Effect of phase filtering on interferometry based displacement analysis of cultural heritage sites

Abstract

Synthetic Aperture RADAR Remote Sensing technique has widely used in mapping and monitoring of cultural and natural heritage sites. In today’s scenario, SAR Interferometry (InSAR) is being used to measure the elevation and displacement of manmade and natural features. This study was focused on phase filtering to detect displacement in cultural heritage site using space borne repeat pass SAR Interferometry. Rohtasgarh fort, was chosen as study site and Sentinel -1, C-Band interferometric pairs of March and April 2017 were used. Interferometric processing for interferogram generation, coherence and phase unwrapping was performed to generate a displacement map. It was however noticed that phase noise greatly affects and deteriorates the quality of interferogram which in turn affects the phase unwrapping and geophysical measurements such as displacement mapping. In this work, Modified Goldstein filter was used for phase noise suppression and preserving the fringe edges. Using a threshold of 0.3 for coherence, displacement map was generated, and Modified Goldstein Filtering was applied on it. Most of the areas in and around the fort showed no displacement but there was an upliftment seen at few places inside the fort with maximum value of 10mm. Subsidence with maximum value of 10mm was also seen in areas outside the fort. Upliftment could have been due to water accumulation inside the fort or dust storm while subsidence could be possible due to lose debris washed off by rain water.