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Top1. Introduction
Surface wave methods are widely used to generate VS profiles or stiffness variation of subsurface stratification. The dispersive nature of Rayleigh waves allows different frequency waves to travel at different velocity in a vertically layered medium and penetrate up to different subsurface depths. Generally, Rayleigh waves with large wavelength, i.e. low frequency waves, penetrate up to higher depth and reflect high phase velocity in the dispersion spectra because of the higher stiffness of deeper geo-materials. And the waves with low wavelengths, i.e. high frequency waves, penetrate shallow depth reflecting the properties of the shallow subsurface (Figure 1). The applications of surface waves in engineering field started in the 1950s with the Steady State Rayleigh Method (Jones, 1958), but their revolution arrived only in the last two decades with the Spectral Analysis of Surface Wave (SASW) method (Nazarian et al., 1983) and Multichannel Analysis of Surface Wave (MASW) (Park et al., 1999; Xia et al., 1999; Socco and Strobbia, 2004; Park et al., 2006) method.
In site characterisation using surface wave methods, waves are generated using an active source and these generated waves are recorded using vertically mounted geophones at several distances from the source. Although, often not only vertical components, but other components also recorded when Rayleigh wave dispersion curve is inverted with Rayleigh wave ellipticity or H/V ratio. An experimental dispersion curve is extracted from the field data using different processing techniques. Dispersion curve is picked from the energy maxima after converting the time domain trace into frequency-wavenumber (f-k) or frequency-velocity (f-v) domain (McMechan and Yedlin, 1981; Gabriels et al., 1987; Park et al., 1999). Multi Offset Phase Analysis (MOPA) which uses weighted linear regression of phase-offset data can also be used to extract the dispersion curves from a recorded seismogram (Strobbia and Foti, 2006; Vignoli and Cassiani, 2010; Vignoli et al., 2011; Vignoli et al., 2016). This developed dispersion curve for a particular site is used as a target dispersion curve in inversion to extract shear wave velocity variation with depth. The limitation of this active surface wave method lies in the energy content in low frequency region of generated dispersion spectra which consequently limits the depth of extracted soil profile. To overcome this, low-frequency ambient vibration measurements with a two-dimensional array (Horike, 1985; Tokimatsu, 1995; Louie 2001; Strobbia and Cassiani, 2011) are also used to extract the field dispersion curve. Sometimes changing the source type, its position and geophone with different central frequency can be used to increase the low frequency content.