Microtremors are produced by multiple random sources, typically close to the surface of the Earth. They include the effects of multiple scattering and their intensities, which depend upon the strength of random sources, are governed by diffusive-like equations. The involved energy in seismic noise may be relatively small but under appropriate circumstances, it is possible to use seismic noise to assess site structure. It has been established that within an elastic, inhomogeneous, and anisotropic medium subjected an uncorrelated set of random forces (equivalently, one can say that the seismic illumination is equipartitioned), the average cross correlation of the motions at given receivers, in the frequency domain, is proportional to the imaginary part of Green function. For a single receiver, average autocorrelation measures energy density, which is proportional to the imaginary part of Green function at the source. Assuming the seismic field is diffuse and such that equipartition holds also for the energy components, we compute the H/V ratio in terms of the imaginary part the Green function tensor components at the surface. Broad band noise records at Texcoco, a soft soil site near Mexico City, are studied and they are interpreted using the theoretical results herein presented for a horizontally layered medium overlaying a half space.Therefore, our approach naturally allows for the inversion of H/V, the well known Nakamura's ratio without using Rayleigh waves'ellipticity.