Green's Function Retrieval from the Cross Correlation Function of Random Waves
Ambient noise, microseisms and coda waves look random; however, they are a superposition of waves generated at various locations at the surface or within the earth and scattered by medium heterogeneities. Aki (1957) proposed to measure the phase velocity of surface waves from the cross correlation function (CCF) of ambient noise; however, his method was limited to a short separation of receivers compared with the predominant wavelength. Recently, it was found that ambient noise show a good correlation between distant receivers if the time window length is sufficiently long enough. Analyzing CCFs of coda waves of local earthquakes at different station pairs in Mexico, Campillo and Paul (2003) showed a similarity between the CCF and synthesized Rayleigh waves. It was found that the peak lag time of the CCF is a good measure of the travel time of seismic waves between seismic stations.