The spectral analysis of surface waves (SASW) method, which is based on dispersive characteristics of surface waves, is an in-situ seismic method for determining the shear wave velocity (or, maximum shear modulus) profile of a site. Most of the surface wave energy exists within one wave length of depth, and in layered media, the surface wave propagation depends on the frequency (or, wavelength) of the wave because waves of different wave lengths sample different parts of the layered medium. The reliable determination of phase velocities with wavelength (or, frequency) is the most important task in SASW method. In some field conditions when various noises exist in the field, however, determination of experimental dispersion curve is a difficult task and the usual phase unwrapping method can lead to erroneous dispersion curve.
The new method of evaluating the experimental dispersion curve was proposed using harmonic wavelet transform in the time-frequency domain. Harmonic wavelet analysis of waves (HWAW) method mainly uses the signal portion of maximum signal/noise ratio to evaluate the phase velocities and it can minimize effects of noises. HWAW method is less affected by the near field effect and can sample much deeper part of the site than the conventional phase unwrapping method at a given receiver spacing. HWAW method was verified through the numerical simulations at the various kinds of multi-layered system. To apply HWAW method in the determination of experimental dispersion curves, the various field testing setups were investigated for the optimization of testing parameters. The source to first receiver(S-R) spacing and receiver to receiver(R-R) spacing were selected as the important parameters and the optimized S-R and R-R spacings were determined as 5~10m and 0.5~1.0m, respectively. The near field effect, the source energy, the attenuation of high frequency contents, the similarity of the wave groups of two receiver signals were considered in the determination of S-R spacing and R-R spacing. Once the testing procedures are optimized, test results obtained by the HWAW method were compared with the results obtained by other seismic tests including conventional SASW and downhole tests at two sites. Experimental dispersion curves determined by HWAW method match nicely with those obtained by other seismic methods showing the reliability of the proposed method. Also, HWAW reduces the testing and processing times to obtain the experimental dispersion curves compares with conventional SASW method and shows the possibility of future automation.
