GNGTS 2013 - Atti del 32° Convegno Nazionale

of picking such spectrum do not reduce the uncertainty on the extracted dispersion. To pick the fundamental mode, one should peak at 80% of the spectral maximum at 28 Hz, and at 10% of the spectral maximum at 23 Hz. We conclude that also passive ReMi data suffer from the osculation problem. Less ambiguous is the use of a different measurement of the same active wavefield, and the use of the horizontal component can help in solving the osculation issues. Active data. As shown above, the energy shift between modes is related to the different ellipticity for frequencies smaller and larger than the osculation frequency. In particular below the osculation point the Rayleigh wave ellipticity makes the first higher mode much more energetic in the vertical component than the fundamental mode. This implies that if only the vertical component of soil motion is acquired and analysed in the multi channel seismic records, energy seems to shift from the fundamental mode (at frequencies larger than the osculation frequency) to the first higher mode (below the osculation frequency). This can potentially lead to severe errors in dispersion curve identification, but is only due to the traditional use of vertical geophones only. Fig. 3 shows the spectra from the vertical and horizontal component data for the same model of Fig. 2, with a limited length array. The horizontal component allows the identification of the fundamental mode, even below the osculation frequency, with no ambiguity. Note that the horizontal components do not present the disturbing osculation phenomenon so important in the vertical components. In particular, below the osculation frequency, energy maxima still remain on the fundamental mode, except for the very low frequencies, and thus produce an easily identifiable dispersioncurve. This is inagreementwith the ellipticity polarization shown in Fig. 8, in which the horizontal component of the fundamental mode maintains high amplitudes below the osculation frequency. These synthetic results and the supporting theory indicate that the use of horizontal component receivers in multichannel arrays can allow a correct definition of the fundamental mode dispersion down to low frequencies, well below the osculation point. This conclusion can lead to the design of alternative operational practice that can avoid any overestimation of bedrock seismic velocity as a consequence of the osculation problem. The same linear array can be used with multi-component receivers acquiring multiple shots. Conclusions. Below the osculation frequency the f-k spectrum energy maxima of the vertical component of motion do not insist on the only fundamental theoretical mode. This can lead to large errors in the inverted models if modes contribution is not considered, with large over estimation of bedrock velocity. The osculation frequency is directly related to the thickness and the velocities of the layers and a similar behaviour is observed in the theoretical Rayleigh ellipticity. Our synthetic tests show that the osculation frequency is practically the same frequency at which Rayleigh ellipticity Fig. 3 – f-k spectra of the horizontal and vertical inline components over the same array. Top) f-k spectrum of the vertical component active seismic records for the same model of Fig. 2 (bottom) f-k spectrum of the horizontal component seismic records for for the same model of Fig. 2. 26 GNGTS 2013 S essione 3.1