Published online 26 June 2001 | Nature | doi:10.1038/news010628-8

News

Ground swell sensed from space

Satellites spot aquifers filling and emptying

Look your fill: satellites monitor ground water changesLook your fill: satellites monitor ground water changes© USGS

Radar images from orbiting satellites are the modern equivalent of the dowser's rods. They point to hidden water, US Geological Survey scientists have found1.

Using a technique called interferometric synthetic aperture radar (InSAR), Zhong Lu and Wesley Danskin have tracked the ground swelling that occurs when rain recharges depleted aquifers in the San Bernardino area of southern California. The radar images were taken by ERS-1 and ERS-2, satellites that were launched by the European Space Agency.

Interferometry compares images; radar records the height of the ground. Interferometric analysis can therefore pinpoint places in which two images of the same region differ at different times. The researchers spotted differences of just a few centimetres between images taken in 1992-95.

InSAR should, they hope, provide an economical and comprehensive way to track changes taking place within hidden water reservoirs. It might alert geologists to impending subsidence, and allow them to monitor beleaguered aquifers.

Just as water swells a sponge, ground water swells the porous rocks that hold it. Conversely, when water is withdrawn from an aquifer, the rock can collapse, leading to subsidence. If the water level falls too low, the rock becomes compacted, and the subsidence is permanent.

Monitoring such changes from the ground can be an expensive business. It involves drilling wells to assess the state of an aquifer. A variety of airborne or satellite remote-sensing techniques have previously measured subsidence, including InSAR. Because of the social and economic dangers of subsidence, the emphasis has been on looking for signs of imminent collapse.

Lu and Danskin show that swelling caused by aquifer refilling can also be extensive enough to see. They found, for example, that in early 1993 the semi-arid San Bernardino basin rose by more than seven centimetres. They attribute this to ground water, as the discharge from the streams and rivers flowing down from nearby mountains was then unusually large.

Other processes can cause uplift. Tectonic movements in the crust squeeze and pull on the rocks - especially in earthquake-prone southern California. But Lu and Danskin consider this an unlikely explanation for the uplift they saw, because it seems to be restricted to the area around the aquifer. Tectonic forces would alter the surface height of land elsewhere too. 

  • References

    1. Lu, Z. & Danskin, W.R. InSAR analysis of natural recharge to define structure of a ground-water basin, San Bernardino, California. Geophysical Research Letters 28, 2661 - 2664 (2001).