HR: 09:40h
AN: S51D-07 INVITED    [Abstracts]
TI: Using coda waves extracted from microseisms to construct direct arrivals.
AU: * Campillo, M
EM: campillo@obs.ujf-grenoble.fr
AF: LGIT Grenoble, BP 53, Grenoble, 38041, France
AU: Stehly, L
EM: lstehly@obs.ujf-grenoble.fr
AF: LGIT Grenoble, BP 53, Grenoble, 38041, France
AU: Stehly, L
EM: lstehly@obs.ujf-grenoble.fr
AF: CEA/DASE, BP 12, Bruyeres-le-Châtel, 91680, France
AB: The analysis of long range correlations of microseisms has been shown to provide reliable measurements of wave speeds that can eventually be used for seismic imaging.In the case of an even distribution of noise sources, it has been theoretically demonstrated that the correlation is the exact Green function, including all types of waves. In practice this method islimited in its application by the actual source distribution. We present the example of a set of stations in Europe, for which the predominant flux of energy from the northern Atlantic results in a clear azimuthal dependence of the quality of the reconstruction of Rayleigh waves, with a very poor reconstruction in some azimuths. To solve this problem, we use the noise correlations measured on the entire network. Let us consider two stations A and B for which the Rayleigh waves are not efficiently reconstructed by correlation of continuous records. We compute all the correlations between the station A (resp.B) and all the 150 other stations located at regional distances. Theoretically, these correlations must contain direct waves and coda with various signal to noise ratios. We then use these signals as equivalents to seismograms produced by sources acting at station locations and recorded in A and B. We select the time windows corresponding to coda waves and compute the average correlation between virtual seismograms obtained in A and B. As for actual earthquake data (Campillo and Paul, 2003), this correlation function contains the surface wave part of the Green function. We illustrate the efficiency of the reconstruction by comparison with direct measurements. This procedure can be used to perform velocity measurement between stations, even in presence of a very directive and poorly oriented noise. It shows that, in spite of a currently small signal to noise ratio, the cross correlations of microseisms contain more than direct surface waves. Arrivals with complex travel paths as coda waves are also present.
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting