WAVELET SPECTROGRAM ANALYSIS OF SURFACE WAVE METHOD FOR IN-SITU ATTENUATION AND DAMPING RATIO OF SOFT SOIL
ROSYIDI, SRI ATMAJA P
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Soil attenuation and damping ratio are important parameters in soil dynamic analysis and design of structural dynamic. Both parameter can be obtained from the field measurement using the seismic measurement, e.g., the spectral analysis of surface wave (SASW). The SASW is one of seismic methods based on the dispersion of Rayleigh waves (R-waves) which is used to determine the shear wave velocity, shear modulus and depth of each layer of the soil profile. Seismic surface wave data detected in the seismic measurement are non-stationary in nature i.e. varying frequency content in time. Especially in the low frequency range measurement, i.e., in soft soil, the interested frequency of surface wave can be relatively low which can be less than 20 Hz. In these frequency values, the noisy signals may disturb in the identical frequency level of the surface wave signals generated from the source. In this paper, an improved technique of continuous wavelet transform (CWT) of derivative Gaussian function was employed in the seismic surface wave analysis in order to calculate the soil attenuation and damping of soft soils. The technique is then called as the wavelet spectrogram analysis of surface wave (WSASW). There are two procedures used in the analysis, i.e., first, time-frequency wavelet spectrogram was utilized to select the response spectrum of interest from recorded surface waves. Second stage is a time-frequency wavelet filtering which was used to remove noisy distortions in the selected spectrogram based on simple concept of wavelet filtering. Detected strong ground noises were able to be filtered out from the signals. Consequently, denoised signals representing the interested waveform of surface wave were reconstructed by inverse wavelet transform algorithm. Finally, clear pattern of phase spectrum can be easily calculated. Results showed that the wavelet spectrogram analysis of surface wave (WSASW) method is able to identify and reconstruct better spectrum seismic and phase velocity dispersion curve from surface wave measurement. Based on reconstructed seismic spectrum and velocity dispersion curve, soil attenuation and damping ratio can be obtained well. This technique can be applied to solve problems related to non-stationary seismic wave at soft clay soil.