Visual Inspections Get a Helping Hand…

Episode 4 of IAM News explored the possibility of SAR data analysis to assist in-situ monitoring operations. But what exactly do InSAR analysts do, and how does this work assist those monitoring the same assets from the ground?

The work of InSAR analysts involve several steps when handling data, including data preparation, the generation of interferograms, phase unwrapping, and interpretation. Before any analysis can take place, data must be acquired. InSAR analysts obtain this data from satellite missions that provide interferometric capabilities such as Sentinel-1. These satellites capture multiple SAR images of the same area over different time periods.

The acquired data then undergoes preprocessing to remove noise and correct geometric distortions. Steps involved in this process could include radiometric calibration, speckle filtering, and orthorectification for subsequent analysis. Once this process is complete, the generation of interferograms is possible. In order to process interferograms, the analyst must select pairs of SAR images acquired at different times for processing. That’s why sometimes InSAR is also referred to as DInSAR (Differential InSAR). By calculating the phase difference between two SAR images, pixel by pixel, the displacement of the Earth’s surface between two dates can be calculated.

Interferograms contain “wrapped” phase values, meaning that the phase measurements are limited to a range of -π to +π. Phase unwrapping is performed to remove these phase ambiguities and obtain the actual displacement values. This involves determining the correct number of 2π  cycles to add or subtract them from each pixel to achieve a continuous unwrapped phase map. These unwrapped values are converted into elevation information. For this purpose, the analysts combine the perpendicular baseline distance between satellites) and the satellite orbit information, to calculate the topographical component of the phase. What emerges from this process is a Digital Elevation Model (DEM) that represents the height of a surface.

From a detailed study of the interferograms and derived DEM, analysts can identify and quantify surface deformations. This includes detecting and measuring deformations such as subsidence, uplift, and lateral movements in infrastructure or natural features. InSAR analysts often assess a series of interferograms acquired over time to track deformation trends. This allows them to examine the rate and temporal evolution of deformities, providing insight into the stability and health of the structure.

Finally, these findings are compiled into reports and maps, presenting the detected deformities, their magnitude, and their potential impact on infrastructure. Analysts communicate their results to stakeholders, such as infrastructure managers, engineers, or decision makers who can take appropriate actions for maintenance or mitigation.

InSAR analysis provides a wide area coverage of infrastructure networks, allowing for the monitoring of large regions and extensive networks simultaneously. This work complements localized and targeted measurements performed by in-situ monitoring operatives. InSAR analysis enables early detection of infrastructure defects on a regional scale. InSAR can identify areas of potential concern, guiding in-situ monitoring efforts to specific locations that require detailed inspections of further investigations. Moreover, in-situ monitoring data can be used to validate and calibrate the results obtained through InSAR analysis. By comparing the measurements acquired on-site with the remote sensing data, any discrepancies can be addressed. Finally, in-situ monitoring and InSAR data can be integrated to create a holistic picture of the infrastructure’s condition. By combining the strengths of both approaches, a more complete understanding of defects, deformations, and risk can be achieved. The integration of multiple data sources enhances the accuracy, reliability, and effectiveness of infrastructure monitoring and management.