In Search of Cleaner Fuel

Peter Malin’s Seismology Group Uses Earthquake Technology to Explore Sources of Geothermal Energy p.3

Scouting for Wells Near a Hawaiian Volcano
While Kahn was in Switzerland, seismologist Eylon Shalev of the Nicholas School’s Earth and Ocean Sciences Division was installing another array of Duke-built instruments within drill holes in the vicinity of Hawaii’s Kilauea volcano.

As project leader, the Israeli-born research scientist and his team lowered a total of eight seismometers, some equipped with tiltmeters, to depths ranging from as much as 700 and as little as 80 feet deep. They will serve as a network to help the existing Puna Geothermal Venture (PGV) power station scout a location to drill new steam wells for power generation.

“PGV now supplies about 20 percent of the electricity on Hawaii’s Big Island,” Shalev said. “They’re producing 30 megawatts using water from four wells. The average geothermal plant in the world would need about 10 wells to produce that amount."

The Puna site is at an ideal location: Red hot magma feeding the volcano supplies enough residual heat off to its sides to warm abundant quantities of underground water.

“Kilauea is the most active volcano in the world, but most of the recent eruptions haven’t happened at the summit,” he said. “Nowadays the most active place is on the East Rift Zone, about 10 miles from the summit. The PGV plant also sits on that zone, 20 miles from the summit.

“They’re on a dangerous place, no question about it. But, as geothermal plants go, this is one of the most efficient in the world.”

But geothermal well prospecting is never a sure thing, said Stephen Onacha, a former geophysicist employee of Kenya Power Generation Co. in Africa’s geothermally rich Rift Valley.

Now on a postdoctoral fellowship at Duke supported by the government of Iceland, he has developed special geothermal exploration methods using earthquakes and electrical properties of rocks.

“One well costs over $2 million,” Onacha said. “And the success rate for geothermal wells is still the same as for the oil industry—about 25 percent.”

The Nicholas School’s seismology group will try to help increase Puna Geothermal Venture’s odds for its next well with a method they previously used in the vicinity of California’s Mammoth Lakes. There, an uplifting dome at the center of a large volcanic caldera threatens to renew the volcanism that has been a repeat visitor for the past 760,000 years.

In 1997, the Duke team installed a temporary network of 40 surface seismic stations near where Mammoth Pacific LP had built a geothermal power plant in California to tap the underground volcanic systems’ geothermal energy.

That array of seismographs allowed the group to paint a tomographic picture of underground structural features underlying Mammoth Lakes. Somewhat analogous to CAT scans, the computerbased technique involves calculating how waves from swarms of microearthquakes interact with the rock they vibrate through.

“We were lucky to be there at a time when there were a lot of earthquakes,” said Shalev.

A followup analysis of that stored data suggested the presence of a promising array of broken rock through which hot underground water could percolate. “Our new interpretation is that the broken rock forms a kind of pipe, identified in extinct and deeply eroded volcanoes as a ‘rubble chimney,’” Shalev said.

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