18 Scientific American, March 2020 Illustration by Thomas FuchsADVANCES
industry a decade ago, this technique—
known as distributed acoustic sensing
(DAS)—has recently infiltrated the scienc-
es. “The [DAS] community has just explod-
ed in the past couple of years,” says Jona-
than Ajo-Franklin, a geophysicist at Rice
University. A workshop organized by the
American Geophysical Union last Decem-
ber included scientists who had used the
technique to image glaciers, monitor thun-
derstorms and peer into the deep ocean.
One major advantage to DAS is that
fiber-optic cables can be many kilometers
long, and a single one can act like a net-
work of thousands of sensors covering
every meter along its path. Conversely,
conventional seismometers record ground
motion only at a single point—a major
roadblock to imaging the earth’s interior.
When Mount St. Helens started rumbling
ahead of its catastrophic 1980 eruption, for
example, the fact that there was only one
nearby seismometer meant that scientists
could not tell if the quakes were actually
caused by the awakening volcano. “Think
of it like streetlights,” says Nathaniel Lind-
sey, a geophysicist now at Stanford Univer-
sity. “If you only have a few streetlights to
illuminate the entire volcano, it’s not going
to work that well.”
A second benefit is that fiber-optic
cables already crisscross the world. Where-
as some sites, such as Taku Glacier, require
new cables, others—locations from cities
to the bottom of the sea—have unused
cables or ones that can be adapted for
DAS. Much of this availability stems from
the dot-com boom of the 1990s, when
telecommunications companies installed
long stretches of cables; some of them,
known as dark fiber, remain untapped. So
scientists can simply connect one end to
an “interrogator” unit, which fires a stream
of laser pulses toward the other end and
monitors any backscatter—and voilà, a
new seismic wave–sensing network is
ready to go.
Last year Tieyuan Zhu, a geophysicist
at Pennsylvania State University, adapted
unused cables in the college’s existing
fiber network to search for subtle vibra-
tions below campus. He was surprised to
find multiple rumbles in his data on the
night of a thunderstorm. Although scien-
tists have long known that air vibrations
from loud noises can rattle the earth’s
surface, it was unclear whether the new
technique could detect such “thunder-
quakes.” But when Zhu synchronized his
results with data from nasa, there was
no question. “I think there’s a big potential
to ‘light up’ the urban area using this tech-
nology,” he says. “And not just to monitor
earthquakes but also geohazards [suchas landslides or tsunamis] and weather.”
Other scientists are eyeing more
remote targets. For a paper published last
November in Science, Lindsey, Ajo-Franklin
and Craig Dawe of the Monterey Bay
Aquarium Research Institute attached an
interrogator to a 20-kilometer fiber-optic
cable typically used to transmit data from
scientific instruments on the seabed off
Monterey Bay. The system was down for
maintenance, giving the scientists a
chance to look for vibrations. In just four
days they mapped multiple underwater
fault zones and characterized seafloor
trembling caused by waves above. More
detailed seafloor maps will help scientists
make better predictions about earthquakes
and submarine volcanoes—both of which
can cause life-threatening tsunamis.
Then there is the glacier work, for
which Labedz and her colleagues have
transformed a single cable into 3,000 seis-
mic sensors. Early results show a five-hour
stretch with 100 icequakes—many likely
caused by meltwater forcing open cre-
vasses within the glacier. Labedz’s aca-
demic adviser Zhongwen Zhan, a seismol-
ogist at Caltech, hopes to one day place
permanent fiber-optic cables in Greenland
or Antarctica to help researchers learn
more about how glacier melt driven by cli-
mate change contributes to sea-level rise.NEUROSCIENCEA Helpful Hiss
White noise may help listeners
distinguish between similar soundsScientists often test auditory processing
in artificial, silent settings, but real life usu-
ally comes with a background of sounds
like clacking keyboards, chattering voices
and car horns. Recently researchers set out
to study such processing in the presence
of ambient sound—specifically the even,
staticlike hiss of white noise.
Their result is counterintuitive, says Tania
Rinaldi Barkat, a neuroscientist at the Univer-
sity of Basel: instead of impairing hearing, a
background of white noise made it easier for
mice to differentiate between similar tones.
Barkat is senior author of the new study,
published last November in Cell Reports.It is easy to distinguish notes on oppo-
site ends of a piano keyboard. But play two
side by side, and even the sharpest ears
might have trouble telling them apart. This
is because of how the auditory pathway
processes the simplest sounds, called pure
frequency tones: neurons close together
respond to similar tones, but each neu-
ron responds better to one particular fre-
quency. The degree to which a neuronresponds to a certain frequency is called
its tuning curve.
The researchers found that playing
white noise narrowed neurons’ frequency
tuning curves in mouse brains. “In a simpli-
fied way, white noise background—played
continuously and at a certain sound level—
decreases the response of neurons to a
tone played on top of that white noise,”
Barkat says. And by reducing the number
of neurons responding to the same fre-
quency at the same time, the brain can
better distinguish between similar sounds.
To determine whether the mice could
differentiate between tones, the research-
ers used a behavioral test in which the
rodents had to react to a specific frequen-
cy. Like humans, the mice easily recognized
very different tones and struggled with
similar ones. But with white noise added,
the mice could better tell similar tones
apart. The researchers investigated further© 2020 Scientific American