But nature has a habit of hiding her riches: It's not easy to figure out how much steam remains untapped, and misjudging the size of a reservoir can be costly. For instance, the steam production of The Geysers, a geothermal field near Santa Rosa, California, that provides electricity for 1.7 million homes, has dropped 10% annually since the mid-1980s--much faster than many power companies 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.

USGS

In the 15 January issue of Geophysical Research Letters, Julian and Foulger report on a new, relatively cheap way to map reservoirs: seismic tomography, a technique often used to image Earth's crust and mantle.

The team compared the speeds of two waves produced by an earthquake. S-waves are relatively unaffected by the type of rock they ripple through. But P-waves, which travel by compression (like shudders passing down a line of braking rail cars), slow down in more compressible material.

Because rock filled with steam is more compressible than that filled with water, slow P-waves should identify where the hot water has boiled away. Foulger and Julian have studied when P- and S-waves from 146 earthquakes arrived at 22 seismograph stations surrounding The Geysers, and they report that seismic tomography can act as a sort of fuel gauge, monitoring the emptying of a reservoir. Between 1991 and 1994, the ratio of P- to S-wave speeds dropped in the area of greatest steam extraction, showing that the cracks and pores in the rock were filling with steam as the hot water--the "fuel" of the reservoir--boiled away.

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