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Air Guns Shake Up Earthquake Monitoring
from ScienceNOW Daily News
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.
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