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Multi-modal determination of {R}ayleigh dispersion and attenuation curves using the circle fit method

Journal Contribution - Journal Article

This paper introduces the circle fit method for the determination of multi-modal Rayleigh dispersion and attenuation curves as part of a Multichannel Analysis of Surface Waves (MASW)experiment. The wave field is transformed to the frequency–wavenumber (fk) domain using a discretized Hankel transform. In a Nyquist plot of the fk-spectrum, displaying the imaginary part against the real part, the Rayleigh wave modes correspond to circles. The experimental Rayleigh dispersion and attenuation curves are derived from the angular sweep of the central angle of these circles. The method can also be applied to the analytical fk-spectrum of theGreen’s function of a layered half-space in order to compute dispersion and attenuation curves, as an alternative to solving an eigenvalue problem. A MASW experiment is subsequently simulated for a site with a regular velocity profile and a site with a soft layer trapped between two stiffer layers. The performance of the circle fit method to determine the dispersion and attenuation curves is compared with the peak picking method and the half-power bandwidth method. The circle fit method is found to be the most accurate and robust method for the determination of the dispersion curves. When determining attenuation curves, the circle fit method and half-power bandwidth method are accurate if the mode exhibits a sharp peak in the fk-spectrum. Furthermore, simulated and theoretical attenuation curves determined with the circle fit method agree very well. A similar correspondence is not obtained when using the half-power bandwidth method. Finally, the circle fit method is applied to measurement data obtained for a MASW experiment at a site in Heverlee, Belgium. In order to validate the soil profile obtained from the inversion procedure, force–velocity transfer functions were computed and found in good correspondence with the experimental transfer functions, especially in the frequency range between 5 and 80 Hz.
Journal: Geophysical Journal International
ISSN: 0956-540X
Issue: 3
Volume: 212
Pages: 2143 - 2158
Publication year:2018
Keywords:Geosciences & technology