 By LEE BOWMAN Scripps Howard News Service June 28, 2006
The good news: Seismic waves make rock more permeable and might make it easier to extract oil and gas from natural reservoirs. No one's really looking for an energy boom to be the bright side of the Big One out West. But researchers in California studying how quakes affect water levels in two test wells have found a striking pattern of quakes making rock more or less spongy.
The study was based on two decades of data from a geophysical observatory at Pinon Flats in Southern California, run by the University of California-San Diego's Scripps Institution of Oceanography. The monitoring includes records of water levels that rise and fall gradually with changes in local weather, including rainfall, but they also change regularly in a pattern similar to those seen in oceanic tides. That's because the gravitational effect of the moon squeezes and stretches rocks in Earth's crust. The speed and extent of this ebb and flow in a well depend on the permeability of the surrounding rock. "We know the tidal strain very well, so we can measure the lag between the imposed tidal strain and the response in the well to get a precise measure of the permeability of the rock," said Emily Brodsky, an assistant professor of earth sciences at the University of California-Santa Cruz and a co-author of the study, published Thursday in the journal Nature. "Every time there's a big earthquake in Southern California, the permeability jumps," Brodsky added. "We saw this in two different wells for more than seven different earthquakes." After each quake, recorded between 1988 and 2005, the rock surrounding the wells became as much as three times more permeable to groundwater, said the researchers, who also included Jean Elkhoury of UCLA and Duncan Agnew of UCSD. And the size of the increase was proportional to the peak amplitude of the seismic waves. The rock then gradually relaxed back to its original state over a period of several weeks to months following the quake. This enhanced permeability from seismic waves could potentially be channeled toward extracting oil and gas, Brodsky said. "Permeability governs how fluid flows through rocks, whether it's water or oil, so this has practical implications for oil extraction." In lieu of earthquakes, seismic waves might be artificially generated using underground explosions or "vibroseis" trucks - large hydraulic rigs that put rubber pads on the ground that vibrate at a specific frequency for a prolonged period of time. The trucks are commonly used for seismic imaging studies looking for specific underground features, such as oil and gas deposits underground. "If we understood the physics of the permeability enhancement well enough, the vibrations could be used to increase the flow of oil," Brodsky said. The researchers also speculate that changes in the permeability of rocks along fault lines may play a role in triggering earthquakes.
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