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How to Avoid Running Out of Steam

Underground steam is a great alternative source of energy with a huge drawback: It's hard to judge the size of a geothermal field before power plants start losing steam. Now, however, scientists have found that they can better plumb a field's contents by analyzing earthquakes that ripple through it. The new technique, reported in the latest issue of Geophysical Research Letters, could act as a sort of "fuel gauge," leading to better estimates about how much money to sink into particular geothermal sites.

There's no doubt that better predictions are needed. Take The Geysers, a 75-square-kilometer geothermal field near Santa Rosa, California, that provides power for about 1.7 million households. Steam production there has dropped 10% each year since the mid-1980s--much faster than many power companies had expected. "They've had to tear down brand-new power plants," says Bruce Julian, a seismologist with the U.S. Geological Survey in Menlo Park, California.

Hoping to avert future financial misfortunes, Julian and Gillian Foulger, a geophysicist at the University of Durham in England, have developed a cheap way for companies to map their reservoirs: seismic tomography, a technique used to image Earth's crust and mantle. Seismic tomography measures variations in the speed of two kinds of natural earthquake waves: P-waves, which travel like the shudders down a line of braking rail cars, and S-waves, which move like a billowing flag.

Over time, steam in a geothermal field fills more and more of the cracks and pores in the surrounding rock; this affects the speed of each kind of seismic wave differently. By studying the arrival times of P-waves and S-waves at 22 seismograph stations, the researchers could pinpoint the shrinking production area of The Geysers. Between 1991 and 1994, Julian and Foulger found that the P-waves tended to slow down in the area of greatest steam extraction. This, the researchers say, suggests that seismic tomography can monitor the emptying of a reservoir.

The technique could also help find new reservoirs, says UNOCAL Corp. geophysicist Bill Cumming. "They have demonstrated that you can see geothermal processes," he says, "and that would be of interest in any geothermal field."