午夜福利1000集合

SATELLITES could pinpoint plentiful new sources of groundwater in arid
regions of the world by spotting the slight swelling of the Earth鈥檚 surface
caused by underground reservoirs.

Just as a sponge swells when it gets wet, the Earth鈥檚 crust can plump up when
underground aquifers fill with water. Now Wesley Danskin and Zhong Lu of the US
Geological Survey have shown that satellites can spot this swelling. At the
moment, water managers have to drill a series of wells to work out the size of
an aquifer. 鈥淥ne of the main advantages of this technique is you can see the
entire landscape at once,鈥� says Danskin.

The technique maps the Earth鈥檚 surface using 鈥渟ynthetic-aperture radar鈥�. SAR
takes advantage of the satellite鈥檚 motion to produce more detailed radar images.
By scanning the same area at different times and comparing the data, researchers
can detect changes in the vertical elevation of the Earth鈥檚 surface as small as
about 5 centimetres鈥攁 technique known as interferometric SAR, or
InSAR.

Danskin and Lu looked at InSAR images of a region around San Bernardino in
southern California that contains a huge aquifer. They detected a bulge of 4 to
7 centimetres during summer of 1993, which followed a particularly rainy
winter.

Because the area is riddled with faults in the Earth鈥檚 crust鈥攊ncluding
the infamous San Andreas Fault鈥攖he researchers analysed the location of
the faults to check that the uplift was not caused by seismic activity.

As well as helping to find water sources in arid areas, the technique may
help scientists predict earthquakes, says Kenneth Hudnut, a geophysicist with
the US Geological Survey posted at the California Institute of Technology in
Pasadena. 鈥淚f we can separate out the changes in the Earth due to groundwater,
we can get a better picture of the seismic activity.鈥�