Wavelet Spectrogram Analysis of Surface Wave Technique for Dynamic Soil Properties Measurement on Soft Marine Clay Site

Abstract

The surface wave measurement is one of in-situ seismic methods based on the dispersion of Rayleigh waves (R-waves) which is used to determine dynamic soil properties, i.e., the shear wave velocity (VS), shear modulus (G), damping ratio (D) and depth of each layer of the soil profile. Much of the basis of the theoretical and analytical work of this method for soil investigation has also been developed (Stokoe et al., 1994). Seismic data used in surface wave analysis are non-stationary in nature, i.e., varying frequency content in time. Especially in the low frequency range measurement, i.e., in soft soil deposit, the interested frequency of surface wave can be relatively low, i.e., less than 20 Hz. In these frequency values, the noisy signals from the natural or man-made sources may disturb the identical frequency level of the surface wave signals generated from the source. Therefore, a timefrequency decomposition of a seismic signal is needed to obtain the correct information of phase spectrum generated from signal transformation. In most of surface wave methods, the data analysis from time to frequency domain has been carried out by using Fourier transformation. However, some information of non-stationary seismic data in analysis maybe lost due to any arbitrary periodic function of time with period which is expressed as sum a set of sinusoidal in Fourier transform. The Fourier analysis is unable to preserve the time dependence. In addition, it also can not describe the evolutionary spectral characteristics of non-stationary processes. Thus, a new tool, i.e., wavelet analysis is required which allows time and frequency localization of the signals in the surface wave measurement beyond customary Fourier analysis.