An approach for retrieving complete three-dimensional ground displacement components from two parallel-track InSAR measurements

In the investigation and evaluation of geological hazards such as those caused by co-seismic displacement, displacement due to underground mining, landslides, volcanic eruptions, it is important to retrieve the complete three-dimensional ground displacement components. However, both conventional D-InSAR (differential synthetic aperture radar interferometry) and MT-InSAR (multi-temporal InSAR) can only measure the displacement component along the radar LOS (line of sight) direction, which greatly limits the application of InSAR technique. Over the past decades, great endeavors have been made to overcome its limitation of the one-dimensional LOS measurements, and several methods have been proposed to retrieve complete three-dimensional displacement components from InSAR measurements. On the whole, the commonality of the existing methods is characterized by using both ascending and descending SAR data. In this paper, we put forward an approach for retrieving complete three-dimensional ground displacement components from two parallel-track InSAR measurements and present the analytical expressions for the vertical, the North-South, the East-West displacement components. It reveals that the vertical displacement component is just a function of the two parallel-track InSAR LOS displacements, i.e., the vertical displacement component can be retrieved from the two parallel-track InSAR LOS displacements, while the North-South or the East-West displacement component is a function of both the two parallel-track InSAR LOS displacements and the azimuthal displacement. What is more, the proposed approach can effectively improve the accuracy of the azimuthal measurement by using a redundant measurement. As a result, the accuracies of the derived North-South and East-West displacement components are improved as well, which can provide more reliable results for characterizing ground movement associated with geological hazards. In terms of their geographic coverage, there is a strong complementary relationship between the proposed approach and the existing methods. Thus, they can be integratedly utilized in the practical applications. Finally, the approach is illustrated through the retrieval of the complete three-dimensional co-seismic displacement components in an area to the west of Bam, Iran, where a strong earthquake occurred in 2003.