The seismic shaking could enhance the permeability of underground rocks and potentially be harnessed to help extract oil from natural reservoirs, scientists reported on Wednesday.

According to a research team from the University of California, the rocks' permeability governs how fluid flows through, whether it is water or oil, so this has practical implications for oil extraction.

These findings were published in the June 29 edition of British journal Nature.

The study was based on two decades of data from the Pinon Flat Observatory in southern California, where the researchers maintain an extensive geophysical observatory.

The monitoring includes records of fluctuating groundwater levels, which fluctuate in response to tidal effects similar to oceanic tides. In this case, the gravitational effects of the Moon on the solid Earth squeeze and stretch the rocks in the crust, forcing water in and out of the wells from the surrounding rocks.

The speed of the response of the groundwater in a well depends on the permeability of the surrounding rocks. Thus, the researchers took the response time as an index of permeability.

"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, the study co-author at the University of California, Santa Cruz.

When the researchers analyzed the data in relation to earthquakes, they saw a striking correlation -- the water's response to tidal forces became more sensitive after a temblor. That means the permeability of the rocks increased.

After an earthquake, the rock surrounding the wells became as much as three times more permeable to groundwater, the researchers found. Furthermore, the size of the increase in permeability was proportional to the peak amplitude of the temblor.

The changes were transient, with permeability returning to the original level within a few months after an earthquake.

"Every time there's a big earthquake in southern California, the permeability jumps. We saw this in two different wells for more than seven different earthquakes," Brodsky said in a statement.

The oil industry might be able to exploit this phenomenon by using different devices, such as vibroseis trucks, to send seismic waves into the ground, the researchers indicated.

Vibroseis truck, which can vibrate at a particular frequency for a prolonged period, is currently used as "man-made epicenter" for seismic imaging studies.

"If we understood the physics of the permeability enhancement well enough, the vibrations could be tuned to increase the flow of oil," Brodsky said.

Source: Xinhua