The Horizontal-to-Vertical Spectral Ratio (HVSR) methodology is a passive seismic technique that uses the environmental noise present everywhere in nature. These environmental micro-tremors are recorded in their three spatial components, allowing to obtain the fundamental resonance frequency of the ground. This parameter is very important in earthquake engineering to properly understand site effects.

The passive seismic technique known as HVSR (Horizontal to Vertical Spectral Ratio) is completely non-invasive and can be applied wherever without any kind of drilling nor external energization different from environmental noise which is present everywhere in nature.

The seismic ambient noise is characterized by small ground movements present everywhere and consist mostly of superficial waves (Rayleigh and Love) produced by constructive interference of P and S waves in the superficial layers. It is also produced by wind and sea waves and at high frequencies by sources of anthropic character (industries and vehicle traffic). It is called a passive method because vibrations are not excited ad hoc, such as in other active seismic methods.

Ambient-noise vibrations are recorded in the three spatial components and the processing consists of estimating the ratio between the Fourier amplitude spectra of the horizontal (H) to vertical (V) components.

The results that can be obtained from HVSR surveys are: the resonance frequency characteristic of the site (main parameter for a correct dimensioning of buildings in order to avoid resonance effect); the fundamental resonance frequency of a building (if measurement is performed inside it) or an estimate of the shear wave velocity Vs (if there is additional information on subsurface geological conditions) .