-windows SP will only exceptionally be an exact divisor of the size LP, so that when dividing the windows of size LP into several windows of size SP, at least one window smaller than SP will remain. Easy HVSR makes it possible to include these residual windows in the filtering process as well, by selecting the appropriate option in the filter section of the Selection Tab. The same section also provides the values defining the validity range of the above ratio.

One further consideration: the size SP must obviously be smaller than the value LP, and should be about one order of magnitude smaller than LP itself.

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