A more than month-long field experiment by a Lawrence Livermore National Laboratory (LLNL) seismologist has demonstrated that a new technology could offer a major breakthrough in seismology.
The technology, called distributed acoustic sensing, allows an instrument to turn buried fiber-optic cable into thousands of virtual seismometers that can be used to measure ground motion of the Earth and structures.
Lab seismologist Gene Ichinose and his team plugged an instrument known as interrogator into an unused fiber-optic cable network that runs 80 kilometers (or 50 miles) from Moscone Center in San Francisco to Sunnyvale on Feb. 20.
The interrogator was left in place for a little more than a month, through April 3. On March 17, Ichinose and his team struck seismological paydirt as a magnitude 3.9 earthquake rattled through the hills west of Dublin.
"The detail of the seismic wave field was unprecedented," Ichinose said. "We've never been able to get this high a resolution in the Bay Area before.
"If I was looking at the publicly available data for the March 17 Dublin earthquake, I could see data from eight seismometers. With the distributed acoustic sensing technology, I'm seeing data from the equivalent of 8,000 seismometers. It's amazing because you can track the seismic wave propagating across the whole Bay Area."
Ichinose and Lab computer scientist Tim Brandt collaborated on the project with two companies - Roanoke, Va.-based Luna Technologies and San Jose-based Viavi Solutions - as well as a researcher from the California Institute of Technology and OPCnet, part of the of the Optic Fiber Communications society.
A revolutionary technology that changes how seismic data is recorded, distributed acoustic sensing, or DAS, provides the capability to use fiber-optic cable to measure motion.
With DAS, an interrogator unit sends pulses of laser light down the fiber, then uses interferometry to measure tiny changes in strain from the backscattered light. The seismic waves shrink and stretch the fiber-optic cable, and, in turn, the interrogator measures the motion.
With their interrogator, about the size of a small microwave, the scientists were able to operate about 1,000 seismometers for every 10 kilometers, or about 8,000 seismic sensors for the 80 kilometers from the Moscone Center to Sunnyvale.
In addition to permitting an exponential increase in the number of seismic sensors, the DAS technology provides seismometers for significantly less cost.
"For $160,000, it's possible to purchase 8-16 seismometers," Ichinose said. "But you can buy an interrogator for that same price and have the use of 8,000 seismometers. You receive much more information because you have so many more virtual seismometers.
"The DAS technology is cost effective and it's much simpler logistically to deploy these sensors because you don't need an army of people to bury the seismometers."
Using the DAS technology solves a problem that scientists have faced in seismology since the discipline's earliest years, in Ichinose's view.
"In the past, we haven't had enough seismic sensors to measure the whole wave field, so we have had to interpolate between the sensors to see what is going on," he said. "People are excited about this research opportunity because there is plenty of fiber-optic cable laid all over the Bay Area and everywhere. It's amazing that by sending a pulse of light down a glass fiber, you're able to measure nano-strains of motion."
Before the field experiment started, Ichinose said that he and the team's other scientists were skeptical about the quality of the data they would receive.
"But then when we started recording, we saw vehicle traffic and even trains. The following day, there was a small earthquake, and we were all impressed by the quality of the data our instrument produced."
Interrogators originated about 15-20 years ago in the oil and gas industry for monitoring drilling and fracking operations.
For the future, it is believed that interrogators could find applications in helping to better understand earthquakes and mitigate damage from earthquakes, understand the Earth's structure and detect damage to roads and bridges.
