Interferometric Synthetic Aperture Radar (InSAR)

Science Center Objects

Interferometric Synthetic Aperture Radar (InSAR) is an effective way to measure changes in land surface altitude. InSAR makes high-density measurements over large areas by using radar signals from Earth-orbiting satellites to measure changes in land-surface altitude at high degrees of measurement resolution and spatial detail (Galloway and others, 2000).

Synthetic Aperture Radar (SAR) imagery is produced by reflecting radar signals off a target area and measuring the two-way travel time back to the satellite. The SAR interferometry technique uses two SAR images of the same area acquired at different times and "interferes" (differences) them, resulting in maps called interferograms that show ground-surface displacement (range change) between the two time periods.

Envisat - a satellite used in InSAR studies

The European Space Agency's (ESA) ENVISAT satellite was used to map and measure displacement. The satellite is side-looking, orbits the Earth at an altitude of approximately 500 miles (800 kilometers), and has 35-day repeat cycles. (Illustration by the European Space Agency.)

Advantages of InSAR

InSAR is ideally suited to measure the spatial extent and magnitude of surface deformation associated with fluid extraction and natural hazards (earthquakes, volcanoes, landslides). It is often less expensive than obtaining sparse point measurements from labor-intensive spirit-leveling and global positioning system (GPS) surveys, and can provide millions of data points in a region about 10,000 square kilometers. By identifying specific areas of deformation within broader regions of interest, InSAR imagery can also be used to better position specialized instrumentation (such as extensometers, GPS networks, and leveling lines) designed to precisely measure and monitor surface deformation over limited areas.

Satellites

Satellites are an integral part of InSAR. In March 2002, the European Space Agency (ESA) launched Envisat, an advanced polar-orbiting Earth observation satellite which provides measurements of the atmosphere, ocean, land, and ice. The Envisat satellite has an ambitious and innovative payload that will ensure the continuity of the data measurements of its predecessor, the ESA European Remote Sensing (ERS) satellites. Envisat data supports earth science research and allows monitoring of the evolution of environmental and climatic changes. Furthermore, the data will facilitate the development of operational and commercial applications. Description and illustration courtesy of the European Space Agency.

Interferograms

Interferograms are maps of relative ground-surface change that are constructed from InSAR data to help scientists understand how tectonic or human activities, such as groundwater pumping and hydrocarbon production, cause the land surface to uplift or subside. Interferograms require 2 images taken at intervals in time to determine if there has been any shift in land surface levels. If the ground has moved away from (subsidence) or towards (uplift) the satellite between the times of the two SAR images, a slightly different portion of the wavelength is reflected back to the satellite resulting in a measurable phase shift that is proportional to displacement. The map of phase shifts, or interferogram, is depicted with a repeating color scale that shows relative displacement between the first and the second acquisitions. The direction of displacement - subsidence or uplift - is indicated by sequence of the color progression of the fringe(s) toward the center of a deforming feature.

Interpreting Interferograms

Reading an interferogram isn't as complicated as it might seem. The process can be broken down into a few steps:

Illustration of a subsidence cone with an interferogram underneath it.

Step 1: Mapping InSAR displacement. In this illustration, two InSAR fringes are equal to 56 mm of deformation. (Public domain.)

 

 

Step 1

Count the number of InSAR fringes between two points on the interferogram, where one fringe is one complete color cycle (i.e. red, orange, yellow, green, blue, purple).

The figure illustrates how land-surface displacement, in this example it's subsidence, is represented on an interferogram. In this example, each fringe, or color cycle, represents 28 mm of range change

 

 

 

 

 

 

 

Example of an interferogram with the color bands forming a depressed cone

Step 2: Processing interferograms. In this example, two InSAR fringes are equal to 56 mm of subsidence. (Public domain.)

 

Step 2

Multiply the number of fringes by 28 mm (1.1 in.).

Because there are 2 fringes, the maximum displacement (at the bottom of the bowl), is 56 mm. Depending on processing, one fringe could represent a different magnitude of displacement. Generally, about 1/3 of a fringe (two colors) is discernible displacement; in this example, 10 mm of displacement. If we process the data such that 14 mm of displacement is represented by 1 fringe, then about 5 mm of displacement is discernable.

 

 

 

 

 

Scale of subsidence.  A rainbow of colors denotes up to 28mm of either subsidence or uplift

The third and final step in interrpeting interferograms is determining deformation. An increase in range means subsidence is depicted. A decrease in range denotes uplift. (Public domain.)

Step 3

Determine if the ground moved closer (uplift) or farther away (subsidence) from the satellite by matching how the colors change between the two points with the InSAR scale bar. An increase in range (i.e. red, orange, yellow, green, blue, purple) signifies subsidence, and a decrease in range indicates uplift.