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SAR: A Versatile New Tool for Earth Science
Rosalind T. Helz, John LaBrecque and William G. Pichel


Sidebar: SAR survival


Invented in the 1950s to permit radar to achieve fine spatial resolution in the cross-track direction, synthetic aperture radar (SAR) played a key role in NASA’s Shuttle Imagery Radar experiments flown in the 1980s. Since then, widespread availability of data from SAR has permitted the technology’s application to a broad range of geoscience problems. The field has blossomed in the last decade, benefiting from the wealth of SAR data collected from a series of satellites launched in the early- to mid-1990s: the European Remote Sensing (ERS) satellites launched by the European Space Agency, the Japanese Earth Resources Satellite (JERS), and the Canadian RadarSat (see story, this issue). The most dramatic results include detailed imagery of Antarctica’s ice sheets and images of the strain associated with earthquakes, such as the Hector Mine quake of Oct. 16, 1999, in California.

Like all other kinds of radar, SAR sensors send out a radio wave and collect the reflected signals for interpretation. SAR data can provide accurate digital topography. Scientists depend on SAR’s all-weather, day and night capability to detect and monitor floods, oil spills or storm damage as quickly as possible, without having to wait for a clear day. Perhaps the most exciting applications of SAR data involve using pairs of images to detect and measure changes in the shape or position of the Earth’s surface. This application, called interferometric SAR, or InSAR, has the potential to let us anticipate volcanic eruptions and earthquakes, and allows us to monitor the movement of ice sheets and the effects of movement or withdrawal of fluids (water, oil or gas) in shallow reservoirs. The future holds still other applications, especially as scientists begin to explore the capabilities of new SAR sensors that operate at different wavelengths. Further progress in these areas will depend on a (yet-to-launch) second generation of radar satellites (see sidebar).

Mapping Earth

Earth scientists need good topographic data as a base on which to plot all other geologic information, whether bedrock or surficial geology, fault locations or other structural information. The advent of GIS (geographic information systems) and digital mapping requires that the topographic information also be digital. Modeling of mass flow (whether water, landslides and debris flows, or volcanic materials) also requires digital topography as a key input for calculations. In spite of the acute need, good digital topography is not widely or easily available for most of the Earth’s surface. SAR can help.

By measuring the time between transmitting a signal and receiving its return, SAR sensors can measure the distance between the source/receiver and the reflecting surface, and so can be used to generate topography. Recently, NASA and the National Imagery and Mapping Agency used a SAR sensor carried aboard the space shuttle to image more precisely the Earth’s land surface topography between 60 degrees north and 56 degrees south. When fully processed, these data will provide greatly improved digital topography for most of the Earth’s land cover.

Satellite radar interferogram draped over the topography of Los Angeles, Calif. The repeating color bands of the large oval show about two inches of subsidence throughout the basin between May and September 1999. The Newport-Inglewood fault is the straight line located just inside the coast line. The small isolated ‘bull’s eye’ feature (left center) is from pumping activity in the Inglewood oilfield. Image courtesy of G.W. Bawden (Bawden et al., Nature, 412, 812-815, 2001).

Monitoring hazards

The largest class of applications of SAR involve simply looking at a single image (or scene) or a mosaic of scenes. Because it uses radio waves, the SAR signal penetrates clouds, whether the result of weather, dust storms or volcanic ash. SAR does not depend on sunlight because the sensor sends out its own beam of energy, so SAR imagery can be obtained at any time of day or night.

SAR imagery applications are becoming routine, limited only by availability of data (see sidebar). The International Charter for Space and Major Disasters, recently organized by a consortium of space agencies to provide free satellite imagery in response to a disaster, has been activated 16 times since it began in November 2000. In 42 percent of the cases, the request was triggered by a flood or an oil spill, and in each of these events, SAR imagery was specifically requested.

Sea Ice Hazards. All-weather SAR imaging of the high-latitude seas provides information that is vital to minimizing the danger of sea ice to shipping and other maritime activities. Observing the edge of the ice pack as well as the position of icebergs with SAR imagery is becoming the accepted technique for monitoring this hazard. SAR is also a useful tool for tracking spring breakup of major rivers in northern regions (for example in the Yukon in Alaska and the Yellowstone in Montana). European and Canadian SAR imagery has been used to monitor the onset of breakup, ice runs, ice jams and flooding due to ice jams.


SAR-derived surface wind-speed image at 1-kilometer resolution from a RADARSAT-1 image taken Dec. 13, 2001, of the Gulf of Alaska near Kodiak Island. Color represents wind speed. Image courtesy of Frank Monaldo, the Johns Hopkins University Applied Physics Laboratory, and Christopher Wackerman, General Dynamics Advanced Information Systems. Click here to get more details and see a larger image.

Wind Tracking. The National Oceanic and Atmospheric Administration (NOAA) uses SAR in coastal regions to monitor strong wind events such as gap winds, downslope winds and barrier jets. SAR imaging is uniquely capable of measuring winds close to the coast and even in bays, rivers and lakes. The resulting data can be used to issue alerts for maritime traffic and low-flying aircraft.

Oil Slicks. Because oil slicks affect the roughness of the ocean’s surface, researchers can use SAR imagery to detect and track anthropogenic and natural oil slicks. This ability is especially useful in the enforcement of pollution laws because SAR can constantly monitor potential violations — a critical capability, as most large oil spills at sea occur as a result of severe weather. Oil slick tracking is also of great importance in the containment of oil spills. Winds derived from SAR can provide local wind conditions for use in oil spill trajectory models.

Floods and Hurricanes. SAR images show areas inundated by rising flood waters in support of disaster response efforts. Such images can be used for rapid damage assessment during and immediately after a flood, when the area may still be obscured by clouds. The inundation patterns recorded by SAR can assist in quantitative damage assessment and mitigation planning. SAR images can also provide rapid damage assessment after major hurricanes, when cloud cover and damaged infrastructure (telephones, roads, bridges) make conventional surveys difficult.

Seeing Earth move

The SAR application of most importance to the geosciences is the use of SAR interferometry to detect deformation of Earth’s surface. This new technique requires two radar images of the same area of Earth’s surface, taken from the same point in space at different times. When scientists process the reflected signals and compare the phase history of the reflected waves for the two images, they can obtain a very detailed look at any changes in the shape or position of the reflecting surface that occurred in the time elapsed between taking the two scenes. The image of the phase differences is called an interferogram. The pattern and amount of deformation is customarily shown by using color fringes from the visible spectrum. If too much change has occurred before the second image is taken, we get “decorrelation” of the surface. What we see in the image is areas with random speckles, instead of an interpretable pattern of deformation.

The resolution of SAR and InSAR is a function of the wavelength and bandwidth of the signal and the sensitivity and power of the satellite system. Shorter wavelength SAR imagery (for example, C-band SAR, with a wavelength of 5.66 centimeters) generally produces higher-resolution images. However, decorrelation is more of a problem because the shorter wavelengths are more sensitive to small changes at Earth’s surface, including changes in vegetation. Longer wavelength SAR imagery (L-band SAR, with a wavelength of 23.53 centimeters) usually produces lower-resolution images, but it is more forgiving of the small-scale instability of many natural surfaces. It also penetrates vegetation more effectively than the shorter SAR signals.

Modern SAR satellites can image thousands of square miles in a few minutes, so it is possible, in principle, to use InSAR to monitor surface changes with great precision over large areas. InSAR detection of surface deformation complements the results obtained from ground-based GPS networks, which allow monitoring of deformation continuously in time, but only at preselected points. InSAR is not continuous in time, as repeat views are typically obtained only every 35 to 45 days. However, it does document areal patterns of deformation. Such imaging offers new promise in the forecasting of volcanic eruptions, earthquakes, landslides, the direct measurement of erosion, the motion of ice streams and glacial retreat, and other forces that sculpt Earth’s surface.

Earthquakes and Regional Deformation. The most challenging goal for InSAR is mapping slow surface deformation. This includes the accumulation of strain leading up to earthquakes, as well as transient strain relaxation following earthquakes. Scientists supported by NASA, the U.S. Geological Survey and the National Science Foundation, under EarthScope and other initiatives, are seeking repeated measurement of surface change in seismically active areas along the Earth’s plate boundaries, where the highest probability of earthquakes exists. This new knowledge will refine our understanding of earthquake hazards along major seismic zones. Repeated InSAR imaging of active fault zones will help illuminate the strain patterns associated with these fault zones, before and after earthquakes. We do not expect to actually “catch” the moment of the quake itself. If InSAR ever did take an image exactly at the time of surface shaking, scientists would only see a decorrelation pattern.

A strong earthquake swarm on Akutan Island that occurred in March 1996 produced extensive ground deformation, including an area of visible ground cracking (dotted lines). The two interferograms show how the deformation field looks in C-band imagery (below, over a rainbow scale of 2.83 centimeters) and in L-band imagery (above, over a rainbow scale of 11.76 centimeters). The longer L-band wavelength provides a much more complete look at the complex deformation field. Images by Zhong Lu, EROS Data Center, US Geological Survey.

Volcanic Activity. Sequences of InSAR images for Etna in Italy and for Okmok and Westdahl volcanoes in Alaska dramatically show both the inflation of their surfaces before the eruptions and the subsidence that followed. These images hold great potential for predicting an impending eruption and analyzing the volume of erupted magma afterward. Applying the same technique to volcanoes thought to be dormant has allowed us to see that some of them — including Three Sisters in the Cascades and Peulik in Alaska — are actively changing shape in response to the movement of magma and gases beneath the volcano. Thus InSAR data can help us define both the long-term and near-term risk to citizens and infrastructure from volcanic hazards.

The all-weather capability of InSAR makes it an important addition to the existing monitoring systems (weather satellites and seismic and GPS networks) used to watch the more than 100 volcanoes of the Aleutian, Kamchatkan and Kurile volcanic arcs. This new technique should further mitigate the potential damage from eruptions and volcanic ash clouds to all aircraft that fly these busy North Pacific jet routes. Because SAR satellites can image all parts of the Earth, InSAR data also can provide important new warning capabilities to help mitigate the effects of eruptions at volcanoes in areas where ground-based monitoring systems are sparse or lacking.

Ground Subsidence. InSAR has demonstrated its ability to measure surface subsidence and rebound in response to aquifer discharge and recharge in regions, such as Los Angeles, Las Vegas, Houston and other cities, dependent on subsurface water. It can also be used to monitor withdrawal of gas and petroleum and has the potential to monitor subsidence associated with karst or collapse of abandoned mines. Some of these applications, especially monitoring oil and gas withdrawal, are already commercially significant.

Ice Sheets and Glaciers. Ice sheets and mountain glaciers contain 80 percent of the world’s fresh water. As their volume decreases, they contribute to rising sea level, which endangers heavily populated low-lying areas around the world. SAR, and especially InSAR, can help monitor the volume and dynamics of the Earth’s glaciers and ice caps because the technology can see through the clouds that might shroud them. SAR imagery provides an unprecedented series of snapshots of the ice sheets, and InSAR documents short-term changes in them. The recent (and unexpected) disintegration of large portions of the ice shelves of the Antarctic Peninsula emphasizes the importance of ice-sheet monitoring, whether for safety of human activities in adjacent areas or for understanding the effects of longer-term changes in sea level and climate.

SAR survival

Many civilian scientists who rely on SAR or InSAR data for their research might dream of complete satellite coverage of Earth’s surface, but the reality is that the data available are spotty — and are potentially about to get spottier.

A total of three working SAR satellites orbit the Earth at the moment, one from Canada (RADARSAT-1) and two from the European Space Agency (ERS-2, ENVISAT). But the ERS-2 is on its last legs, and RADARSAT-1 is well into its operational lifetime. Canada and Japan have plans for two new satellites in the next few years. Purchasing images from the other satellite-managing countries, however, can be expensive, ranging from several hundred to thousands of dollars for one image.

In the 1990s, U.S. SAR-based researchers formed WInSAR, a consortium of 25 institutions — including NASA, the U.S. Geological Survey and the National Science Foundation — to obtain data from European SAR satellites. “Going to foreign suppliers, which is what we’ve been doing the last 10 years, has worked reasonably well,” says Howard Zebker, a geophysicist at Stanford University in California, and good research has come entirely from those data. However, having a U.S.-run satellite would make things better, Zebker says.

But development of a U.S. SAR satellite cannot happen overnight. Even if the decision were made today to develop such a satellite, the earliest that it could be launched would be 2008 or 2009. “It is a problem,” says John Pallister, a research geologist at Cascades Volcano Observatory and former USGS Volcano Hazards Program coordinator, “in the sense that the uses of SAR data are increasing dramatically, in part because we’ve discovered what a good tool it is for monitoring ground deformation from space, whether it’s groundwater or volcanoes.”

But time, power and downloading capabilities are limited for active satellites, and target selection requires careful planning. Countries with satellites, Pallister says, give priority to their own researchers. “The ideal, looking toward the future, would be a constellation of SAR satellites.”

However, getting even one U.S. satellite launched has proven difficult, says Jean-Bernard Minster, a geophysicist at the University of California, San Diego. Over the past few years, NASA has judged several proposals for satellite missions (co-authored by Minster and others) to be budgeted unrealistically, Minster says; such missions can cost several hundred million dollars or more and take years to plan. However, Minster says another opportunity to propose an earth-observing satellite with SAR equipment will come this February, with another round of NASA applications.

Says Zebker: “Sometime soon, researchers are going to get to large problems that will require more data than can be begged, bought or borrowed from foreign satellite owners.”

Naomi Lubick

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Helz is associate coordinator of the USGS Volcano Hazards Program. LaBrecque manages NASA’s Solid Earth and Natural Hazards Program. Pichel is with NOAA’s Office of Research and Applications. We appreciate review comments from John Pallister and R.J. Thompson (USGS) and from Craig Dobson (NASA).

Links:
"Snapshots from Space of the World's Continents," Ahmed Mahmood et al., Geotimes, November 2003


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