Application of Surface wave methods for seismic site characterization

Surface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages. Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this topic

2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

Shear wave velocity imaging of the Avignonet landslide (France) using ambient noise cross correlation

International audienceThe Avignonet landslide affects a 2 by 2 km area covered by clayey deposits. This paper presents the use of the seismic ambient noise cross-correlation technique to retrieve a 3-D model of the shear wave velocity of the area. Seismic ambient noise was recorded during 15 days at 13 stations located on the landslide. Cross correlations computed between the vertical components of all station pairs allow the retrieval of the Rayleigh wave Green's functions and the estimation of their group velocity dispersion curves in the 1.7-5 Hz frequency range. At frequencies lower than 1.5 Hz, the anisotropy of the wavefield strongly influences the apparent Rayleigh wave velocities. Moreover, the analysis of the convergence of the cross correlations shows that at frequencies higher than 5 Hz, the recording time length was not sufficient for the cross correlation to be stable. These 1.7-5 Hz passive group dispersion curves are complementary to the ones computed from shot signals in the 3-7 Hz frequency range. A tomographic inversion of the resulting 1.7-7 Hz Rayleigh wave group dispersion curves provides local group dispersion curves at each cell of the tomographic grid. These are inverted with a neighborhood algorithm to retrieve the 3-D model of the landslide. Despite the complex wave propagation in the eastern part of the landslide and the sparse ray coverage, estimated velocities and first-order features are in good agreement with previous investigations

CHARACTERIZATION OF FRENCH ACCELEROMETRIC PERMANENT NETWORK STATIONS WITH SURFACE-WAVE BASED METHODS: IMPORTANCE OF JOINT USE OF ACTIVE AND PASSIVE METHODS, LOVE AND RAYLEIGH WAVES

International audienceData provided by accelerometric networks are important for seismic hazard assessment. They are key to derive Ground Motion Prediction Equations (GMPEs). The correct use of accelerometric signal is also linked to the station site metadata that include reliable information about site class (VS30), velocity profiles, and other relevant information that can help to quantify the site effect associated to stations. In France, the permanent accelerometric network consists of about 150 stations. A recent project led to the characterization of around 30 stations, especially in South East of France. This characterization project was performed using surface-wave based methods that allow the derivation of velocity profiles from dispersion curves of Rayleigh and Love waves. We implemented both active acquisitions (Multichannel Analysis of Surface Waves) along lines from 50 to 100 m length and passive acquisitions (Ambient Vibration Array) using multiple circle arrays (apertures from 10 to 1000 m). The computation of dispersion curves, then their inversion in terms of shear wave velocity profiles (taking into account the non-uniqueness issue of such inversion) allowed the estimations of VS30 values and the designation of soil classes including the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) Perform both active and passive measurements in order to derive dispersion curves for an adequate frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities, which reduces the risk of misattribution of modes and thus, mitigates errors when modeling velocity profiles. Even when logistical contexts are sometimes difficult, the use of surface-wave based methods are suitable for station site characterizations, even on rock sites (where the applicability of these methods was sometimes disputed). Typically, it is possible to achieve a complete survey for one station in one working day, by 5 to 6 motivated operators. Conversely, the processing is time consuming (one working week for one geophysicist) and the inversion procedure has to be supervised by an expert in surface wave methods

Clayey Landslide Investigations Using Active and Passive VS Measurements

Clay slopes frequently are affected by gravitational movements. Such movements generate complex patterns of deformation that have slip surfaces located at different depths and are likely to modify geophysical parameters of the ground. Geophysical experiments performed on the large clayey Avignonet landslide (Western Alps, France) have shown that shear-wave velocity (VS) is most sensitive to clay deconsolidation resulting from the slide. Values of VS at shallow depths exhibit an inverse correlation with the GPS-measured surface-displacement rates. Compared with measurements in stable zones, VS values in the most deformed areas of the slide can be reduced by a factor of two to three. Laboratory measurements on clay samples set in triaxial cells have shown that a strong decrease of VS values accompanies an increase in the void ratio, in a velocity range similar to that measured in situ. Although other factors (stress change, cementation, granularity) can modify VS values, these results justify the potential of VS imaging to map spatially the deformation induced by a landslide. Several active and passive techniques for measuring VS are tested and compared on the kilometer-size and 50-m-deep Avignonet landslide. The crosscorrelation technique, applied to seismic noise recorded by a large-aperture array and associated with shot records, turns out to be an effective tool for imaging the landslide in three dimensions. If permanent stations are installed, the same method also can be used to monitor the evolution of seismic velocity with time, as an indicator of landslide activity

Influence of parameterization on inversion of surface wave dispersion curves and definition of an inversion strategy for sites with a strong V-S 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 contrast. 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 second step, this information is introduced in inversions with linear velocity laws. Synthetic tests showed that applying this procedure requires the dispersion curve over a frequency range from F-0 to 10F(0), where F-0 is the site resonance frequency. The strategy was tested on two real cases for which Rayleigh wave dispersion curves were measured over this frequency range using passive and active seismic methods. The strategy was successful at the first site, while the bedrock depth was overestimated by 15% at the second site, probably resulting from the existence of a higher mode affecting the dispersion curve at low frequency

A new simple neural-network based approach to predict the seismic response of levees and small height earth dams

International audienceThe issue of the seismic stability of embankments has been raised by several recent events. Given their very large cumulated length, together with the large variability of their owners, it is rarely possible to perform detailed investigations and complex numerical simulations, especially in moderate seismicity areas. Analyzing their capacity to withstand seismic loading thus requires the use of simple tools, based on a few easily available parameters. This work aims at providing an easy-to-use tool to assess peak acceleration at crest of embankments and peak acceleration of potential sliding blocks then allowing to evaluate their stability in case of earthquake. It is based on a 2D numerical parametric study combining 135 realistic configurations of embankments and natural soil layers, and four loading levels, characterized by peak acceleration at outcropping bedrock (0.01g, 0.1g, 0.3g and 0.5g). For each configuration and loading level, nonlinearity is taken into account by using equivalent shear modulus and damping consistent with the strain level (derived from a set of 1D linear equivalent computations). Artificial neural networks are then used to identify the parameters that best control the seismic response of the embankment while being easily available. They also provide a relation between these input parameters and the researched outputs (peak accelerations at embankment crest and for potential sliding blocks). A few abacus based on those neural networks are also provided as a visual tool to help engineers understanding the main trends

Joint inversion of Rayleigh wave ellipticity and spatial autocorrelation measurements

The local soil structure (i.e. shear and pressure wave velocities) can be obtained by inversion of dispersion curves ranging over a sufficiently large frequency band. However, measurements of such dispersion curves using ambient seismic vibrations require a large number of seismometers and a long measuring time. As a simple alternative, we propose to invert Rayleigh wave ellipticity obtained by ambient seismic noise measurements at a single site using a method based on the random decrement technique (Hobiger et al., 2009). Indeed, the freSeismological Research Letters Volume 81, Number 2 March/April 2010 305 quency-dependency of Rayleigh wave ellipticity is tightly related to the shear wave profile of the soil. However, as different soil structures can result in the same ellipticity curve (e.g. homothetic structures in velocity and thickness), the inversion of ellipticity curves alone is ambiguous. Therefore, additional measurements fixing the shearwave velocity in the superficial layers have to be included into the inversion process. We suggest using a small number of seismic stations to measure spatial autocorrelation curves. In this way, three seismic sensors and one hour of measurements can be sufficient to invert the local soil structure. We will present the method to extract the Rayleigh wave ellipticity curve, show which parts of the ellipticity curve have to be included in the inversion process and demonstrate the benefit of the additional spatial autocorrelation curve measurements. Then, we will present an example application to real noise data collected within the framework of the European NERIES project at well-known European accelerometric sites and the results will be compared to thPublishedMontpellier, France4.1. Metodologie sismologiche per l'ingegneria sismicarestricte

REVISITING SARMA'S METHOD FOR SEISMIC RESPONSE OF EMBANKMENTS: FIRST RESULTS

International audienceGiven the very large length of embankments along rivers and channels, there is a need for a reliable, simple and affordable method to assess the stability of these structures in case of earthquake. However, most of the existing simplified methods have been developed considering earth dams. They are usually not representative in terms of height and frequency range of the particular case of a river embankment. Moreover, the river embankments do not lie directly on the rigid bedrock but on a (soft or stiff) soil foundation. For this last reason, and because it provides an analytical formulation, the Sarma's method is considered as the first simplified method to apply for assessing the dynamic response of a river embankment. However, it is based on several assumptions that have never been qualified. Therefore, in order to assess the reliability of this method, it is applied on 18 configurations of embankments and soil foundation loaded by 26 accelerograms. The results are compared with those obtained by direct numerical simulation with the spectral element method for the same configurations. The comparisons show that Sarma's simplified method generally leads to an overestimation of the peak response of the embankment. The discrepancies are mostly explained by the less accurate predictions of higher modes with Sarma's method and by the assumption of a rigid bedrock under the foundation layer. Moreover, an analysis of the strain distribution indicates that Sarma's (1979) assumption of a uniform damping in the embankment and the foundation layer is far from being always justified, especially for equivalent linear computations. Finally, the main perspectives of this work is to provide an affordable methodology to estimate the peak response of an embankment taking into account the interaction with the underlying soil, which may be more complex than a simple horizontal layer