The unwanted amplification of monochromatic signals in seismic noise cross-correlation functions by spectral whitening

The estimation of the Green’s function between two points on the Earth’s surface by the cross-correlation of seismic noise time series requires, in general, very long time series (months to years) as well as massive normalisation. Spectral whitening is a widely used powerful normalisation to improve the emergence of broad-band signals in seismic noise cross-correlations. Nevertheless, we observe spectral whitening to depend strongly on the time window length necessarily used to fragment very long time series. An unwanted amplification of a persistent microseism signal is observed on the continental scale with time windows shorter than 12 hours.