DENSE MEDIUM RADIATIVE-TRANSFER THEORY - COMPARISON WITH EXPERIMENT AND APPLICATION TO MICROWAVE REMOTE-SENSING AND POLARIMETRY

The dense medium radiative transfer theory is applied to study the multiple scattering of electromagnetic waves in a slab containing densely distributed spherical particles overlying a homogeneous half-space. This theory is used to explain phenomena observed in a controlled laboratory experiment. The experimental data indicate that, in a dense medium with small particles, both the coherent attenuation rate and bistatic intensities first increase with the volume fraction of the particles until a maximum is reached, and then decrease when the volume fraction further increases. Thus attenuation rates and bistatic scattering exhibit a peak as a function of the concentration of particles. The magnitudes of both are also less than those predicted by the independent scattering assumption and the conventional radiative transfer theory. These phenomena cannot be explained by the conventional radiative transfer theory. It is shown that the dense medium radiative transfer theory is in agreement with these experimental features. The dense medium radiative transfer equations are derived from the Dyson equation under the quasi-crystalline approximation with coherent potential (QCA-CP) and the Bethe-Salpeter equation under the ladder approximation of correlated scatterers. The extinction rate, albedo, and phase matrix are related to the physical parameters of the medium. The dense medium radiative transfer equations are solved numerically by using Fourier series expansion and discrete eigen-anal-ysis approaches. The numerical results account for all multiple scattering mechanisms included in the dense medium radiative transfer theory. The numerical results are also applied to interpret the backscattering measurements of active microwave remote sensing of snow. In this paper, the polarimetric properties of dense media are also studied. The co-polarization signatures and degree of polarization are calculated as a function of the orientation and ellipticity angles of the polarization of the incident wave. This study demonstrates that the multiple scattering effect can significantly decrease the degree of polarization so that the returned signal is no longer completely polarized. Multiple scattering can also cause a pedestal in the co-polarization signature which has been observed in recent radar polarimetry. © 1990 IEEE