GNGTS 2013 - Atti del 32° Convegno Nazionale

Masoomzadeh H., Barton P. J. and Singh S. C.; 2010: Nonstretch moveout correction of long-offset multichannel seismic data for subbasalt imaging: Example from the North Atlantic. Geophysics, 75 , no. 4, pp. R83-R91. Mazzotti A., Stucchi E. and Clementi M.; 2005: Normal Moveout through partial corrections. 67th EAGE Conference & Exhibition, EAGE, Extended Abstracts, Z-99 pp. Perroud H. and Tygel M.; 2004: Nonstretch NMO. Geophysics, 69 , no. 2, pp. 599-607. Rocchi S., Mazzotti A., Marroni M., Pandolfi L., Costantini P., Di Biase D., Bertozzi G., Federici F. and Lo Papa G.; 2007: Detection of Miocene saucer-shaped sills (offshore Senegal) via integrated interpretation of seismic, magnetic and gravity. Terra Nova, 19 , no. 4, pp. 232-239. Taner M.T. and Koehler F.; 1969: Velocity spectra-digital computer derivation and applications of velocity functions. Geophysics, 34 , no. 6, pp. 859-881. The impact of Rayleigh waves ellipticity in mode misidentification J. Boaga 1 , G. Cassiani 1 , C. Strobbia 2 , G. Vignoli 3 1 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy 2 Total E&P, Pau, France 3 Aarhus University, Hydrogeophysics Group, Aarhus C, Denmark Introduction. The surface wave method is widely diffused for shear wave velocity estimation. It is based on the dispersion properties of vertically heterogeneous media (Arai et al. , 2004; Foti, 2002; Tokimatsu, 1995; Strobbia, 2002; Socco and Strobbia, 2004). Every surface wave technique requires firstly an high quality recorded seismograms to be analysed, From the data we can estimate the dispersive properties of surface waves, and then go forward with an inversion process of the dispersion curve, controlling the observed Rayleigh wave dispersion, in order to estimate a realistic shear wave velocity profile. One of the most neglected problem in surface wave analyses is the complexity of the multimodal wave propagation which can lead to modes misidentification (Tuan et al. , 2011). Even if several issues must be fronted, e.g. attention must be paid to the presence of subsurface structure more complex than simple one-dimensional layering (Strobbia and Foti, 2006; Vignoli et al. , 2009, 2011), the inversion of the dispersion curve remains one of the most critical aspect of all the procedure (see Lai et al. , 2005, Maraschini et al. , 2010). Commonly dispersion curves are identified as energy density maxima in transformed domains, for example the frequency-wavenumber or the frequency- velocity domain. For each frequency the energy maxima are identified in correspondence of certain modal wavenumbers k or velocities v, and it is commonly assumed that the highest modal k, corresponding to the lowest phase velocity v, belongs to fundamental mode of propagation (Weaver et al. , 1982; Karray et al. , 2008; Park et al. , 1999). Generally common surface wave users consider only the largest spectrum energy frequency by frequency, and the fundamental mode is assumed as dominant even if this is not always verified (Foti et al. , 2002; Zhang et al. , 2003). Often from surface wave analyses we observe only an apparent dispersion curve which represent the contribution of several modes. The focus of this study is on evaluating the effects of mode mis-identification, in particular as a consequence of the well known ‘osculation’ phenomenon (Malischewsky et al. , 2008). Osculation points are point at which two modal dispersion curves get very close and often have the appearance of crossing each other (Cercato, 2009). We want here to show as the osculation phenomenon is strictly related with the polarization of Rayleigh motion in the case of strong impedance contrast in the first subsoil (Boaga et al. , 2013). The use of a multi components analyses of motion can be really useful in certain particular geological conditions. Osculation and velocity contrasts. The errors in terms of depth and velocity of the bedrock can be large if modes identification is wrong, and these parameters are of interest in engineering surveys. When strong impedance contrast is present energy shift from the fundamental to the first higher mode at the so call ‘osculation’ point. In the case of ‘perfect osculation’ even with ideal noise-free modal synthetics, a standard data analysis will extract an apparent continuous curve resulting from the two adjacent modes. But in the applied practice of multi-channels analysis of Rayleigh wave (MASW, Park et al. , 1999), given the measurement noise, and the 22 GNGTS 2013 S essione 3.1