IDENTIFICATION OF DAMPING FACTOR CONSIDERING ITS LOWER LIMIT BY SPECTRAL RATIO INVERSION : Application to borehole array records at hard rock sites and evaluation of attenuation characteristics

A spectral inversion method adopting new functional model of damping factor with its lower limit is proposed for identifying more sophisticated attenuation characteristics of rock sites. The proposed model is applied to borehole array data recorded at hard rock sites. The identified lower limits of damping factor agree well with experimental material damping factors derived from laboratory test of rock samples. Therefore it can be interpreted that the lower limit of proposed model may be corresponding to actual material damping factor. Moreover, we indicate that frequency dependent characteristics of the damping factor at rock sites could be interpreted as a theoretical damping due to scattering of inhomegeneous rock.