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Seismic ground vibration

Heard today on the radio that the latest idea is all the buried fiber optic cables can be used to sense and measure quakes. The cables bend, flex, and stretch as the ground heaves. Somehow this can be measured in real time.
Bill D
 
... The cables bend, flex, and stretch as the ground heaves. Somehow this can be measured in real time.

Basically, they send out a train of powerful optical pulses, and listen for the little echoes off tiny imperfections in the fiber. The longer the delay between pulse out and echo return, the farther away the relevant bit of fiber cable. These pulses are sent periodically, and the echoes from each patch are tracked over time, the differences telling one how much each patch of cable has physically moved.
 
Years ago i read that bank vault concrete has buried fiber optic cables. If the robbers break the concrete and break a fiber the signal is lost and the alarm goes off.
Bill D
 
Basically, they send out a train of powerful optical pulses, and listen for the little echoes off tiny imperfections in the fiber.
To add a little bit more. This is Time Domain Reflectometry (TDR). Usually TDR is used to locate big imperfections like breaks in the fiber, bad splices, etc. This seismic sensor application is doing three things that the usual TDR doesn't do. First, they are paying attention to really tiny echoes, many times smaller than what a fiber break will generate. Second, they are building a profile or signature for each piece of cable so they know what the "relaxed" or "normal" position looks like. Third, they are comparing the echo pattern in real time against the signature to identify where the cable is bent differently from its relaxed position.
We are talking really, really tiny and subtle echoes here.
 
Heard today on the radio that the latest idea is all the buried fiber optic cables can be used to sense and measure quakes. The cables bend, flex, and stretch as the ground heaves. Somehow this can be measured in real time.
Bill D

One of the more interesting jobs I did was about 10 years ago, I made about a hundred of these pretty elaborate titanium ROV torque tool front ends that had mass and bouyancy elements attached to them, they clamped on to a subsea fibre cable at specific intervals to damp harmonics caused by seismic activity.
 
To add a little bit more. This is Time Domain Reflectometry (TDR). Usually TDR is used to locate big imperfections like breaks in the fiber, bad splices, etc. This seismic sensor application is doing three things that the usual TDR doesn't do. First, they are paying attention to really tiny echoes, many times smaller than what a fiber break will generate. Second, they are building a profile or signature for each piece of cable so they know what the "relaxed" or "normal" position looks like. Third, they are comparing the echo pattern in real time against the signature to identify where the cable is bent differently from its relaxed position.
We are talking really, really tiny and subtle echoes here.

Yes it is a form of TDR for sure, but it's not traditional TDR. The difference is that the processing of the echoes is coherent, likely done in the complex math (I+Q) domain. Hmm. I've read some of the earlier papers, but it's time to find the actual paper.
 
Heard today on the radio that the latest idea is all the buried fiber optic cables can be used to sense and measure quakes. The cables bend, flex, and stretch as the ground heaves. Somehow this can be measured in real time.
Bill D

I've dug a bit, finding what I think is the underlying paper on how undersea fiber optic seismic sensing works. The paper assumes considerable knowledge of optical physics and mathematics.

Optical polarization–based seismic and water wave sensing on transoceanic cables

It is open access - just click on the red PDF symbol.
 
It is not clear if this is being used today or if it just a theory with some short term experiments having been done. Does it work fast enough to give sesimic warnings? Or is it just after the fact to get results for theoretical studies.
My Shake Alert phone ap is real time warnings up to 15 -20 second advance warnings depending on distance.
Bill D
 
Does it work fast enough to give sesimic warnings?
Evolving from a process where they ship the raw numbers off to a computer to crunch for hours toward real-time analysis. Here's a press release from Caltech discussing a fiber sensor they put into place fast enough to detect aftershocks from the 2018 Ridgecrest earthquake that also mentions that complete processing of the collected data would take months.

Here's a late 2022 page from Pacific Northwest Seismic Network discussing it. The summary down at the bottom includes a list of limitations and problems to overcome.

A more recent 2023 article in Wired says the investigators are (working toward) getting results in real-time.
 
Evolving from a process where they ship the raw numbers off to a computer to crunch for hours toward real-time analysis. Here's a press release from Caltech discussing a fiber sensor they put into place fast enough to detect aftershocks from the 2018 Ridgecrest earthquake that also mentions that complete processing of the collected data would take months.

Here's a late 2022 page from Pacific Northwest Seismic Network discussing it. The summary down at the bottom includes a list of limitations and problems to overcome.

A more recent 2023 article in Wired says the investigators are (working toward) getting results in real-time.
I dug into how existing undersea fiber optic cable can be used to read out seismic motion along the cable, and it's very clever, very complex, and pretty much immune to simple explanation.

It depends on the fact that practical (read, imperfect) optical fibers are birefringent under strain, and causes the optical polarization vector to rotate more or less as a function of strain, which varies along the cable. This is done in a way that cancels some very large noise sources, allowing the otherwise swamped seismic changes to be measured.

Birefringence
 








 
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