THE EFFICIENCY OF USING THREE COMPONENT ARRAY-BASED ELLIPTICITY METHODS FOR SITE CHARACTERIZATION

Many researchers believed that the spectral ratio between horizontal and vertical components has been introduced by Nakamura for the first time in 1989. But, started from 1971 and based on the assumption that the noise wavefield is predominantly consists of surface waves, many authors expressed that the H/V ratio is related to the ellipticity of Rayleigh waves (Konno and Ohmachi, 1998). The fact is, in contrast with Love waves, Rayleigh waves are polarized in both horizontal and vertical directions, so the peak observed in the H/V may be related to the polarization of Rayleigh waves. In this regards, Bard (1999) noticed that the existence of Love waves will not affect the interpretations of H/V ratio because it does not affect the vertical component and also due to the airy phase of Love waves, the peaks is get more strength near the resonance frequency. He also mentioned that the sensitivity of this ratio to body waves is not important if the wavefield is not significantly including the body waves.Since the sources are unknown, it is necessary to consider some assumptions concerning the spectral content of SH waves, because the SH part of the wavefield contributes to the horizontal component and if it could be removed, the H/V ratios would better determine the ellipticity (Fah et al., 2001). For this purpose two main methods has been introduced: The first one is reducing the SH wave influence by identifying P-SV wavelets from the signal in this regard, the most energetic parts of vertical component is identified in time for each frequency and therefore P–SV wavelets are located. This method is called HVTFA (Fah et al., 2001). The second method using the vertical component as a master trigger and stacking a large number of horizontal and vertical signals based on random decrement technique to extract Rayleigh waves respect to Love and body waves. This method is called RayDec (Hobiger, 2009). From 2001 as part of NERIES project (2004-2008) using ellipticity information for site characterization has been popularized. The first attempt to get additional information like shear wave velocity from this parameter comes back to Fah et al. (2001) paper. Two years later he succeeded to invert the stable part of ellipticity to retrieve shear wave velocity (Fah et al., 2003). As respects to the non-uniqueness of the inverted results from single station methods there is some difficulty for using this kinds of method, for example it is difficult to find the exact correction for the distribution of SH and Love wave’s energy in the horizontal components. Also, difficulty in identifying and separating the contribution of higher modes is another disadvantage of this set of techniques, and generally it is not possible to identify the propagation direction and the sense of rotation with single station methods (Hobiger et al., 2013).

Keywords:

Ellipticity , Rayleigh waves , Ambient noise , RTBF