Toting their surveying instruments to the tops of mountains or across continents, 20th century geodesists might have dreamed of an easier way to measure precise locations and track changes in ground motion over time. Today that dream has been realized with GPS and InSAR. Neither was the result of fanciful dreams or foresight. Two of today’s most effective techniques for studying ground deformation were fortuitous afterthoughts. (The GPS story was covered in an earlier Caldera Chronicles article.)
The InSAR story requires a brief introduction. Radars come in two basic types: tracking radars and imaging radars.
Tracking radars “ping” a target, record the reflected signal and measure how long it took for the ping’s round trip, as well as the frequency of the return signal. The travel time is a measure of distance to the target. The target’s velocity can be determined from the frequency of the return signal, which differs from that of the transmitted signal as a result of the Doppler effect (this is the effect that makes a siren sound different when it is coming toward you versus moving away from you). Air traffic control radars and police speed detectors work on this principle.
Imaging radars reveal much more about their targets than “how far and how fast.” They illuminate a target and record the pattern of reflected energy as an image, akin to a photograph, but different in some important respects. The pattern depends on properties (size, shape, orientation, etc.) of many individual reflectors that contribute to each pixel in the image. Radar images look different than conventional photographs and they reveal different things about the target.
There are two types of imaging radars: real-aperture radars and synthetic-aperture radars (SARs). The resolution of an imaging radar depends on several factors, including antenna size — bigger is better. To achieve useful surface resolution with an orbiting real-aperture radar would require an enormous antenna, much too large to be practical. But radar engineers are a clever bunch, able to conjure up (“synthesize” is the technical term) enormous virtual antennas by taking advantage of the orbital motion of a real-aperture radar and the Doppler effect mentioned earlier. The details aren’t important here. The point is rather that SARs with sufficient resolution to “see” your car from orbit are a reality.
Recognizing that SAR satellites could provide unique and valuable information about surface features and characteristics, the European Space Agency launched the ERS-1 satellite in 1991 with a mission to acquire radar images of the entire globe.
Scientists at the Jet Propulsion Laboratory realized that under certain circumstances SAR images could be combined to produce derivative images called interferograms. SAR images show how the surface would look to a pair of radar-sensitive eyes. An interferogram shows how much the surface moved during the time interval between SAR images — with millimeter-scale precision.
With InSAR (interferometric synthetic-aperture radar), scientists can create snapshots of surface deformation without ever trudging to a mountaintop or across a continent. A new branch of geodesy was born.
A remarkable demonstration of InSAR’s capability came in 1993 when several French scientists produced an interferogram from ERS-1 images showing in glorious detail the pattern of surface disruption caused by the 1992 magnitude 7.3 Landers, California, earthquake. In the decades prior, geologists had measured fault scarps, geodesists had surveyed nearby benchmarks and modelers had calculated how the Earth might move during a major earthquake. For the first time ever all could marvel at a “picture” of what actually happened. It was a snapshot taken from space by a satellite designed to do something else.
Today, scientists are using InSAR, the “magic deformation camera” to study deformation at Yellowstone and elsewhere around the globe. Using the technique, it has been possible to get an overall picture of Yellowstone deformation, revealing some interesting patterns of ground motion. We’ll discuss some of these patterns in future issues of Yellowstone Caldera Chronicles.