Technical description¶
The following description is not intended as a treatment of the HVSR technique itself for the analysis of microtremors, for which reference should be made to the relevant scientific literature, but simply as a way of illustrating the various stages of the analysis carried out by Easy HVSR, so that the user has a clearer and more complete understanding of it.
Once the recording with the traces in the three directions is available, with a duration of at least 20 minutes, these are divided into several time windows of the order of a few tens of seconds.
The windows are then selected so as to remove parts of the recording containing transients or other sources of disturbance that could invalidate the analysis.
The Fourier transform of all the windows is computed, so as to obtain average spectra in all directions within a frequency range chosen by the user. By computing a first H/V spectral ratio for each window and placing them side by side in chronological order, a map indicating the stationarity of the spectral ratio is obtained. Similarly, by varying the angle between the reference system and the direction of the trace axes, a map indicating directionality is obtained, useful to assess whether the sources are actually uniformly distributed.
In the evaluation of the H/V spectral ratio, the two horizontal components can be summed according to various criteria chosen by the user. The Fourier transforms are also smoothed according to various criteria, again chosen by the user.
Checks are carried out on the reliability of the average H/V curve and of its peak, based on the indications of the SESAME project, to which reference should be made for further clarification.
Once these checks are passed, the frequency at which the peak (the maximum value) of the H/V spectral ratio is located identifies the natural vibration frequency of the site where the recording was carried out.
A further step performed by Easy HVSR is the identification of a stratigraphic model, together with the user's experience, that closely approximates the recorded behaviour. This allows the further evaluation of the code parameter Vs,eq.
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