Real and Synthetic Ambient Noise Recordings in the Grenoble Basin: Reliability of the 3D Numerical Model

Abstract eng:
The Grenoble basin is a 3D Y-shaped deep basin filled mostly with late-Quaternary postglacial deposits overlaying Jurassic marls and a marly limestone substratum. More than ten years of geophysical surveys performed within the basin (vertical and offset seismic profiles at a deep borehole, refraction and reflection seismics and gravity measurements) has allowed to build a 3D numerical model of the Grenoble basin, which was adopted for the numerical benchmark test on third international ESG symposium held in 2006. Besides these measurements, intensive microtremors surveys have been carried out from 1999 to 2005: extensive H/V measurements, microtremor arrays, simultaneous ambient noise recordings along linear profiles perpendicular to the valley branches axes, and temporary seismological recordings. In this paper we compare recorded ambient noise with noise synthetics produced in the 3D model of Grenoble in order to assess the reliability of numerical 3D model used for ground motion prediction. The noise modelling was performed within the [0.2 1.1 Hz] frequency band by using a 3D finite-difference scheme and was aimed to model ambient noise generated by human activity (i.e. by surface sources randomly distributed in space and time). We observe that noise synthetics capture most of the features of the actual ambient noise. H/V peak frequencies and Rayleigh waves phase velocities are indeed well reproduced, outlining the reliability of the 3D velocity model of Grenoble for ground motion numerical prediction at low frequency.

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