Modelling the impact of moisture changes in a heterogeneous soil on differential interferometry

Changes in soil moisture between the two radar acquisitions can impact the observed coherence 7 in differential interferometry: both correlation |γ| and phase φ are affected. The influence on the latter potentially biases the estimation of deformations. These effects have been found to be variable in magnitude and dependent on polarization, as opposed to predictions by existing models. Such diversity can be explained when the soil is modelled as a half-space with spatially varying dielectric properties and a rough interface. The first-order perturbative solution achieves - upon calibration with L band data - median correlations ρ at HH of 0.77 for the phase Φ and of 0.50 for |γ|. The depth distribution of the scattering heterogeneities within the soil impacts the sensitvity of the observables to soil moisture changes in a way similar to a changing relative importance of the surface and the volume components, thus leading to similar qualities of fit when the latter is estimated. The first-order expansion does not predict any impact on the HV coherence, which is however empirically found to display similar sensitivities to soil moisture as the co-pol channels. These results indicate that the first-order solution, while not able to reproduce all observed phenomena, can capture some of the more salient patterns of the effect of soil moisture changes on the HH and VV DInSAR signals.