Abstract
Interferometric synthetic aperture radar (InSAR) technology has been widely applied to measure Earth surface motions related to natural and anthropogenic crustal deformation phenomena. With the widespread uptake of data captured by the European Space Agency’s Sentinel-1 mission and other recently launched or planned space-borne SAR missions, the usage of the InSAR technique to detect and monitor Earth surface displacements will increase even more in the coming years. However, InSAR can only measure a one-dimensional motion along the radar line of sight (LOS), which makes interpretation and communication of InSAR measurements challenging, and can add ambiguity to the modelling process. Within this paper, we investigate the implications of the InSAR LOS geometry using simulated and observed deformation phenomena and describe a methodology for multi-geometry data fusion of LOS InSAR measurements from many viewing geometries. We find that projecting LOS measurements to the vertical direction using the incidence angle of the satellite sensor (and implicitly assuming no horizontal motions are present) may result in large errors depending on the magnitude of horizontal motion and on the steepness of the incidence angle. We quantify these errors as the maximum expected error from simulated LOS observations based on a Mogi deformation model. However, we recommend to use LOS observations from several image geometries wherever data are available, in order to solve for vertical and E–W oriented horizontal motion. For an anthropogenic deformation phenomenon observed in seven independent InSAR analyses of Envisat SAR data from the Sydney region, Australia, we find that the strong horizontal motion present could lead to misinterpretation of the actual motion direction when projecting LOS measurements to vertical (uplift instead of subsidence). In this example, the difference between multi-geometry data fusion and vertical projection of LOS measurements (at an incidence angle of 33.8°) reach up to 67% of the maximum vertical displacement rate. Furthermore, the position of maximum vertical motion is displaced horizontally by several hundred metres when the LOS measurements are projected.
Highlights
In the past three decades, Interferometric synthetic aperture radar (InSAR) has proven to be a valuable geodetic technique to detect and quantify motion of the Earth’s surface
We present the results of multi-geometry data fusion of InSAR-derived velocity observations from Envisat SAR data
Our investigations are based on velocities since these are mostly used to describe slow motion events measured with InSAR, but all results and conclusions are applicable to the case of displacement data
Summary
In the past three decades, InSAR has proven to be a valuable geodetic technique to detect and quantify motion of the Earth’s surface (either defined as a displacement or a displacement rate, aka velocity). Surface motions (resulting from subsurface deformation processes) generally occur in the three spatial dimensions (i.e., east (E), north (N) and up (U)) This means that the InSAR analysis of a stack of synthetic aperture radar (SAR) images acquired in a single viewing geometry is not able to fully capture the magnitude and direction of surface motions In many scientific and commercial studies LOS measurements are converted into the vertical direction by projecting the data using the sensor incidence angle This procedure neglects the horizontal components of motion that would be mapped into the LOS. To the authors’ knowledge, the error made when projecting LOS to vertical and neglecting any horizontal motion has not yet been quantified with observations
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