Explanation of InSAR Phase Disturbances by Seasonal Characteristics of Soil and Vegetation

Rogier Westerhoff,Moira Steyn-Ross

doi:10.3390/rs12183029

Abstract

Seasonal phase disturbances in satellite Interferometric Synthetic Aperture Radar (InSAR) measurements have been reported in other studies to suggest sub-centimetre land surface terrain motion. These have been interpreted in various ways because they correlate with multiple other (sub-)seasonal signatures of, e.g., clay swelling/shrinkage and groundwater level. Recent microwave radar studies mention the occurrence of phase disturbances in different soil types and soil moisture. This study further explored this topic by modeling phase disturbances caused by both soil and vegetation surface characteristics and aimed to interpret what their possible effects on InSAR-interpreted terrain motion is. Our models, based on fundamental microwave reflection and transmission theory, found phase disturbances caused by seasonal variation of soil and vegetation that have the same magnitude as interpreted seasonal land movement in earlier InSAR studies. We showed that small, temporal differences in soil moisture and vegetation can lead to relatively large phase disturbances in InSAR measurements. These disturbances are a result of waves having to comply with boundary conditions at the interface between media with different dielectric properties. The findings of this study explain the seasonal variations found in other InSAR studies and will therefore bring new insights and alternative explanations to help improve interpretation of InSAR-derived seasonal terrain motion.

Highlights

The Interferometric Synthetic Aperture Radar (InSAR) technique has become widely applied for measuring land terrain motion caused by earthquakes [1,2]; volcanism [3,4]; landslides [5,6]; or excessive pumping or abstraction of gas, oil, or water resources [7,8,9,10,11]
Our research used simplifying assumptions because basic microwave theory is already deemed enough to prove the point of this study, i.e., that the phase disturbance effect of slightly varying dielectric properties of soil, roots, and vegetation on the InSAR signal could be misinterpeted as actual ground deformation and that we hope that this study gives more physics-based explanations to help improve future InSAR land motion interpretation
This study has further explored phase disturbances mentioned by De Zan et al [26] but with inclusion of both soil moisture as well as vegetation

Summary

Introduction

The Interferometric Synthetic Aperture Radar (InSAR) technique has become widely applied for measuring land terrain motion caused by earthquakes [1,2]; volcanism [3,4]; landslides [5,6]; or excessive pumping or abstraction of gas, oil, or water resources [7,8,9,10,11]. By repeatedly measuring the phase and amplitude of the electromagnetic (microwave) radar backscatter, observed interferometric phase differences lead to information on whether the target has moved in the line of sight (LOS). This LOS displacement is interpreted as terrain motion. Atmospheric noise in InSAR data is well documented: statistics, physics-based analyses, and ground observations show that the effect of atmospheric circumstances on microwave radar propagation can cause sub-centimetre uncertainties in estimates of average terrain motion [15,16,17,18]

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Discussion

Conclusion