Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE

Abstract:

A seismic recording of the non-proliferation experiment (NPE) contains the first break of the regional P phases followed by a three minute long coda. The frequency-domain result of seismic interferometry is studied. This procedure is analogous to the spatial auto-correlation (SPAC) method, devised for studying microtremors by Aki (1957). Cross-correlating two receiver stations retrieves, under favorable circumstances, an approximation of the Green's function between these two stations. To first order, this Green's function consists of a direct event traveling between the receivers. In the frequency-domain, the lowest mode in the Green's function is a weighted and scaled zero-order Bessel function of the first kind, $J_0$. The cross-spectrum from the coda of the NPE is estimated using multitaper spectral analysis. The retrieved Green's functions are fitted to damped $J_0$ functions to recover phase velocity and estimates of the attenuation coefficients. Only energy between 1-4 Hz can be fitted unambiguously with $J_0$ functions, because higher frequencies contain too much spurious energy. This result shows the equivalence of the SPAC method and seismic interferometry for the lowest mode in the Green's function. This study also demonstrates that the coda of a regional event, seemingly unfavorably positioned, can contain energy useful for seismic interferometry.

Seismic interferometry versus spatial auto-correlation method on the regional coda of the NPE