INVESTIGATING POLARIZATION OF RAYLEIGH WAVES IN SINGLE METHOD MEASUREMENTS BASED ON THE REVISED RAYDEC

The knowledge of the site effects is important to the assessment of seismic hazard studies. The horizontal-to-vertical spectral ratio (H/V) method (Nakamura, 1989) has been introduced as a convenient technique to estimate the fundamental frequency of soft deposits. The ellipticity curve analysis can be described as the elliptical motion of Rayleigh waves using a single seismic station. Since Love and body waves are present in the wavefield as well, the H/V curve would not completely correspond to the ellipticity curve. The effect of Love and body waves should be minimized or removed before considering the H/V spectral ratio.Horizontal-to-Vertical Time-Frequency Analysis (HVTFA) method can reduce the SH-wave influence by identifying P-SV wavelets from the signal and computing the spectral ratio from these wavelets (Fäh et al., 2009).The RayDec method was proposed to determine the Rayleigh wave ellipticity curve emphasizing the effect of Rayleigh waves and minimizing other types of waves based on the random decrement technique (Hobiger et al., 2009). The random decrement technique was introduced at NASA (Cole, 1973) as an alternative to the FFT algorithm. The principle is to estimate random decrement functions by averaging time segments of the measurements, which have been selected under certain conditions (Asmussen, 1997). The RayDec includes applying narrow-band Chebyshev filter on the three components of a signal. The vertical component is considered as a master trigger using zero crossings with positive slope triggering condition for all time steps. Where this condition is hold, a signal of given length could be stored for all the three components. In order to compensate the phase shift of between vertical and horizontal components, both of the horizontal signals are shifted. For each stored signals, the horizontal components are projected into the direction which maximizes the correlation between the horizontal and vertical components. Then, the obtained signals are stacked together and the ratio between the horizontal and vertical motions is estimated by analysing the energy content of the vertical and horizontal stack. The final ellipticity measurement is then obtained by averaging the RayDec results of these time windows. In this method, the length of the buffered signal Δ and the width of the frequency filter can be chosen arbitrarily.

کلیدواژه ها:

Seismic noise ، Ellipticity ، Rayleigh wave ، RayDec ، HVTFA