Simulation par la méthode des éléments spectraux des formes d’onde obtenues par corrélation de bruit sismique

Paul Cupillard. ( 2008 )
Université Paris 7 - Institut de Physique du Globe de Paris

Abstract

The waveform we can obtain by correlating ambient seismic noise recorded at two different stations gives an interesting signature of the media between these stations. This provides a new type of data that can be used to investigate the Earth's structure, as the earthquakes records do. Only the phase information has been considered from noise correlations for now and we would like to know in this work whether the amplitude can also be used or not. Ultimately, our aim is to process a spectral element simulation of the entire waveforms. In a first time, we study the features of correlations generated with synthetic seismic noise. Both uniform and non-uniform sources distributions are investigated and the effect of attenuation is carefully detailed for raw, 1-bit and whitened noise. In a second time, we compute a spectral element simulation of these synthetic correlations. The main difficulty is to take into account the amplitudes of the anisotropic noise flux that goes across the stations network. To do so, we use the time-reversal technique and we create a extended source that is positioned at a station and then propagated with the SEM in a certain Earth's model to retrieve waveforms corresponding to correlations between this station and the other receivers of the network. The result is demonstrated numerically as well as theoretically using the representation theorem. Moreover, the spectral element code we use is presented, with an accurate validation and two examples of wavefields generated by earthquakes in a 3D model of Europe. To end up, we apply our time-reversal process to correlations from real seismic noise. We discuss problems due to space-time variations of the noise sources and we show very encouraging results.

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BibTeX Reference

@PHDTHESIS{,
    author = { Cupillard, Paul },
     title = { Simulation par la méthode des éléments spectraux des formes d’onde obtenues par corrélation de bruit sismique },
      year = { 2008 },
    school = { Université Paris 7 - Institut de Physique du Globe de Paris },
  abstract = { The waveform we can obtain by correlating ambient seismic noise recorded at two different stations gives an interesting signature of the media between these stations. This provides a new type of data that can be used to investigate the Earth's structure, as the earthquakes records do. Only the phase information has been considered from noise correlations for now and we would like to know in this work whether the amplitude can also be used or not. Ultimately, our aim is to process a spectral element simulation of the entire waveforms.

In a first time, we study the features of correlations generated with synthetic seismic noise. Both uniform and non-uniform sources distributions are investigated and the effect of attenuation is carefully detailed for raw, 1-bit
and whitened noise. In a second time, we compute a spectral element simulation of these synthetic correlations. The main difficulty is to take into account the amplitudes of the anisotropic noise flux that goes across the stations network. To do so, we use the time-reversal technique and we create a extended source that is positioned at a station and then propagated with the SEM in a certain Earth's model to retrieve waveforms corresponding to correlations between this station and the other receivers of the network. The result is demonstrated numerically 
as well as theoretically using the representation theorem. Moreover, the spectral element code we use is presented, with an accurate validation and two examples of wavefields generated by earthquakes in a 3D model of Europe. To end up, we apply our time-reversal process to correlations from real seismic noise. We discuss problems due to space-time variations of the noise sources and we show very encouraging results. }
}