In early 80s, a two-receiver approach was introduced by investigators at the University of Texas (UT), Austin, that was based on the Fast Fourier Transform (FFT) analysis of phase spectra of surface waves generated by using an impulsive source like the sledge hammer (Fig. 1). It then became widely used among geotechnical engineers and researchers. This method was called Spectral Analysis of Surface Waves (SASW) (Heisey et al., 1982). The fundamental-mode (M0)-only Rayleigh wave assumption was used during the early stages. Simultaneous multi-frequency (not mono-frequency) generation from the impact seismic source and then separation by FFT during the subsequent data processing stage greatly improved overall efficiency of the method in comparison to earlier methods such as the continuous surface wave (CSW) method. Since then, significant research has been conducted at UT-Austin (Nazarian et al., 1983; Rix et al., 1991; Al-Hunaidi, 1992; Gucunski and Woods, 1992; Aouad, 1993; Stokoe et al., 1994; Fonquinos, 1995; Ganji et al., 1998) and a more complete list of the publications on SASW up to early 1990s can be found in “Annotated bibliography on SASW” by Hiltunen and Gucunski (1994). The overall procedure of SASW is as follows (Fig. 1):
Field setup with different separations (D’s),
Data processing for phase velocity (Vph): Vph=2*pi*f / dp (dp=phase difference, f=frequency, pi=3.14159265)), and
Earlier research of SASW method was focused on ways to enhance accuracy of the fundamental-mode (M0) Rayleigh-wave dispersion curve through field procedure and data processing efforts. Then soon came the speculation about the possibility of the curve “being more than M0” and subsequently higher modes (HM’s) were included in the studies (Roesset et al., 1990; Rix et al., 1991; Tokimatsu et al., 1992; Stokoe et al., 1994). In consequence, the concept of “apparent (or effective)” dispersion-curve (Gucunski and Woods, 1992; Williams and Gucunski, 1995) was introduced that accounts for the possible mixture of multiple influences rather than M0 alone (Fig. 2). Once multiple modes were recognized and included, the field approach and data processing techniques attempted to account for the multiple-mode possibilities. Pavement investigation by SASW was regarded quite challenging, especially for base layers, and the possibility of multi-modal superimposition was speculated as being responsible for this. Reported difficulties with SASW fit into the following three main categories:
higher modes (HM’s) inclusion that was previously underestimated,
inclusion of other types of waves (body, reflected and scattered surface waves, etc.) (Sheu et al., 1988; Hiltunen and Woods, 1990; Foti, 2000) that was also underestimated or not considered at all, and
data processing, for example, phase unwrapping (Al-Hunaidi, 1992) during the phase-spectrum analysis to construct a dispersion curve.
Fig. 1. Schematic describing overall procedure of the SASW method.
Fig. 2. The apparent dispersion concept in the SASW method.