Questions raised over volume of leaked BP oil

Initially, officials claimed that the flow could not be measured. Then, as public pressure for information mounted, they looked for ways to measure it and started producing estimates: at first 1,000 barrels a day, then 5,000, then 12,000 to 19,000, then upward from there.

Now, using a new technique to analyse underwater video of the well riser, scientists say it leaked some 56,000 to 68,000 barrels daily - maybe more - until the first effective cap was installed on 15 July.

Their estimate of the total oil escaped into the open ocean is some 4.4m barrels - close to the most recent consensus of US government advisors, whose methods have not been detailed publicly.

’We wanted to do an independent estimate because people had the sense that the numbers out there were not necessarily accurate,’ said Timothy Crone, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory.

After BP and government officials downplayed the possibility or importance of measurements, a number of scientists, environmental groups and legal experts pointed out that the information was needed to determine both short- and long-term responses, as well as monetary liability.

The new study divides the flow rate into two periods: 22 April to 3 June, when oil spurted from a jagged break in the riser, and after 3 June, when the riser was cut and oil temporarily spewed into the ocean unimpeded.

Crone and Lamont marine geophysicist Maya Tolstoy, used a visual analysis technique that Crone recently developed called optical plume velocimetry to make their estimate. They say video from the earlier period indicated a flow of about 56,000 barrels a day. After the pipe was cut, they say the rate went to about 68,000.

After accounting for time elapsed, the authors subtracted 804,877 barrels collected by BP at the site, to come up with a total of 4.4m barrels that escaped.

Given the study’s stated 20 per cent margin of error (plus or minus), this roughly agrees with the US federal government’s Flow Rate Technical Group’s most recent comparable estimate of 4.1m barrels (after subtracting the oil collected by BP).

Crone and Tolstoy say their conclusions rest on just a few short clips of high-resolution video - almost all that has been released by BP and the government so far, and made available by members of Congress to the scientists. The researchers point out that the flows could have varied day to day. And the analysis did not include video of several other leaks from smaller holes further up the pipe, which are thought to have grown with time; thus, the true figures may be larger said Crone.

The researchers said they hope they and others will be able to refine their estimates if the government and BP release more video and other information to independent researchers.