Analysis of three-component ambient vibration array measurements

SUMMARY Both synthetic and observed ambient vibration array data are analysed using high-resolution beam-forming. In addition to a classical analysis of the vertical component, this paper presents results derived from processing horizontal components. We analyse phase velocities of fundamental and higher mode Rayleigh and Love waves, and particle motions (ellipticity) retrieved from H/V spectral ratios. A combined inversion with a genetic algorithm and a strategy for selecting possible model parameters allow us to define structural models explaining the data. The results from synthetic data for simple models with one or two layers of sediments suggest that, in most cases, the number of layers has to be reduced to a few sediment strata to find the original structure. Generally, reducing the number of soft-sediment layers in the inversion process with genetic algorithms leads to a class of models that are less smooth. They have a stronger impedance contrast between sediments and bedrock. Combining Love and Rayleigh wave dispersion curves with the ellipticity of the fundamental mode Rayleigh waves has some advantages. Scatter is reduced when compared to using structural models obtained only from Rayleigh wave phase velocity curves. By adding information from Love waves some structures can be excluded. Another possibility for constraining inversion results is to include supplementary geological or borehole information. Analysing radial components also can provide segments of Rayleigh wave dispersion curves for modes not seen on the vertical component. Finally, using ellipticity information allows us to confine the total depth of the soft sediments. For real sites, considerable variability in the measured phase velocity curves is observed. This comes from lateral changes in the structure or seismic sources within the array. Constraining the inversion by combining Love and Rayleigh wave information can help reduce such problems. Frequency bands in which the Rayleigh wave dispersion curves show considerable scatter are often better resolved by Love waves. Information from the horizontal component can be used to correctly assign the mode number to the different phase–velocity curve segments, especially when two modes seem to merge at osculation points. Such merging of modes is usually observed for Rayleigh waves and thus can be partly solved if additional information from the Love waves and the horizontal component of Rayleigh waves is considered. Whenever a site presents a velocity inversion below the top layer, Love wave data clearly helps to better constrain the solution.