Analysis of dense array noise measurements using the modified spatial auto-correlation method (SPAC): application to the Grenoble area

Recent earthquakes outlined the importance of amplification effects in deep sedimentary basins where many large cities like Mexico city are located. This paper presents an application of the “modified spatial auto-correlation method” based on ambient vibrations analysis in order to obtain a model of the underlying local structure (velocity profiles for compressional and shear waves). We first describe the spatial auto-correlation method itself designed for regular, circular 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, broad band amplification effects and very thick late 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 relative simplicity, and its performance for very thick deposits. However, some further theoretical and experimental investigations are required to clearly assess the capabilities and limitations of the method.