Isolating and Suppressing the Spurious Non-Diffuse Contributions to
Ambient Seismic Field Correlations
Abstract
Cross-correlation of fully diffuse wavefields averaged over time should
converge to the Green’s function; however, the ambient seismic field in
the real Earth is not fully diffuse, which interferes with that
convergence. We apply blind signal separation to reduce the effect of
spurious non-diffuse components on the cross-correlation tensor of the
ambient seismic field. We describe the diffuse component as having
uncorrelated neighboring frequencies and equal intensity at all
azimuths, and an independent (i.e., statistically uncorrelated)
non-diffuse component arising from a spatially isolated point source for
which neighboring frequencies are correlated. Under the assumption of
linear independence of the spurious non-diffuse wave outside the
stationary phase zone and the constructive interference of noise waves
within that zone, we can suppress the spurious non-diffuse component
from the noise interferometry. Our numerical simulations show good
separation of one spurious non-diffuse noise source component for either
non-diffuse Rayleigh or Love waves. We apply this separation to the
Rayleigh-wave component of the Green’s function for 136
cross-correlation pairs from 17 stations in Southern California. We
perform beamforming over different frequency bands for the
cross-correlations before and after the separation, and find that the
reconstructed Rayleigh waves are more coherent. We also estimate the
bias in Rayleigh wave phase velocity for each receiver pair due to the
spurious non-diffuse contribution.