SCIENCE OBSERVER
Sea Change
David Schneider
Oceanographers charting the sea's ever-changing configuration of
water masses struggle with a fundamental problem: The traditional
method—lowering instruments into the briny depths and
recording the temperature and salinity at various levels—is
inordinately time-consuming. Indeed, the features of interest often
evolve faster than they can be tracked in this way. But
oceanographers might soon be able to obtain better snapshots of the
water's internal structure with a novel approach: seismic surveying,
a technique used routinely to map geologic strata beneath the ocean floor.
Marine geologists have for decades exploited this tactic, which
involves recording the acoustic echoes that bounce back from the
various buried layers after a loud blast is set off near the
surface. The primary application of such reflection seismology is
exploring for offshore oil and gas deposits, but academic scientists
also use the method for various geological studies.
One such scholarly investigation was carried out in 2000 using the
Columbia University research vessel Maurice Ewing, near the
Grand Banks off Newfoundland. The chief scientist on board was W.
Steven Holbrook, a geophysicist at the University of Wyoming. During
this particular survey, Holbrook noticed something curious on a
monitor being used to check the quality of the data being collected:
"There were reflections coming back where they shouldn't have
been," he recalls, referring to the echoes he saw emanating not
from the seafloor or the buried layers below but from within the
water itself.
Holbrook, being busy enough with his geological studies, relegated
this observation to his "crazy ideas file." But after he
observed the same phenomenon two years later during another seismic
survey on the Ewing, he assigned Pedro Páramo, a
graduate student in his group, the task of processing the data to
accentuate reflections coming from the body of the ocean itself.
"Within a few days he had the first water-column image,"
Holbrook says, adding, "I just about fell off my chair: There
were these spectacular structures in that data."
At that point Holbrook decided he had better consult with an
oceanographer. His university did not, however, employ anyone in
that capacity. (Oceanography is not a popular area of study in
Wyoming.) So Holbrook sent some feelers to Woods Hole Oceanographic
Institution, where he had once worked, hoping that a little name
recognition might help him to be taken seriously. It did. Raymond W.
Schmitt, a physical oceanographer there, was quite impressed with
what could be seen in Holbrook's seismic-reflection profiles of the
sea near the Grand Banks (an area of much oceanographic interest).
Schmitt immediately saw the advantages of probing the ocean in this
way. Instead of tediously trying to survey the sea by lowering and
raising oceanographic instruments on a cable—a tactic that
limits a ship's speed to about a knot—oceanographers like
himself could image a two-dimensional slice of the ocean by towing
the appropriate seismic equipment behind the survey ship, which can
then travel at several knots' clip. That gives almost an
order-of-magnitude reduction in the time needed to perform a survey,
which, in Schmitt's words, constitutes "a remarkable quantum leap."
Holbrook and Schmitt collaborated in further checking the seismic
results and interpreting their oceanographic significance,
eventually publishing (with Páramo and Scott Pearse, another
Wyoming graduate student) a paper in Science. In the course
of preparing that article, Holbrook discovered two earlier efforts
to use seismic-reflection data to map goings-on within the ocean.
Joseph D. Phillips, for one, had published on this topic in 1991,
when he was at the University of Texas at Austin. Phillips, who is
now retired, says that the use of reflection seismology to image the
ocean has a long history, in fact, but not one that many people know
about. This early work was done in support of a Cold War–era
Navy program called SOSUS (for SOund SUrveillance System), which
took advantage of a relatively deep layer within the ocean that
tends to trap the sound energy coming from ships and submarines. So,
as Phillips explains, "much of that work was classified."
Holbrook's Science paper brought this promising method out
of the shadows last year. Perhaps soon oceanographers will be
competing with geologists for funding to carry out marine
seismic-reflection surveys. But even before that eventuality comes
to pass, oceanographers will want to take advantage of the vast
archive of seismic-reflection data already obtained. Getting hold of
the appropriate data sets might, however, present a challenge, even
for public-domain results collected in the course of academic
studies. Tom Shipley, a geophysicist the University of Texas at
Austin, notes that because the academic institutions that acquired
these data originally had limited resources for archiving them, the
old tape libraries tend to be in poor condition. Fortunately, access
to such archives should soon become open to all: The U.S. National
Science Foundation is currently funding an effort by Shipley and
Suzanne Carbotte at Columbia University's Lamont-Doherty Earth
Observatory to recover and organize seismic-reflection data
collected aboard academic research vessels in past years and then
make these results freely available over the World Wide Web.
Oceanographers intending to dig through this small mountain of
seismic-reflection data looking for scattered gems will surely be
interested also in what the oil and gas industry has to offer,
something Holbrook regards as "a potentially huge gold
mine." Since the 1980s, commercial seismic vessels have been
conducting "3-D" surveys, whereby they take data along
multiple parallel lines simultaneously. (The first vessel with this
capability will not join the U.S. academic fleet for a couple of
years yet.) Collecting the same number of profiles using
conventional oceanographic techniques would take not one but two
orders of magnitude more time. So 3-D seismic data could provide
oceanographers not just with a snapshot of an evolving oceanographic
feature but with the equivalent of a high-speed stroboscopic image.
How might academic oceanographers gain access to such industry data?
Because data on the seismic properties of the ocean itself have no
clear commercial value, the cost should be minimal. Locating the
data might present the greater hurdle. David Bamford, technical
vice-president for exploration at British Petroleum until his
retirement last year, notes that because major oil companies each
typically own only a patchwork of seismic-survey results, an
oceanographer interested in an extended region would have to do
considerable "stamp collecting" to piece together full
coverage. Bamford suggests that the better strategy would be to
approach the contracting companies that do most of this surveying.
Unlike the money-strapped academic institutions with their
deteriorating collections of old tapes, these contracting companies
can be relied on to have their data libraries in good shape: As
Bamford says, "It's their business."