Horizontal to vertical spectral ratio analysis, also referred to as simply HVSR analysis is a method for estimating the natural resonant frequency (and thereby the fundamental site period) of the ground using a measurement of passive microtremors.

The HVSR method is performed by using a seismometer which includes sensitive geophones that record motion in the three orthogonal directions. The equipment is placed on the ground surface and levelled with one of the horizontal geophone orientated in the N-S direction. The device is left to record passive ground motions over a period of approximately 30 mins. The recorded information is transformed to frequency spectra for each of the horizontal and vertical directions using a Fourier transform. The two horizontal frequency spectra are averaged out and then divided by the vertical spectra to create a horizontal to vertical spectral ratio, as illustrated below.

Horizontal to vertical spectral ratio graph example
Horizontal to vertical spectral ratio graph example

The peak in the plots is indicative of the natural resonant frequency of the site, fo_H/V. The fundamental site period, To, can then be determined by, To = 1/fo_H/V.

This information is useful in assessing the seismic response of the site in an earthquake and can be used to help determine the Site Subsoil class in accordance with NZS1170.5.

The method relies on a significant impedance contrast (e.g. a rock surface) at a depth range within the measuring capability of the device and also depends on the properties of the soil above the rock. Multiple impedance contrasts can result in multiple peaks and noise can affect the results. Consequently, the method may not be suitable for all sites and may not always be successful or definitive.

It is advisable to undertake more than one such test per site for comparison purposes. It is also recommended that a different form of in situ test, such as a SCPT, be carried out in conjunction with an HVSR analysis to allow independent verification.

At Ground Investigation we undertake the HVSR testing in general accordance with SESAME 2004, “Guidelines for the Implementation of H/V Spectral Ratio Technique on Ambient Vibrations, Measurements, Processing and Interpretation”. We use the computer software, Geopsy to process and analysis the results. The results are provided in a report. We periodically have our reports peer reviewed by the University of Auckland.

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