MASW and HVSR Seismic Surveys

For non-invasive seismic characterization of sites, MASW and HVSR geophysical surveys can be performed. These two types of surveys differ because the first requires acquisition with an active methodology, while the other uses a passive methodology.

Let’s see what the main differences are between the two types of surveys.

Active seismic MASW geophysical surveys

The MASW (Multi-Channel Analysis of Surface Waves) test, developed in 1999 by researchers at the Kansas Geological Survey, allows for a detailed determination of the variation of shear wave velocity (or S waves) as a function of depth through the study of the propagation of surface or Rayleigh waves.

Therefore, they are very useful for quickly and economically determining the trend of the average shear wave velocity in the first thirty meters of depth (Vs30) and thus the soil category of the foundation.

Furthermore, this methodology is also used for:

• liquefaction analysis;
• determination of soil profiles;
• mapping of the substrate

The MASW method is called “active” because the surface waves are produced by an impulsive source located on the ground surface and are recorded by a linear spread composed of numerous receivers placed at a short distance (inter-geophone distance).

The purpose of the test is to determine the site stiffness profile (shear wave velocity S) through the measurement of the propagation velocity of Rayleigh surface waves (Ve) and a subsequent inversion process. It is assumed that the volume of soil, subject to the investigations, is horizontally stratified and that within each layer the behavior of the soil can be considered elastic, homogeneous and isotropic.

Passive seismic HVSR geophysical surveys

The HVSR (Horizontal to Vertical Spectrum Ratio) methodology was introduced by Nakamura (1989) for the determination of soil vibration frequencies and the estimation of local seismic amplification, elements of great utility as they allow to distinguish areas characterized by the presence or absence of resonance phenomena in the frequency range of engineering and construction interest.

This methodology measures the ambient seismic noise that is present everywhere on the Earth’s surface and is produced by atmospheric phenomena, such as ocean waves, micro-earthquakes, wind, and anthropogenic activity. Ambient seismic noise is also called microtremor as it consists of very small amplitude oscillations compared to those associated with earthquakes.

The HVSR method is called passive seismic because the noise is not generated artificially, as in the energization of active MASW seismic, but is present naturally.

In fact, in any flat place there are always vibrations associated with ocean waves with peaks at 0.14 and 0.07 Hz. To this spectral behavior of “background”, always present in various forms and subject to very little attenuation, are superimposed local sources due to anthropogenic activities (traffic, machinery, etc.) and natural ones. The effect of these local sources is subject to attenuations that are greater the higher the frequency and due to inelastic absorption associated with the internal friction of rocks and soils.

Our equipment

To carry out these geophysical surveys, our study uses a 24-channel PASI GEA24 seismograph. Thanks to its compact size, it can be used for refractions, seismic tomography, MASW, HVSR and downhole and crosshole surveys anywhere.

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