1 SPATIAL SAMPLING ISSUES IN FK ANALYSIS OF SURFACE WAVES Sebastiano Foti, COFS, University of Western Australia, Perth Luigi Sambuelli, DIGeT, Politecnico di Torino, Italy Laura V. Socco, DIGeT, Politecnico di Torino, Italy Claudio Strobbia, DIGeT, Politecnico di Torino, Italy Abstract The geotechnical characterisation with surface waves involves the full-wave recording, the estimation of the experimental dispersion curve and its inversion. In multistation method the dispersion curve is obtained by picking maxima of the f-k spectrum, which is strongly influenced by spatial sampling. As a consequence, an apparent dispersion curve is obtained, resulting from difficulties in separating the energy associated to different modes. The following critical aspects are discussed using numerical and experimental data: - the limit on the maximum k may be critical with wide bandwidth dispersion curves; - the resolution in k affects the precision of the phase velocity determination and the possibility of discriminating different modes; - the k interpolation produced by zero padding increases the accuracy of maxima positioning while the capability of peak separation depends only on the actual array length. - windowing introduces in the k spectrum ripples that can mask the presence of secondary modes. These sampling issues can lead to a quite irregular and misleading behaviour of the experimental dispersion curve if they are not correctly assessed in the design of the testing array. The criteria introduced for spatial sampling issues are also in agreement with the necessity of limiting near field effects. Introduction The use of surface waves for site characterisation is based on the dispersive properties of such wavefield in vertically heterogeneous media (Aki and Richards, 1980). Indeed surface wave data inversion is based upon the dispersive characteristics of such waves inferred by a full-waveform analysis. The acquisition parameters and the data processing influence the results in terms of resolution, accuracy, maximum and minimum investigation depth both in multistation and in two receivers layout. This influence is evident even when the first mode is dominant, but has heavy consequences in stratigraphic situations that make higher modes of propagation important (Foti et al., 2000). Indeed we will show, in this paper, that only the modal curves should be considered a characteristic of the site: the experimental dispersion curve is strongly affected by mode superposition effects and depends also on the acquisition parameters, it can then be considered as an apparent curve. To correctly compare synthetic and experimental data within an inversion scheme, it is necessary to use a modelling that reproduces closely the testing procedure, taking in account the acquisition parameters. Surface Wave Analysis In theory The equation of wave propagation in a linear elastic layered medium with zero stress boundary conditions on the free surface has Surface Waves as non-trivial solution. Considering the corresponding eigenvalue problem, for each frequency ω, uniquely determined wave numbers k 1 (ω), …, k n (ω), and phase velocities v 1 =ω/k 1 ,…, v n =ω/k n can be found. Each eigenvalue corresponds to a mode of propagation. At low frequency (ω→0) only the fundamental mode exists. Increasing ω, other higher