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 Earths 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 Earths 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 Earths 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 bulls 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).
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 oceans 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 Earths surface. This new technique
requires two radar images of the same area of Earths 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
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 Earths 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 Earths 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 Earths 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 worlds 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 Earths 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.
Many civilian scientists who rely on SAR or InSAR data for their research
might dream of complete satellite coverage of Earths surface, but
the reality is that the data available are spotty and are potentially
about to get spottier.