Interferometric synthetic aperture radar InSAR

This Envisat Advanced interferometric synthetic aperture radar (InSAR) interferogram over the Kenyan section of the Great Rift Valley shows small surface displacements that are not visible to the naked eye of the Longonot volcano (front right). In the background is Suswa volcano, which was not deforming at this time. Using InSAR, a group of scientists discovered that from 1997 to 2000 the volcanoes at Suswa and Menengai (not visible) subsided 2cm to 5cm, and between 2004 and 2006 the Longonot volcano experienced uplift of around 9cm (pictured). Interferogram images appear as rainbow-coloured interference patterns. A complete set of coloured bands, called 'fringes', represents ground movement relative to the spacecraft of half a wavelength, which is 2.8cm in the case of Envisat's InSAR.

[edit] Introduction

Interferometric synthetic aperture radar (abbreviated as InSAR or IfSAR) is a technique that uses satellite or aircraft radar signals from multiple sources to capture complex and precise measurements.

[edit] Where it can be used

InSAR is a highly effective way to measure changes in land surface altitude. The technique uses two or more synthetic aperture radar (SAR) images and ‘interferes’ them to interpret differences and produce interferograms. This is an accurate way of detecting ground movements and can be scaled from single pixels (typically 3m x 3m or 4m x 14m) to entire countries.

Radar waves can be transmitted through most weather conditions and do not require daylight.

[edit] Geophysical disaster detection

The deployment of InSAR was first investigated in the 1980s, with additional exploration in the 1990s. One of the earliest practical applications of InSAR came in 1992, when it was used to measure the after effects of an earthquake in California.

Since those early applications, InSAR has been used to record data associated with landslides, volcanoes and other natural occurrences. In these instances, geotechnical engineers use InSAR as a safe, remote sensing technique for assessing and monitoring movement.

[edit] Subsidence

InSAR has proven effective at recording millimetre-scale shifts of land (such as subsidence). Subsidence can be tracked using historic satellite radar data covering previous decades and then monitored throughout the duration of a project and beyond.

InSAR has also been useful in recording the consequences of subsurface mining and natural resource extraction (both current and historic).

[edit] Structural instability

InSAR can also be used to record the stability of infrastructure (including highways, rail, dams and so on) and to monitor buildings. Movements can occur for a multitude of reasons - inadequate design or poor construction of foundations - but it can be difficult to capture the historic data.

NB Earth observation and aerial surveys, RICS professional standard, 6th edition, September 2021, published on 4 January 2022 by the Royal Institution of Chartered Surveyors (RICS), defines interferometric synthetic aperture radar (InSAR) as: ‘The measurement of the differences in the phases of the waves between two SAR images acquired over the same area at different times.’

[edit] Related articles on Designing Buildings

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• Geophysical survey.
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