From noise measurements to site effects: main learning from the SESAME project and ESG2006 blind test

This presentation outlines the main lessons of two major recent accomplishments, the SESAME European project (2001-2004) and the ESG2006 blind test, which were both dedicated to assessing the actual capabilities of techniques using ambient vibration measurements to provide reliable and useful information on site conditions and/or site effects. Two techniques - the very simple H/V ('Nakamura's') technique, and the more advanced array technique - have been thoroughly considered under different viewpoints, in order to better understand their physical basis, assess their actual capabilities in view of site effect estimation, and, whenever possible, propose user guidelines and processing software to ensure a correct use. This broad set of investigations indicates basically that ambient noise measurements, when performed, processed and analyzed 'with caution', can provide useful, though limited, qualitative and quantitative information on site conditions and amplification effects. More specifically, results prove to be very satisfactory for estimations of the fundamental frequency, and rather satisfactory for the the band-limited estimations of Rayleigh wave dispersion curves. The inversion of the shear wave velocity profiles remains however a very uneasy and non-unique task, which can be significantly improved by incorporating external information. Microtremor based methods can therefore be recommended as an inexpensive and fast tool for microzonation and/or site/specific studies, provided the ambient noise measurements are performed, processed and analyzed 'with caution', while it is also highly recommended to combine them with other survey types. The software, guidelines and recommendations available from the web sites http://sesame-fp5.obs.ujf-grenoble.fr and http://www.geopsy.org, which represent the consensus view of a large community of European scientists, are intended to explicit clearly and practically the fuzzy expression 'with caution'. (authors)