The Nakamura technique or spectral ratio method applied to environmental noise is a non-invasive passive method that records environmental noise through the measurement of seismic waves propagated in the three orthogonal components. This technique consists of determining the Fourier spectra of the horizontal and vertical components of environmental vibration records, obtained with a triaxial geophone (figure 1). From these, the H/V ratio (spectral quotient) is obtained, which is considered as the approximate transfer function of the soil strata on the basement. This technique is useful to determine the fundamental period (T0) or fundamental frequency (f0) of the soil or structures (figure 2), and can be complemented with the values of Vs obtained by other seismic prospecting techniques, such as MASW and ReMi methods.
Currently there are different seismic design codes that consider the parameter T0, where the DS No. 61/2011 of the Minvu stands out, carried out in response to the consequences caused by the earthquake that originated on February 27, 2010.
To obtain the spectral ratio H/V it is necessary to carry out the corresponding field measurements. The number of measurements depends on the area to be studied, and it is recommended to carry out a minimum of 3 measurements, which should have a minimum duration of 20 to 30 min. Logging can be done for short intervals of time, for example 10 measurements of 2-3 minutes or a continuous measurement of 30 minutes.
Regarding the equipment used, it should be taken into consideration that since it is a triaxial geophone, it can be affected by unwanted noise sources, such as wind or the presence of poorly compacted vegetation or sand cover that can hinder the quality of the seismic record. ,therefore, it is recommended to isolate the triaxial geophone by excavation.
Some applications of this technique are:
- Determination of the fundamental period and fundamental frequency of the soil and structures.
- Determination of surface wave velocity Vs and basement depth by ellipticity curve.
Figure 1: H/V spectral ratio graph versus period and frequency (own elaboration).