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Recent earthquakes outlined the importance of amplification effects in deep sedimentary basins where many large cities like Mexico City are located. This p aper presents an ap plic ation of the “modi fied spatial auto-corre l ation method” based on ambient vibrations analysis in order to obtain a model of the underlying local stru ctu re (velocity pro files for compressional and shear waves). We fi rs t desc ribe the spatial auto-corre l ation method itself designed for reg ular, ci rc ular arrays. Then an improvement is presented that allows the use of irregular, almost arbitrarily shaped arrays. We then apply this technique to data obtained at two sites during a temporary experiment in Grenoble (March 99). Grenoble is an interesting site because of large, b road band amplifi c ation effects and ve ry thick lat e quaternary deposits. The results compare relatively well with existing geophysical information from gravimetric, seismic reflection and borehole data. The relevance of the method for urban areas is therefore emphasized, considering its low cost and re l at ive simplicity, and its perfo rmance for ve ry thick deposits. Howeve r, some further theoretical and experimental investigations are required to clearly assess the capabilities and limitations of the method.

C o rresponding author: P.-Y. B a rd; Lab o rat oi re de Géophysique Interne et Te ctonop hy sique, O bse rvat oi re de G re no ble Unive rsité Jo s eph Fo u ri er, BP 5 3, 3 8 0 4 1 G re no ble Cedex 9, France; phone:+ 33 476828061; fax:+ 33 476828101; e-mail: pierre-yves. bard@ obs. ujf-grenoble. fr