Depth-Resolved Backscatter and Differential Interferometric Radar Imaging of Soil Moisture Profiles: Observations and Models of Subsurface Volume Scattering

Microwave observations of soils at L-to X-band are commonly interpreted in terms of surface scattering. However, recent observations of the backscatter and the differential interferometric response indicate that subsurface volume scattering may often be more important than generally thought. The dependence of the subsurface backscatter and interferometric return on soil moisture remains poorly understood as most observations are indirect in that they are confounded with the surface contribution. Here, we aim to elucidate the impact of soil moisture on subsurface volume scattering by employing depth-resolved observations at C-band with a depth resolution of 5-10 cm, thus isolating the subsurface contribution. For the sandy soil analyzed in the experiment, the volumetric subsurface backscattered power is strongly affected by soil moisture (>15 dB): It appears to be governed by the local soil moisture content (local dielectric constant and its spatial variability) and the soil moisture content above (absorption). When the soil moisture changes, the observed interferometric response is consistent with the notion that the subsurface return is mainly governed by the changing optical path through the soil. By modeling and inverting this relation of the interferometric phase to soil moisture, we derive the first spatially continuous estimates of soil moisture profiles based on the clutter. These findings indicate the potential of depth-resolved observations to provide direct, spatially extensive information about the vertical variability of soil moisture, and to inform models of the subsurface response.