1 Background Ambient seismic noise cross-correlation has far-reaching implications for the passive seismic surface wave imaging from regional to continental scales (e.g. Campillo & Paul, Shapiro & Campillo, 2004). Spectral analysis of cross-correlations is the key to retrieving reliable dispersion information about surface waves. Based on the Fourier spectra of event seismic surface wave data, however, we need to shift the cross-correlation time function before doing FFT to obtain the right spectra.