Easy HVSR > Description of the technique

Description of the analysis technique in Easy HVSR

This description is not intended as a discussion of HVSR technique itself to the analysis of microtremors, for which reference should be made to the scientific literature, but simply a way to illustrate monolithically various phases of analysis that Easy HVSR fulfills so that the user can have a clearer and more complete understanding.

Once provided with the recording traces in the three directions, of duration at least equal to 20 minutes, these are divided into time windows of dimensions of the order of some tens of seconds.

These windows are then filtered to remove parts of the recording containing any transients or other sources of interference that could undermine the analysis.

It is performed the Fourier transform of all the windows in order to obtain the average spectra in all directions within a range of frequencies chosen by the user. Performing a first spectral ratio H/V for each window and by a chronological order approach the user gets an indicative map of the spectral ratio stationarity. Similarly, by varying the angle between the reference system and the direction of the traces' axes is obtained an indicative map of the directionality, useful to measure the actual uniform distribution of sources.

In the evaluation of the H/V spectral ratio, the two horizontal components can be added together according to various criteria chosen by the user. In addition, the Fourier transforms are also smoothed according to various criteria also at the user's choice.

Checks are performed on the reliability of the H/V average curve and of its peak according to the directions of the SESAME project, to which reference is made for any further clarification.

Overcome the above checks, the frequency in which is located the peak (maximum value) of the spectral ratio H/V indicates the natural vibration frequency of the site for which the recording was made.

A further step that Easy HVSR performs is the identification of a stratigraphic model, in collaboration with the user's experience, which well approximates the recorded behavior. This allows further evaluation of normative parameter Vs30.