A càrrec de Martin Schimmel (ICTJA-CSIC, Barcelona)
Organitzat conjuntament CSIC-UB

Data: 11-05-2011
Hora: 12:00
Lloc: Sala d'actes de l'Institut Jaume Almera

Descripció: Ambient seismic noise is observed everywhere on Earth. It is mostly generated in the oceans or at the coasts by harmonic forcing through wind-driven ocean swell. This noise (and other background noise) can be "turned into signals" for seismic imaging and monitoring as shown by a large number of recent noise studies. Advantages of using seismic noise are that no active sources or earthquakes are required and that the experiments are repeatable.

Stacks of seismic ambient noise cross-correlations are more and more routinely used to extract empirical (mostly surface wave) Green's functions between station pairs. The success of the cross-correlations is due to waves which are recorded on both stations and which constructively sum at lag times which equal their propagation time between the station pair. Stacking cross-correlations for different time spans improves the azimuthal noise coverage and further enhances the signals.

In our presentation we summarize the basic concepts related to the extraction of empirical Green's functions from seismic noise. We discuss the classical processing flow and show how the instantaneous phase coherency can be used for a more efficient signal extraction from ambient noise cross-correlations. The instantaneous phase coherence is obtained by analytic signal processing and can be employed through the phase cross-correlation and/or through the time-frequency domain phase-weighted stack.

The phase cross-correlation is more sensitive to waveform similarity but less sensitive to strong amplitude features than the conventional cross-correlation. The time-frequency domain phase-weighted stack cleans the ambient noise cross-correlations by attenuating incoherent noise and permits an improved signal identification. We show that both approaches are powerful tools in the recovery of signals from ambient noise data and show examples where they improve the extraction of P and Rayleigh waves considering regional and global scale applications.