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Just as a sponge expands when absorbing water, so too does the Earth bulge slightly where groundwater aquifers absorb unusually large amounts of water from stream runoff or heavy rains. Scientists using satellite data have been able to measure these bulges on the land surface and believe they can use the technique to study the location and size of aquifers in remote regions.
Writing in the July issue of Geophysical Research Letters, published by the American Geophysical Union, Zhong Lu and Wesley R. Danskin of the U.S. Geological Survey describe their interferometric analysis of imagery from the synthetic aperture radar (SAR) instruments aboard the European Space Agency satellites, ERS-1 and ERS-2. This technique, known as InSAR, produces an image, called an interferogram, showing differences in land features between two SAR images taken at different times. Synthetic aperture radar is essentially a high resolution radar.
Using the well documented San Bernardino groundwater basin of southern California for their test case, Lu and Danskin detected an uplift of more than three inches during the first half of 1993. This period was marked by unusually high runoff from surrounding mountains and high levels of water in nearby wells.
The use of InSAR techniques, supplemented by other remote sensing techniques, to define the extent of an aquifer system simplifies the traditional process, which relies on expensive, ground-based data collection. Traditionally, scientists have had to study a large number of wells to define the boundaries of an aquifer system and its internal structure. By using InSAR to resolve vertical distances to inches, and by comparing images taken at different times to detect expansion or subsidence of Earth's surface, they can detect changes in an underlying aquifer.
The San Bernardino area is semi-arid, and groundwater levels rise dramatically in response to recharge from intermittently flowing streams that originate in the San Bernardino and San Gabriel Mountains, which are composed mainly of granite and absorb little water. The area lies between the San Andreas and San Jacinto earthquake fault lines. From 1950 to 1970, groundwater levels in one area fell by about 160 feet and the land subsided by as much as 12 inches. From then until 1980, natural and artificial recharge brought water levels in that area to within three feet of the surface.
With these and other available data, Lu and Danskin compared 13 InSAR images taken between 1992 and 1995 and produced a series of interferograms. They observed that the three inch uplift occurred between December 1992 and August 1993, with two inches occurring in only three and a half months, during the period of greatest runoff. Although significant land subsidence as a result of groundwater pumpage is a well known scientific phenomenon, the magnitude of the observed uplift caused by recharge was unexpected.
The researchers were concerned that the observed land deformation might have been caused by movement of tectonic plates deep below the land surface or by earthquakes, rather than by recharging of the aquifer. They concluded, however, that this was not the case, based on the interferometric patterns acquired from the satellites.
The study was funded by the U.S. Geological Survey and the San Bernardino Valley Municipal Water District.