Walk around downtown or any central business district of any city and you’ll probably not notice that there’s ground underneath you (well, unless you’re walking through some designated green space). This is probably because you’re out walking the pavement, driving on a road, or inside a building. We don’t often think about the ground beneath the concrete jungle, which doesn’t always remain still. For one thing, this could happen:
Sinkholes occur mainly when soil beneath the surface is dissolved by water; here’s an article from The Independent that goes into it with greater depth (*groan*) and with some nifty-yet-terrifying pictures and links, such as when a sinkhole swallowed up a few collectible Chevrolet Corvettes on display. I will not link to those pictures, it’s just too painful to watch…anyway, these are at least costly and at most deadly and at most deadly, and it’s not just coastal or earthquake-prone areas that may be subject to sinkholes. Even if we can’t save buildings or streets (or sports cars) from being swallowed up by sinkholes, perhaps we could save the people using them. Maybe if we had some kind of early warning system? Fortunately, a team of scientists from the University of California – Berkeley, Stanford University, and the U.S. Army Cold Regions Research & Engineering Laboratory tackle this problem in this week’s Featured Article. Their solution? In short, a bunch of cables in the ground.
OK, it’s a little more involved than that (and I’m mostly salty because I have to clean up around the living room later, and the back of my entertainment center is a nightmare). In what the authors call distributed acoustic sensing (DAS) techniques, fiber-optic cables could be monitored using laser light to measure how much strain the cables are experiencing. Lasers may be necessary because these strains may be very minute, and you would want this to be sensitive to minute changes if you’re going to monitor the strain over time. I know a person walking on a sidewalk probably feels the same way on a DAS arrangement as an ant walking across your arm feels on the muscles on your arm, but multiply that by a large amount like, say, one billion, which is about the number of people who use Shinjuku Station in Tokyo in a year, and maybe then you would be concerned. Well, maybe not as much as if you had a billion ants crawling all over your arm. Sleep tight.
Nightmare fuel aside, the authors attest that this is probably a cheaper solution than how subsurface phenomena are monitored now, particularly with geophones if one wanted to monitor a large area. A DAS system could work with the network of fiber-optic cables already in place that allows us to Internet. One would imagine that South Korea would definitely have an edge on this one, and would probably need it – have you seen Seoul’s urban sprawl?
OK, enough of these asides, how well did their system actually work? Probably well – it did get published and everything. Of course, they had to create their own mini-grid (well, two 100-m wires) built along a couple of roads to capture ambient noise in terms of stretching wires. The authors attest that this process is easily repeatable and sensitive enough that changes in soil near the surface can be captured by monitoring the sudden changes in the strain on the wires. This isn’t a way to stop sinkholes from happening, but at the very least, if something is about to happen, the right people can just cordon off an area and keep you from walking into a sinkhole or probably get you out of your house because a Bad Thing is about to happen.
Or maybe it’s a Good Thing if you’ve condemned your town as a toxic waste dump. How do I know how your life?
Featured Article: Dou S, Lindsey N, Wagner AM, Daley TM, Freifeld B, Robertson M, Peterson J, Ulrich C, Martin ER, Ajo-Franklin JB. (2017). Distributed Acoustic Sensing for Seismic Monitoring of the Near Surface: A Traffic-Noise Interferometry Case Study. Scientific Reports 7:11620. DOI: 10.1038/s41598-017-11986-4.
Featured Image: Public Domain (Author: Free Photos, 2012)