Petroleum geologists have long used air guns in their search for oil and gas deposits. Sudden blasts from the devices generate seismic waves that they use to map underground rock formations. Could the same technique be used to study earthquakes? A team of Chinese scientists thinks so. The researchers have designed an air gun that could be useful in monitoring changes in stress buildup along fault zones.
Although geologists typically use dynamite or some other method of creating seismic waves in their land-based explorations, for exploration over water they often use air guns. When these devices—which are often towed behind boats—are set off, sharp blasts of pressurized air send shock waves through the water and into underlying sediments, where they trigger seismic vibrations. As doctors take CT scans of the human body, geologists gather such data with a network of seismometer-like receivers and use it to map the structure of rock deposits or other features, such as geological faults, in the region's crust.
Similar networks can be used to monitor subtle, long-term changes in the velocity of seismic waves. These are just the sort of variations that can signify changes in stress buildup along a fault zone, says Baoshan Wang, a geophysicist at the China Earthquake Administration in Beijing. Although studies have noted many such changes in fault zones and volcanic areas, they have relied on natural sources of seismic waves. But those sources vary in strength and occur infrequently, at irregular intervals, and in unpredictable places. As a result, the resolution of images is relatively low, Wang says.
"The challenge is to develop a source of seismic energy that's consistent," says Fenglin Niu, a seismologist at Rice University in Houston, Texas, who is not involved in the team's research. Hence the need for the device that Wang and his colleagues designed. The apparatus, which the researchers call a "transmitting seismic station," consists of four large air tanks, each of which can hold almost 33 liters of air at a pressure of 15 megapascals (or about 153 times atmospheric pressure at sea level).
The team's field tests were conducted last April at a site in China's Yunnan Province, near the center of a 10,000-square-kilometer array of seismometers used to monitor two major faults that intersect nearby. The researchers also temporarily deployed an extra 40 seismometers in the surrounding region during their trials to ensure good coverage, Wang says.
During the tests, the air-gun device was lowered into a small reservoir by a crane and fired 111 times. Data gathered by instruments near the reservoir indicate that all shots generated nearly identical ground motions, equivalent to those that would be produced by a magnitude-0.5 earthquake. Ground motions triggered by a single shot were picked up at a seismometer 112 kilometers away, the researchers report in the current issue of Eos, Transactions, American Geophysical Union. When data gathered in the wake of all of the shots were superimposed, the ground motions stood out from background seismic noise at instruments more than 240 kilometers away.
Besides having great consistency from one test to another, neither the air-gun blasts nor the ground motions generated by the shots apparently had any effect on wildlife or nearby structures. None of the lake's fish were killed or stunned by the shots, and instruments installed at a dam 1.4 kilometers away from the test site showed that peak ground accelerations were far below those detectable by humans, Wang says. Other possible effects need to be evaluated, he admits, but they are believed to be minimal.
The researchers propose to monitor changes along the nearby faults by firing their device once a week for the next 3 years or more. When data gathered by the regional network of seismometers is combined with that gleaned by global positioning systems and other instruments, scientists may be able to determine how changes in the speeds of seismic waves are related to changing stresses along the faults, Wang says. Those changes, in turn, may give early warning of stress buildup along the faults, one possible sign of a future earthquake.
The team's results are promising, especially in light of recent advances in seismic sensors, Niu says. But he notes potential problems. For one thing, changes in water level at the reservoir could cause changes in stress in the underlying rocks, thereby affecting the speed of seismic waves generated by air-gun blasts. Data gathered during future tests will reveal whether that's an issue, he suggests.