The Earth has the same problem as the rest of us--if it drinks a little too much, it tends to bulge. Using satellite radar, researchers determined that the ground swells as rainwater fills aquifers below. In a type of geologic liposuction, these bulges may sink as water is pumped out by farms and towns. Hydrologists hope to use relatively inexpensive remote-sensing to map aquifers in remote or inhospitable places in Africa, Iran, or Southern California.
When rain and ground water seeps into the ground, they end up in semipermeable regions of rock called aquifers. Characterizing the ins and outs of these systems has always been labor-intensive and expensive. To determine the extent and activity of a typical aquifer, researchers usually dig more than a dozen wells, install sensitive monitors, and visit the sites every few months to take measurements. Last year, however, a research group showed satellite radar could reveal how far land subsides after water is pumped out.
Now another team of researchers has watched an aquifer rise--by about 7 centimeters--apparently because it absorbed water runoff. Led by Zhong Lu and Wesley Danskin of the United States Geological Survey in Sioux Falls, South Dakota, and San Diego, respectively, the team examined 13 radar images of the land above a well-studied aquifer system near San Bernardino in Southern California. They used a technique called synthetic aperture radar (SAR) interferometry to see height changes on the centimeter scale.
The maps showed two regions of 3- to 7-centimeter uplift between mid-1992 to late 1993 in the valley below the San Bernardino and San Gabriel Mountains, the researchers report in the 1 July issue of Geophysical Research Letters. The bulges emerged after particularly heavy rain and snowmelt doused the aquifer, which the researchers verified by conventional well readings.
"I'm quite impressed with their analysis," says Jet Propulsion Laboratory geophysicist Eric Fielding, who says the team's supporting well data plus careful elimination of other causes for the bulges made for "excellent work." He adds, "we're going to see plenty more applications for SAR interferometry that we haven't even thought of yet."