crust boundary. Probing such deep regions, tens or hundreds of kilometers
down, requires arrays with sensors spaced
similar distances apart—such as the U.S.
Transportable Array, a system of 400 seismometers, spaced at intervals of 70 kilometers, that has been marching eastward
across the continent over the past decade
(Science, 14 June 2013, p. 1283).
Much denser arrays, however, can sample
higher frequency ambient noise to answer
questions about the upper 10 kilometers of
crust. “The potential to
put out thousands of sensors over a very small area
lets you resolve structure
in ways you couldn’t before,” says Robert Detrick,
president of the Incorporated Research Institutions for Seismology
(IRIS) in Washington,
D.C., which operates the
Transportable Array. Until recently, only oil and
gas companies could afford to deploy such arrays. “We in the academic
community thought a big
experiment was putting
out a few dozen sensors or maybe a hundred sensors,” Detrick says.
Robert Clayton, a seismologist at the Cal-
ifornia Institute of Technology in Pasadena,
was in the right place at the right time to
take advantage of an industry-scale array.
In 2011, an oil company, Signal Hill Petro-
leum, surveyed a small oil field in Long
Beach, California, deploying 5300 ZLands
and a fleet of “thumper trucks” to gener-
ate seismic signals. In between thumps,
however, the seismometers also caught the
sounds of cars on the Los Angeles freeways
and the Pacific surf. The data would have
normally been tossed out as junk, but Clay-
ton thought he could make use of it, and the
company shared it with him.
Ambient noise, Clayton realized, makes
strong surface waves—waves confined to
shallow rock layers, in contrast to the deep-diving body waves associated with earthquakes
and active shots. Analyzing the Signal Hill data,
he found that he could
use these surface waves to
image the top kilometer
of crust. The result was
valuable to the company,
serving as a sort of corrective lens for the deeper
part of the company’s survey, he says. And because
the upper kilometer of
crust has an outsize effect on ground shaking
in an earthquake, the information should also be
extremely useful for the city of Long Beach
and its engineers. “I think it’s going to revolutionize how we do seismology in urban
areas,” he says.
Clayton’s success has sparked wide interest: A half-dozen other universities have
asked for the Signal Hill data. In the past
3 months, IRIS and the American Geo-
physical Union have each hosted work-
shops on the new techniques. New data,
from a 5400-node survey of shale and lime-
stone formations near Sweetwater, Texas,
will be uploaded to IRIS databases later
this month, freely available for scientists
wanting to experiment with the passive
techniques. In June, Yehuda Ben-Zion of
the University of Southern California in
Los Angeles packed 1100 ZLand sensors
into less than a square kilometer along
an active portion of the San Jacinto fault,
a major offshoot of the San Andreas fault.
With sensor spacings as little as 10 meters,
Ben-Zion hopes for insights into how earth-
quakes get started and how properties of
the rock along the fault change before and
after an earthquake.
Oil and gas companies are getting more
comfortable with the passive techniques,
and they too would like to avoid some of
the hassles of permitting shots and thumps,
says Dan Hollis, vice president of market-
ing and technology at Nodal Seismic in Sig-
nal Hill, California, which performs dense
surveys for industry clients. “I see this as
the way that the oil exploration industry is
going to go.”
As industry money pours in, sensors like
the ZLand are getting better and cheaper.
For now, Hollis is happy to lend his collec-
tion of ZLands to scientists like Schmandt
when they aren’t being used by industry.
And Schmandt is happy to take advantage.
“If someone like me can scrap this thing to-
gether in the past few months, we should be
capable of some cool things as a community
in the next few years,” he says. ■
15 AUGUST 2014 • VOL 345 ISSUE 6198 721 SCIENCE
Robert Clayton with sensor.
have recorded ambient
noise around the Mount St.
Ears on the Earth
A dense array of
seismometers has allowed
detailed imaging below
Long Beach, California.
Source: Robert Clayton/Caltech 1000 meters
Phase A, 5400 sensors
Phase B, 4000 sensors