1915 Bulletin of the Seismological Society of America, Vol. 96, No. 5, pp. 1915–1933, October 2006, doi: 10.1785/0120060119 Deriving Wavefield Characteristics and Shear-Velocity Profiles from Two- Dimensional Small-Aperture Arrays Analysis of Ambient Vibrations in a Small-Size Alluvial Basin, Colfiorito, Italy by Giuseppe Di Giulio, Cecile Cornou, Matthias Ohrnberger, Marc Wathelet,* and Antonio Rovelli Abstract We analyze the dispersion characteristics of ambient noise vibrations. For this purpose, two-dimensional (2D) seismic array data were acquired in four different sites in the Colfiorito plain, an alluvial intramountain basin that exhibits strong site effects. Assuming seismic noise being mainly composed of surface waves, we derive one-dimensional (1D) shallow shear-velocity profiles through the inversion of dispersion curves measured by frequency–wavenumber ( f-k) methods. The in- verted shear-wave velocity profiles are consistent with a priori information for those sites that can be approximated by 1D simple models. In these cases, the use of passive records of seismic vibrations can be a valuable tool for determining the shallow velocity profile if a detailed depiction of velocity structure is not required. The theo- retical dispersion curves for Rayleigh and Love waves were compared with the mea- sured dispersion curves for vertical and horizontal components, respectively. This allows us to discuss qualitatively the composition of ambient vibrations (outlining a large proportion of Love waves in the noise wave field) and the effects of higher modes. We also use the single-station method for investigating the origin of the horizontal-to-vertical (H/V) peak in the plain of Colfiorito in terms of ellipticity of the fundamental Rayleigh mode. Introduction The geotechnical characterization of the soil conditions is a crucial task for seismic hazard assessment. Very attrac- tive tools for the determination of velocity and thickness of the soil layers are passive methods based on ambient noise recordings (Tokimatsu et al., 1992). Such methods may in- deed represent a low-cost and noninvasive exploration so- lution. However, many issues concerning techniques based on ambient noise are still in debate within the scientific com- munity. A research project (Site Effects Using Ambient Ex- citations [SESAME], 2001–2004) was recently financed by the European Community with the main purpose to clarify the nature of the ambient noise and to investigate the capa- bility of noise-based techniques in retrieving relevant and quantitative information about the site features. In this article we discuss the results of one of the test sites chosen within the SESAME project (SESAME deliverable no. D06.05, http:// sesame-fp5.obs.ujf-grenoble.fr/SES_TechnicalDoc.htm). The study area is the Colfiorito plain, an alluvial basin of small size in central Italy (Fig. 1). Important site effects (Rovelli et al., 2001; Di Giulio et al., 2003) and the fairly well known geological structure of the basin (Fig. 1) are the two aspects that make this site attractive. We show the re- sults of five small-aperture two-dimensional (2D) arrays de- ployed in the Colfiorito plain that recorded ambient vibra- tions. We analyze the variation of the resonance frequencies within the basin. Then we compute the phase-velocity dis- persion curve for each array. The inversion of the dispersion curve, under the basic assumption that noise is composed of surface waves, provides near-surface shear-velocity profiles (e.g., Horike, 1985; Ohrnberger et al., 2004a). Comparison between the inverted soil profile with the known geotech- nical information allowed us (1) to assess the reliability of array techniques in estimating the local soil profile, and thereby (2) to check the adequacy of the starting assumption on noise composition. Finally, using the inverted shear pro- files, we derive the theoretical one-dimensional (1D) site re- sponse. For those sites with an available data set of local earth- quakes, we find a good agreement between the theoretical and the empirical transfer function estimated through clas- sical spectral ratio on the S-wave part of seismograms. *Present address: Laboratoire de Ge ´ophysique Interne et Tectonophy- sique (LGIT/IRD), Grenoble, France.