The techniques of passive seismic HVSR are aimed to define the mechanical properties of the subsoil starting from measurements of the field of environmental vibrations generated from uncontrolled sources of different kinds. The environmental noise has by nature an irregular trend.
Passive seismic methods are based on the observation of a wave field physically linked to that generated by the earthquake. The advantage of these techniques is that the seismic waves used are characterized by wavelengths and propagation depths much greater than those generated artificially and this allows to reach depths of exploration much larger than those achievable with active seismic techniques, a worse spatial resolution but to the detriment of and a difficult interpretation.
They can operate in an urban environment even in the presence of strong anthropic disturbances. Passive seismic technique at a single station (H / V spectral ratio or HVSR – Horizontal to Vertical Spectral Ratio technique) has been spreading rapidly in recent years as it is totally non-invasive, very fast, applicable everywhere, low costs because it is not it requires neither drilling, nor external energizations other than environmental noise that in nature exists everywhere.
In principle, this is a simple measure that requires only one operator. In fact, it is a matter of measuring with a three-component seismometer (digital tromograph) the environmental vibrations present in a point for several minutes and then constructing a representative curve of the spectral ratios (H / V) as a function of the vibration frequency. Any maximums will be indicative of the possible presence of resonance phenomena.
The main fields of application concern:
• analysis for the definition of the seismic action of the project in the execution phase of direct building interventions.
• seismic microzonation studies in support of general urban planning tools during the territorial and urban planning phase;
The HVSR technique is used for:
1. determine the fundamental frequency of subsurface resonance. It is a very important parameter if analyzed together with the frequencies of the works that fall on it, since in case of coincidence of the two frequencies the phenomenon of “double resonance” can occur, extremely dangerous for the same structures, which would be subjected to excitations not supported;
2. carry out stratigraphic investigations of the subsoil; the HVSR technique for stratigraphic purposes can be used preliminarily to identify the most representative points in which to refine the survey with classic point techniques or to verify, where there are already other specific investigations, significant heterogeneity around the investigated points or as a support element to other investigations if they present interpretative doubts;
3. estimate the value of VS,eq, in the presence of a constraint, in the depth of a reflector, less than 30 m, of which the marker is recognized in the H / V curve or the VS,30 , 30 if the reflector depth is higher at 30 m of the VS estimate of the surface layer.
Easy HVSR is the Geostru software for the analysis of the spectral ratio of microtremors with single-station recordings. It is greatly appreciated for its simplicity of use and the speed of execution of the calculations. The program complies with the directives suggested by the SESAME project.