The Society for Exploration Geophysicists (SEG) Reginald Fessenden Award is given annually “in recognition of specific technical contributions to exploration geophysics, such as an invention or a theoretical or conceptual advancement, which merits special recognition.”
The SEG award cites Ajo-Franklin for making “notable and significant contributions to expanding the application of Distributed Acoustic Sensing (DAS), which uses telecom fiber for applied geophysics research in near-surface imaging and seismology. His work has had a substantial impact in advancing this growing field, which is increasingly relevant to environmental and energy-related studies.”
How it works
The DAS technique uses fiber optic cables that are deployed in deep bore holes, laid along or buried under the ground, and strung across the sea floor. Think of it as a long wire that acts like a continuous microphone, one that can record a variety of signals, from strong ground motions (like earthquakes) to subtle background vibration caused by ocean waves or vehicles, all in real time using light.
“Wide-spread application of DAS is relatively recent. I started using DAS around 2010 in the context of borehole sensing, where you're trying to make measurements of the seismic waves in deep wells for imaging structures or changes in reservoirs. The last 5 years have seen an explosion of applications in different domains.”
The cables are connected to an instrument called an interrogator that both sends and receives pulses of laser light. Variations in the fiber itself causes scattering in the light pulses; changes in this scattering can be processed and converted to measurements of strain or “stretch” in the fiber. These measurements are then processed to construct a picture of the proximal subsurface.
Discovering the potential of ‘dark’ fiber
“One of the areas my group has worked on is expanding DAS applications beyond just being a tool for borehole geophysics towards broader use on the surface, for environmental monitoring and earthquake seismology. That basically had not been done at scale, and so around 2013-2014, we were one of a few groups who pushed for DAS measurements on surface arrays,” says Ajo-Franklin.
For DAS surface arrays, fiber needs to be buried just below the ground surface. “We were in Alaska, deploying surface fibers with one of my graduate students, Nate Lindsey. The cables themselves are very thin, but to get them to couple with the ground, you often deploy them in a shallow trench. We were having lots of problems with roots and gravel and then having to backfill them.”
Across the road, the team noticed workers doing telecom repair. That sparked a realization; “There's all this fiber underground for communications already. Why can't we use that for seismic acquisition?”
The infrastructure and complexity of deploying fiber is expensive, so telecom companies anticipate expansion of users and utilities and lay what is called ‘dark fiber’; extra fiber that is not yet ‘lit’ for consumer purposes.
“The idea was to find fiber which is in the ground and not being utilized at any given moment. Our first big test of the idea was using unlit telecom fiber near Sacramento, CA, and then recording a bunch of data from an offshore cable which went out from Monterey Bay.” Proof that it worked appeared in Science (2019), around the time Ajo-Franklin transitioned from Lawrence Berkely National Laboratory to Rice University.
The future of DAS
One of the advantages of glass fiber is its robust physical properties, flexibility of deployment and it’s spatially continuous data collection. This means it can be used in a wide range of environments and places that are difficult to access.
“It's becoming a much bigger field than it was 10 years ago when only three or four academic groups in the US were working on it. Now there are thousands of people who are doing this kind of research around the world, in every type of environment.”
Among the future applications Ajo-Franklin looks to see improvements is work on transoceanic cables. “For a long time, there were limitations to how far you could actually make these measurements. As a function of distance, maybe you could make them at 50 kilometers, but no farther, which means that in a marine setting you can't really look at things like mid-ocean ridges. There's been a lot of work to find ways of making measurements over much longer distances to extend these seismic networks farther into the ocean, into places where we have basically no sensors.” Ajo-Franklin says the sky is the limit, with some groups even looking to deploy fiber on the Moon and Mars.
Closer to home, Ajo-Franklin’s students are helping expand DAS environmental applications on Rice Campus. “I'm still really interested in hydrologic and near-surface monitoring. My PhD student Valeriia Sobolevskaia is using fiber DAS to monitor local aquifers, soil moisture, and the inputs to the hydrologic cycle. We really can build anything with DAS that you can do seismology for. It's really amazing. It's one of the more flexible and useful tools in geophysics at the moment.”