The nature of noise wavefield and its applications for site effects studies: A literature review
Introduction
Over the last two decades, many cities have grown considerably and demographists expect a similar trend for at least the two next decades. These urban areas are very often built on soft sediments, and a large number is unfortunately located in seismic areas, emphasizing the need for a careful and reliable assessment of site amplification phenomena. This issue has been addressed for a long time by scientists and engineers who developed many techniques to identify the main characteristics of site responses for soft deposits (i.e., resonance frequencies and amplification factor). These techniques may be grouped into three main categories. The first is based on a numerical simulation approach (see Panza et al., 2001 for a review of numerical simulation methods), and is coupling with classical geophysical and geotechnical tools (such as seismic refraction, seismic reflection, boreholes, penetrometers, etc.) in order to provide reliable estimates of the required input parameters (sediment depth, S and P wave velocities, etc.). However, many such classical geophysical tools suffer severe limitations in urbanized areas, mainly because of their cost (seemingly prohibitive especially in developing countries which face many other priorities), and their environmental impact that is less and less accepted by the community (use of explosives, drillings, etc.). The second category of technique consists in directly measuring the site response on the basis of earthquake recordings on specific stations located on carefully chosen sites. Although this technique provides an unbiased experimental estimation of the site transfer amplification factor, its use in areas of low to moderate seismicity is limited by the time required to gather a significant number of recordings with satisfactory signal to noise ratio. Finally, the latest category of methods, based on ambient noise recordings, became more and more popular over the last decades as it offers a convenient, practical and low cost tool to be used in urbanized areas. Two techniques are predominantly used to determine site response parameters: the simple horizontal to vertical Fourier amplitude spectral ratio (HVSR), and the more advanced array technique.
The ability of the HVSR technique to provide a reliable information related to site response has been repeatedly shown in the past (Nakamura, 1989, Lachet and Bard, 1994, Kudo, 1995, Bard, 1998). However, its theoretical basis is still unclear as two opposite explanations have been proposed. Nakamura, 1989, Nakamura, 2000 claims that the horizontal to vertical spectral ratio mainly reflects the S-wave resonance in soft surface layer (removing effects of surface waves), and hence that HVSR curves provide a consistent estimate of the site amplification function. This “body wave” interpretation has been contradicted in several papers highlighting the relationship between the HVSR and the ellipticity of fundamental mode Rayleigh waves (Lachet and Bard, 1994, Kudo, 1995, Bard, 1998), and thus seriously questioning the existence of any simple direct correlation between HVSR peak value and the actual site amplification factor (the HVSR peak would then be associated with the vanishing of the Rayleigh waves vertical component, instead of amplification of S-wave on horizontal components). This brief summary about the two hypothetical origins of the HVSR peak shows the close link between the composition of the seismic noise wavefield (body or surface waves) and the interpretation of the HVSR curve.
To go one step further, when one assumes that the seismic noise wavefield is mainly constituted by surface waves, then new questions arise concerning the type of surface wave (Rayleigh or Love waves) and propagation mode (fundamental or higher modes). Answering these questions is very important for the processing and interpretation of array microtremor recordings, which mainly focus on the derivation of surface waves dispersion curve. For instance, processing of horizontal components should definitively take into account the simultaneous existence of both Rayleigh and Love waves (Arai and Tokimatsu, 1998, Arai and Tokimatsu, 2000, Yamamoto, 2000, Bonnefoy-Claudet, 2004), while improper identification of surface waves type (Rayleigh or Love) or order (fundamental or harmonics) might severely bias the inversion of S-wave velocity profile (Tokimatsu et al., 1992, Tokimatsu, 1997, Beatty et al., 2002, Zhang and Chan, 2003, Bonnefoy-Claudet, 2004, Wathelet, 2005).
Despite the lack of theoretical agreement about the nature of the ambient seismic noise wavefield, the number of site-specific studies based on noise recordings has increased dramatically in recent years. Time has come thus for a clear assessment of the HVSR and array methods, in order to better identify their actual possibilities and limitations. This was one of the main goals of the SESAME European project (Site EffectS using AMbient Excitations, EESD project n° EVG1-CT-2000-00026). Within this framework, a wide set of numerical and experimental researches work has been conducted over the last 4 years to investigate the nature of the ambient noise wavefield in the frequency range of interest for site effects estimation purposes (i.e., from 0.2 to 10 Hz). The first step of this SESAME project has been to update a survey of the scientific literature dealing with seismic noise, in order to establish a state of the art about the knowledge of the nature of the ambient noise wavefield. The present paper is devoted to a presentation of the main outcomes of this comprehensive survey. After reviewing the evolution of concerns about seismic noise over the year (Section 2) we will focus on its origin (Section 3) and the composition of the noise wavefield (Section 4), with special attention to the respective proportion between a) surface and body waves, b) Rayleigh and Love waves, c) fundamental and higher modes.
Section snippets
The use of seismic noise: main historical periods
Seismic noise is observed very early on, from the very beginning of instrumental seismology in the nineteenth century. A pendulum led Bertelli (1872), in Italy, to observe that it was continuously moving for hours or days with regional weather conditions. He noticed a correlation between the long period noise and disturbed air pressure. Since this date many studies about seismic noise have been carried out. We can distinguish three predominant time periods in subsequent studies of ambient
Origin of the seismic noise
Noise is the generic term used to denote ambient vibrations of the ground caused by sources such as tide, water waves striking the coast, turbulent wind, effects of wind on trees or buildings, industrial machinery, cars and trains, or human footsteps, etc. Clearly classifying all noise sources is not an easy task. Gutenberg (1958) established a list of the different types of sources according to their frequency. Asten (1978) and Asten and Henstridge (1984) reached the same conclusions in a
Composition of the seismic noise wavefield
Ideally the goal would be to split up the noise wavefield into body waves (P, SV, SH) and surface waves (Rayleigh and Love waves), and quantify the proportion of each type of waves. The literature survey does prove that achieving that goal is not an easy task, and that many issues are still wide open. This section will thus summarize the relatively few results that have been obtained concerning 1) the relative proportion of surface waves and body waves, 2) the relative proportion of Rayleigh
Conclusions
This review outlines that, while an overall agreement is reached concerning the origin and gross characteristics of seismic noise, this is not the case for the composition of seismic noise wavefield, mainly because of the lack or scarcity of data.
It seems well established now that the spatial and temporal characteristics of seismic noise are closely related with its natural or cultural origin (low frequency microseisms or higher frequency microtremors, respectively). The amplitude variations of
Acknowledgements
This work was supported by the European Union through the research program Energy, Environment and Sustainable Development (EC-contract No: EVG1-CT-2000-00026), the EUROSEISRISK project (contract No: EVG1-CT-2001-00040) and the RDT-SismoDT for the seismic noise measurement in Grenoble. We thank P. Bodin and M. Asten for their helpful comments on this study. Their suggestions have strongly improved the present manuscript.
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