Distributed Acoustic Sensing (DAS) has been embraced by the global seismology community as a transformative tool for studying Earth systems. It can change the way we measure a variety of signals, from ground motion to animal sounds, in real time. But how does it work, and how can it help us detect these things in a new way?
How it works
The DAS technique uses a long, fiber optic cable that is laid along or buried under the ground. Think of it like a long wire with many microphones attached to it. The fiber optic cables are connected to an instrument called an interrogator that repeatedly sends pulses of light, like a laser, through the cable. At imperfections or variation in the fiber, this light pulse is disrupted and goes in multiple directions. Some of that light will go back to where it came from (towards the interrogator) for processing and analysis. This is called Rayleigh backscattering. Let’s say seismic waves from an earthquake reach the cable. This seismic energy deforms the cable as it passes through it – including the imperfections responsible for backscattering.
When the data is processed, we can look at the phase shift of the light wave that comes back compared to what it looked like before the cable was deformed. We can use information about the wave, like amplitude, frequency, and arrival time, to see the location in the cable where the strain occurred and turn that into measurements of seismic waves. Using data from multiple areas along the cable (or multiple cables) over time can then allow you to determine the location of the earthquake.
These signals that DAS picks up can range in forms, and we are able to figure out what it is based on the frequency, duration, and other characteristics. For example, for a fiber optic cable on the seafloor, a blue whale call has a frequency of 10 – 40 Hertz and lasts around 10 – 30 seconds, whereas seismic waves cover a much wider frequency range and follow distinct patterns. DAS can detect many things, including drilling activity, ships, or marine life in the ocean, as well as seismic activity, landslides, or even movements of glacial ice.
Why use DAS?
In the past we have used many instruments that make similar measurements at a single point, however none match the unique characteristics of DAS. Hydrophones deployed on the seafloor, tethered to floats, or towed by a boat are commonly used in the ocean. While great for one-shot measurements, they aren’t a spatially continuous measurement like DAS. The cable itself is the sensor in the case of DAS, so it is able to pick up any disturbance along the length it covers, which can be tens of kilometers. To collect similar data with hydrophones, you would need a huge number to cover every single little point that a DAS cable could. Because of all of these sensor points of the DAS cable it can be more sensitive to movements and disturbances than traditional hydrophones and can record higher signal frequencies, giving it more applications.
DAS does output a large volume of data (often terabytes) compared to conventional seismometers, and while figuring out how to effectively process this data is an ongoing challenge, these unique and flexible datasets have the potential to yield new insights.
Applications
DAS can be applied to monitoring hazards like earthquakes, volcanic eruptions, and landslides. Seafloor cables, in particular, can provide data on offshore earthquakes where seismometers are rarely present. This could help improve the performance of earthquake and tsunami early warning systems.
It can also be used to study other Earth systems. Research on glaciers, for example, has utilized DAS to listen for lurching movements in the ice or to image structures hidden below the surface. DAS cables have even been deployed in streams to study flow behavior and sediment transport.
There are generally two ways DAS can be deployed. Existing telecommunications fiber, like on the seafloor or on land, can sometimes be tapped into to gather the data. Another option is laying out a length of cable specifically for use in a temporary project. Cables can be short or tens of kilometers long and can be located in shallowly buried trenches, in boreholes, or in some other configuration required by a project.
The National Science Foundation funded a Distributed Acoustic Sensing (DAS) Research Coordination Network (RCN) beginning in 2020. The original RCN proposal included mainly geophysical and engineering topics, but was later expanded to include more disciplines as more research was being done. An even broader scope of DAS applications can be expected in the future that includes computer science, aerospace, medical fields, atmospheric sciences, climate change, and biodiversity. DAS could become the go-to tool for high-resolution data collection in a wide variety of situations.
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