F. Renalier

Influence of parameterization on inversion of surface wave dispersion curves and definition of an inversion strategy for sites with a strong VS contrast

Inversion of the fundamental mode of the Rayleigh wave dispersion curve does not provide a unique solution and the choice of the parameterization (number of layers, range of velocity, and thickness values for the layers) is of prime importance for obtaining reliable results. We analyzed shear-wave velocity profiles derived from borehole tests at 10 sites where soil layers overlay bedrock in various geologic contexts. One to three seismic layers with linear velocity laws could model all of them. Three synthetic models defined from this preliminary study were used to understand the influence of parameterization on the dispersion curve inversion. This analysis resulted in the definition of a two-step inversion procedure for sites exhibiting a strong impedance con-trast. In the first step, the dispersion curve is inverted with an increasing number of layers over half space. The evolution of the minimum misfit and bedrock depth with number of layers allows the estimation of the true bedrock depth range. In the...

Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.

Characterization of site conditions (soil class, VS30, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, VS30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of VS30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated VS30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.