High frequency multi-modal ellipticity from array active experiments

6 months from March, 2022

Context
During the last decade, a number of techniques have been proposed to extract the Rayleigh ellipticity from ambient vibration experiments (Raydec : Hobiger et al. 2009, 3C HRFK : Poggi et al. 2010, WaveDec : Marano et al. 2017a, and RTBF : Wathelet et al. 2018). The 3C array methods are all based on a high-resolution beamforming (Capon, 1969) which requires a full-rank cross-spectral matrix, usually obtained thanks to a block averaging, that is, an average of the cross-spectral matrix over several time windows. Whilst ambient vibration are mostly stationary and available on long records, active experiments are much shorter in time and block averaging is almost impossible. Active sources have often been observed with linear arrays but recently active 2D arrays have also gained interest (Marano et al., 2017b).

Objectives
We are interested in testing the retrieval of the ellipticity from the processing of active 2D arrays. Active 2D arrays, thanks to their broad source distribution, can deliver more realistic uncertainty estimations than linear arrays, used so far since decades. Including the three components may also have a positive impact on the dispersion curve reconstruction. These aspects will be approached with synthetic cases (1D Hisada modelisations) and several experimental datasets recently acquired : on Maca landslide (Peru), on alterites acquifers in the region of Djougou (Benin), site characterization of Argostoli basin (Greece) and ice-quakes recorded on the Argentiere glacier (France). Ambient vibrations and active recordings are available for all of these sites, which makes them ideal candidates for a comparison between passive and active techniques.

References
Hobiger, M., Bard, P.-Y., Cornou, C. & Le Bihan, N., 2009. Single station determination of Rayleigh wave ellipticity by using the random decrement technique (RayDec), Geophys. Res. Lett., 36(14), 0–4.
Maranò, S., Hobiger, M. & Fäh, D., 2017a. Retrieval of Rayleigh wave ellipticity from ambient vibration recordings, Geophys. J. Int., 209(1), 334–352.
Maranò, S., M. Hobiger, P. Bergamo, and D. Fäh, 2017b. Analysis of rayleigh waves with circular wavefront : A maximum likelihood approach, Geophys. J. Int. 210, 1570–1580.
Poggi, V. & Fäh, D., 2010. Estimating Rayleigh wave particle motion from three-component array analysis of ambient vibrations, Geophys. J. Int., 180(1), 251–267.
Wathelet, M., Guillier, B., Cornou, C., Roux, P., & Ohrnberger, M., 2018. Rayleigh wave three-component beamforming : signed ellipticity assessment from high-resolution frequency-wavenumber processing of ambient vibration arrays, Geophysical Journal International, 215(1), 507–523.