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Importance of Using 3-component F-K Methods for Processing Ambient Vibration Array (AVA) Measurements for Improved Site Characterization

Description: 

The characterization of sites hosting seismological stations is important for the optimal use of ground motion databases. Over the past decade, the use of non-invasive methods based on the recording and analysis of surface wave dispersion properties has imposed itself as an efficient, quick to implement and inexpensive approach for developing shear wave velocity (Vs) profiles. Several methods can be cited. One example is the MASW (Mutichannel Analysis of Surface Waves) method, which is an active-source method involving the use of a seismic source (often a sledgehammer) and several geophones (typically 24 to 48) placed on a line. This method is effective for characterizing the part of the soil column closest to the surface (typically the top 15 – 20 m). However, passive-source methods involving the use of sensors arranged in 2D arrays and recording ambient vibrations (e.g., 'Ambient Vibration Array' methods: AVA or 'Microtremor Array Methods': MAM) are necessary in order to estimate the properties of the subsurface at greater depth, which is often required if seismological bedrock is to be reached.

AVA data is often processed using the SPAC (Spatial Autocorrelation) method. The use of frequency-wavenumber (FK) methods (e.g., beamforming) is less widespread, although they do have certain advantages related to resolving the frequency-dependent direction of ambient noise propagation, among others. Recent developments in 3-component FK methods, such as the Rayleigh Three-component Beamforming (RTBF) method developed by Wathelet et al. (2018) and Wathelet (2024), present a real qualitative step forward in accurately resolving experimental dispersion data and a better separation of the different modes. We present characterization results obtained by 3-component FK analysis on complex sites. Examples of very large aperture networks (up to 10 km) will be discussed, enabling Vs profiles to be obtained at depths of up to several kilometers. We will also show how these methods can be used to highlight velocity anisotropies, and provide some recommendations on array geometry and minimum acquisition durations.

Session: Station Installations and Site Conditions, a Quest for Improved Strong Motion Database - I

Type: Oral

Date: 4/16/2025

Presentation Time: 02:45 PM (local time)

Presenting Author: Fabrice

Student Presenter: No

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