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SURFACE WAVES

MASW is the acronym for Multichannel Analysis of Surface Waves. This indicates that the phenomenon that is analyzed in detail with this technique is the dispersion of surface waves, that is, the fact that different frequencies - characterized by different wavelengths - travel at different speeds.
Since the dispersion of the surface waves depends on the vertical variations of the subsoil, it is possible to obtain the characteristics of the medium (i.e. velocity of shear waves and thickness of the layers) from the determination of the dispersion curves. As, typically, the speed of the seismic waves increases with depth, the lowest frequencies of the surface waves will travel at a higher speed than the higher frequencies. MASW is traditionally performed by analyzing Rayleigh waves, which are recorded using common vertical component geophones, the same used also for seismic refraction waves and the Re.Mi. On the other hand, making use of the dispersion of Love waves together with the dispersion of Rayleigh waves using horizontal component geophones is an interesting development in MASW analysis, simplifying the interpretation of the most complex situations. 

Consider instead the "passive MASW", the Re.Mi. surveys ("Refraction Microtremors") use the same instrumentation used for the MASW, but exploit as a source of energy the microtremors generated spontaneously in the ground. These can be generated from isotropic anthropogenic sources (i.e., arranged in all directions) with respect to seismic spreading. If the source is not isotropic, other particular methods (such as ESAC, SPAC, etc.) can still be used. The advantage of all these methods is that they can be used in highly urbanized areas, where high background noise negatively affects the results of other seismic investigation methods.

The study of surface waves through the application of all these methods allows - in a simple and economical way - to derive the parameter Vs30 required by the new seismic regulations.
Finally, the use of triaxial surface terns in combination with single component geophones has allowed the development of so-called "unconventional" methods.