Laboratory Measurement of the DInSAR Response to Spatiotemporal Variations in Soil Moisture

Differential interferometric synthetic aperture radar (DInSAR) has traditionally been used for the detection and accurate monitoring of surface movement in a scene and has found applications in fields such as mining subsidence and earthquake deformation. In these studies, the phase is understood to directly relate to the radial component of the physical deformation of the surface. In this paper, however, we use a novel combination of microwave and optical laboratory measurements to demonstrate the presence of persistent and coherent phase changes in a temporal sequence of DInSAR images, related solely to moisture change in a soil. This is confirmation of recent reports suggesting that, in some circumstances, the DInSAR signal may include a significant soil moisture signal. Laboratory measurements were used to obtain a set of high-resolution C-band DInSAR images of a sandy soil sample of an area of 2.0 m x 1.8 m and a depth of 0.2 m, with the fractional volumetric water content varying between 0.1 and 0.4. To independently monitor the soil surface for physical movement, a time-lapsed set of high-resolution digital optical images was continuously acquired. Although the soil underwent a large moisture change, the soil surface was static to within +/-0.1 mm over the majority of the experiment. The DInSAR sequence displayed dynamic and complex variations of the phase, although a linear relationship with moisture change was evident when the mean phase change was considered. The work raises the possibility that DInSAR could be used for the monitoring of soil moisture change in a scene, a parameter of significant economic and environmental importance.