Soil moisture effects on temporal subsidence estimates using InSAR

The reflectance of geological materials to radar waves is used to estimate earth structure in Interferometric Synthetic Aperture Radar (InSAR) and Ground Penetrating Radar. Repeat use over a time window has great potential use in appraising the dynamic behaviour of geological, geotechnical or mining-related structures, such as subsidence profiles. Temporal changes in structure can be masked by temporal changes in physical properties due to changes in vegetative cover or in soil moisture of the reflecting surface overlying the structure of concern. The effect on reflectance of radar waves by seasonal soil moisture variations is demonstrated by numerical modelling using parametric models of soil conductivity and by dielectric/analytic modelling of the plane wave response of a half-space. More elaborate modelling using layered earth or periodic earth is also possible. This study demonstrates that the use of InSAR for subsidence studies is proscribed if soil moisture varies markedly between passes unless the variation in soil moisture can be removed. One technique would involve the use of inverse modelling to estimate the temporal variation of moisture and to correct for it. Inverse mixing bound theory takes an effective dielectric permittivity and uses it to estimate bounds on soil moistures. Testing this on field data represents a possible next step. Another technique would use artificial reflectors to baseline the reflectance of a specified area.