For more information

E-Mail:

Devin Galloway, USGS, Sacramento, CA
dlgallow@usgs.gov

Website:

http://southport.jpl.nasa.gov/links/links.html

Satellite Watch: New Ways to Detect Changes on Earth's Surface

Data from satellites continually orbiting the Earth are giving scientists more effective and far less costly and time-consuming methods for detecting quite small changes in the elevation of the Earth's surface. These measurements can show how much land has subsided from the pumping of ground water, or how the land surface has been deformed, or raised, by the movement of magma deep within the Earth. Images created from these satellite data -- interferometric synthetic aperture radar, or INSAR for short -- provide a higher level of accuracy over a larger area than ever before.

INSAR color maps and models of the land surface, combined with other more traditional sources of information, give research scientists and resource planners and managers an unprecedented ability to determine the effects and risks of natural and human activities.

The level of detail and accuracy of INSAR technology allows researchers not only to have accurate current data, but also to compare changes and conditions against predictive models. USGS researchers, for example, are able to obtain more information, more frequently over larger areas and at lower costs than has been possible using only traditional surveying methods. INSAR imaging provides an additional and valuable tool in the scientists' arsenal that complements and enhances other assessment techniques.

INSAR data enable scientists to measure minute changes such as the increased subsidence (warmer colors) from 1993 to 1995 in the Yellowstone caldera. This satellite technology allows researchers for the first time to monitor the entire caldera and discern whether the deformation is a response to pressurization and depressurization of a hyperthermal system or a response to magma movement.

Interferometric Synthetic Aperture Radar - How Does It Work?

INSAR images are created from data collected at different times from a satellite in the same orbital position. Satellite data that USGS uses are collected by the European Space Agency. The satellites orbit at altitudes of about 300 miles and transmit radar data that are recorded at ground stations. These data are then digitally analyzed on computer workstations to obtain refined radar images of the Earth.

The spatial resolution is quite detailed, with an individual picture element, or pixel, of data representing an area of 1,025 square feet on the Earth's surface. INSAR images are essentially a measurement of the differences when radar scans are compared. The pairs of radar images are compared to detect ground movements of four-tenths of an inch or less that occurred during the time interval between acquisition of the two images.

INSAR relies on changes in the Earth's reflective properties over time. This means that heavily vegetated lands, such as forests, cultivated fields, and other areas that have changing land cover, are not well suited to INSAR technology because they absorb, rather than reflect, infrared signals. Suitable areas include deserts and areas with minimal vegetation, urban areas, and other areas with extensive paved surfaces. Agricultural areas with fixed surface areas of more than 33 feet across, and such features as freeways, paved runways, urban clusters and factories, therefore, could be monitored for regional changes in surface elevation by using these sites as "markers."

A future generation of radar satellites scheduled for launch around the year 2000 is expected to improve data quality and the capability to combine radar images for analysis.

INSAR and Water Management in Antelope Valley, California - A Model for the Thirsty West

The thirst for water in the Antelope Valley of California follows the pattern of many arid regions of the West: initial development of ground water for agriculture, followed by urbanization and importing of surface water. Natural ground water and seasonal supplies of surface water may not be adequate to meet future demands.

Future water supplies may depend on managing surface water stored in aquifers during low-demand periods, and then pumping and "recovering" the water when needed. Such managed storage requires a detailed understanding of aquifer characteristics and behavior. INSAR is being explored to help provide that information.

In the Antelope Valley -- home of Edwards Air Force Base and the Space Shuttle -- long-term, ground-water pumping has caused irreversible compaction of the clay and silt layers in the aquifer from which ground water is pumped. The resulting land subsidence has caused substantial damage to runways, roads, wells, pipelines, and other structures. Surface fissures or cracks in the land surface 1,300 feet long, 6 feet wide, and 13 feet deep have been reported.

The additional accuracy and timeliness of data that INSAR can provide in measuring the amount, rate, and distribution of subsidence, along with computer models of ground-water recharge, flow, and pumpage, will help managers evaluate how aquifers might be used as potential storage to expand future water supplies and limit the impacts of subsidence.

The USGS is exploring the use of INSAR data in other California watersheds, such as the Santa Clara Valley and Coachella Valley and in Las Vegas and Houston.