Retrieval of Green's function in the radiative transfer regime

The field-field correlation function of an imperfectly diffuse acoustic field is shown to equal the (time derivative of) Green's function times the specific intensity of the noise at the position of the pseudo-source directed toward the pseudo-receiver. The identity is established in a high frequency limit in which stations are separated by distances large compared to a wavelength and in which equal-time correlations vary smoothly in space. The specific intensity is governed by a radiative transport equation. This observation permits interpretation of correlation amplitudes and promises to facilitate the retrieval of attenuation, site amplification factors, and scattering strengths from noise correlations.