The Importance of the Apparent Dispersion Curve and the Higher Modes of Rayleigh Waves in MASW‐Remi Method for Seismic Geotechnical Site Characterization

Abstract

The interest of both the scientific community and the professionals towards the MASW method (Multichannel Spectral Analysis of Surface Waves) has been increasing for the last years. In the '90 several researchers have realized that, when dealing with inversely dispersive sites, the MASW method based only on the fundamental mode can really cause erroneous Vs profiles, hence an erroneous seismic site characterization. When dealing with inversely dispersive sites (i.e. sites where stiffness discontinuities exist, soft layers trapped between stiffer layers or viceversa stiff layers trapped between softer layers) higher modes of Rayleigh waves must be combined together with the fundamental mode to calculate the effective or apparent dispersion curve (Lai 1998, Roma 2001‐2002‐2006), in order to achieve a reliable Vs profile and a reliable seismic site characterization. It is not sufficient to calculate the numerical higher modes and use them separately for the inversion process, because it is practically impossible to distinguish the experimental higher modes from the field data in the geotechnical scale. It is well known that the apparent experimental dispersion curve that is determined from the field data is the result of a superposition of the fundamental and the several higher modes. In order to obtain a reliable Vs profile and hence a reliable site soil characterization not only the fundamental mode, but also the higher modes of Rayleigh waves have to be combined when calculating the apparent numerical dispersion curve. Consider that it is not sufficient to calculate the higher modes of Rayleigh waves and invert them separately, because it is necessary to combine the higher modes with the fundamental mode to calculate the numerical apparent dispersion curve, as it is measured in field. This is due to the fact that from field data it is not possible to distinguish and define the higher modes without a large uncertainty. In this article the potentialities of a new algorithm (www.masw.it, Roma 2001) that calculates the apparent dispersion curve using both the fundamental mode and all higher modes are shown into an application to a real case.