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Sep 23, 2010 3:01 PM

Accurately Estimating How Much Oil Spilled

WASHINGTON (AP) - After several missteps, the federal government
finally got it right, accurately estimating how much oil spilled
into the Gulf of Mexico, an independent scientific study found.
Nearly 185 million gallons of oil spilled from the broken BP
well into the Gulf of Mexico this summer, according to a study by
two Columbia University researchers who made their estimates based
on video of the oil spewing from the well.
The federal government's final estimate was a shade more than
172 million gallons. The Columbia researchers' estimate is 12.6
million gallons more than the federal figure. However, because it's
so difficult to get a precise estimate, there is a large margin of
error for both the government figure and the Columbia number. The
margin is so large that the two estimates essentially overlap, the
researchers said. Their study was published online Thursday in the
journal Science.
U.S. Geological Survey Director Marcia McNutt, who oversaw
federal estimates of the spill size, called the Columbia study "a
completely independent and unbiased verification of the government
result."
Tim Crone of Columbia, the lead researcher who calculated his
estimate based on detailed flow formulas determined by watching
video of the leak, "Our numbers overlap, so sure, we agree."
But Crone said he is more confident in his estimate because it
went through the rigorous independent peer review required to be
published in a respected journal. He added that it is hard to
compare in depth to federal numbers "because few details of their
methods have been released."
Crone used a different technique to study video than most
scientists who analyze flow rates from video. Usually, scientists
track particles and calculate a speed as they travel across a
screen, sort of like watching a car race down a highway. But in
this case the particles were hard to track, so Crone used a
technique he's been working on for a decade. Crone studies
individual points in the video - all the points - and watches their
changes in color and texture. For this study, Crone reviewed video
from two dates, May 15 and June 3, and extrapolated for the spill
estimate.
The Columbia estimate has a margin of error of 20 percent, so
the spill would be somewhere between 148 million and 222 million
gallons. The federal estimate had a 10 percent margin of error, so
the spill would be somewhere between 155 million and 189 million
gallons.
Federal estimates of how much oil spilled and where it has gone
have been criticized by both scientists and Congress because the
agencies involved have not provided detailed information on how
they calculated their figures. But the government numbers were not
based solely on traditional video analysis; they also factored in
measurements of pressure and estimates from photos and infrared
images taken from airplanes and satellites.
Purdue University engineering professor Steve Wereley, who at
first was critical of federal efforts and then joined the
government team, said he trusts the government work as more
comprehensive than Columbia's.
The federal government struggled mightily to calculate the
spill's size at first and was sharply criticized for clinging to a
too-low estimate for weeks.
Ian MacDonald, a Florida State University oceanographer, said
that by his count it took eight tries for the federal government
finally to get its calculations right.
Initially, federal officials adopted BP's estimate that 42,000
gallons a day was gushing out. They upped it to 210,000 gallons a
day and stuck with that number for weeks. Then the government set
up a special team of experts to estimate the spill size; that group
came up with a range of estimates that was blasted by independent
scientists as still too low.
Finally in mid-June, about two months after the oil rig accident
that caused the deepwater gusher, the federal government said the
well could be leaking as much as 2.4 million gallons a day. And in
August, following the shutdown of the spill the previous month,
federal scientists estimated a total leak of 172.2 million.

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