A B S T R A C TWe have cross-checked the conventional theory of the spatial autocorrelation method and the consequence of seismic interferometry: the retrieval of the elastodynamic Green's function. Their mutual consistency is almost complete. The basic formulas of the conventional spatial autocorrelation theory can be derived by an alternative approach based on the retrieval of the elastodynamic Green's function. The only discrepancy is found with the average of the complex coherence function over azimuth in a wavefield dependent on azimuth. It is hypothesized, in discussion, that this discrepancy is due to the way of representing the wavefield in the background theory of seismic interferometry that can produce only wavefields moderately dependent on azimuth and that the mentioned consequence of seismic interferometry can also only make sense in a wavefield moderately dependent on azimuth.Our field experiment with a wavefield dependent on azimuth showed that the consequence of seismic interferometry in the logical framework of the conventional spatial autocorrelation theory is appropriate under such degrees of approximation as the measure proposed in this study, i.e., the deviation of the total dispersion curves is between about 10 and 16 per cent at the maximum from those averaged over azimuth.The acceptance of the retrieval of Green's function gives a proper physical meaning to the complex coherence function: the real part of the elastodynamic Green's function normalized by its zero-offset version. This makes it possible to take a deterministic approach rather than the statistical one on which the conventional spatial autocorrelation method is based and gives fruitful new aspects and perspectives. For example, the formula for the multi-mode case is given and the possibility of exploration of two or three dimensional velocity structures is suggested. I N T R O D U C T I O NExploration of the shear-wave velocity at the shallowest part of sediment is a key issue of engineering seismology and earthquake engineering. As these low-velocity layers amplify the seismic waves considerably and vary site by site abruptly, it is necessary to explore the underground velocity structure at each site. A commercial market was then formed for geotechnical geophysics directly related to civil engineering. This laterally varying amplification takes place not only for seismic * E-mail: tyokoi@kenken.go.jp waves due to tectonic earthquakes but also for the elastic waves produced by a surface shot and reflected by the velocity discontinuities that can be significant for production. If one wishes to extract useful information from the lateral variation of the reflected shear waves' amplitude, the effects of the shallowest sedimentary layers have to be taken into account, namely, they have to be quantitatively estimated and corrected. Among the various useful methods to estimate the underground shear-wave velocity structure, the usage of microtremor (ambient noise) is preferred by engineering seismologists because it does not require a...

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