Based on SkyTruth's latest satellite observations today of the size of the oil slick and published data on the thickness of floating oil at sea that produces a visible sheen (1 micron, or 0.000001 meters) we think the official estimate of the spill rate from the damaged well has been significantly too low.

Immediately after the Deepwater Horizon rig sank on April 22, the Coast Guard estimated that the well was leaking 336,000 gallons (8,000 barrels) of oil per day. But for the past few days they've estimated the rate at 42,000 gallons (1,000 barrels) per day. We think it's actually a lot closer to their original estimate.

We have a visible oil slick covering 2,233 square miles (5,783 km2). Given a minimum thickness of 1 micron (see chart below), that is 5,783 cubic meters of oil, or 1,527,706 gallons (36,374 barrels). The blowout happened almost 7 days ago on April 20. That's at least 5,000 barrels of oil per day - assuming none of it was consumed during the two-day fire that raged before the rig sank on April 22, and none has been collected by the response crews that have been working diligently for days.

Our calculation also assumes the entire slick is a sheen barely thick enough to be visible. Yet the images we've seen so far, especially the ALI image taken on April 21, suggest a strong spectral response from the oil slick, and that in turn suggests a much thicker slick. Today a BP exec claimed that 3% of the slick was 100 microns thick, and the remaining 97% is only one or two molecules thick. We're skeptical: 1 micron is the published, generally accepted lower limit for a visible sheen at sea:

CONCAWE chart of thickness and visible appearance of floating oil at sea. From a Minerals Management Service report, Real-time Detection of Oil Slick Thickness Patterns with a Portable Multispectral Sensor.

So if 3% of today's slick (173.5 km2) is 100 microns thick, and the remainder (5,609.5 km2) is 1 micron thick that's a total of 22,960 cubic meters of oil: 6,065,390 gallons. That's right: more than 6 million gallons spilled into the Gulf of Mexico so far.

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Dr. Ian MacDonald at FSU just produced a new spill-size estimate based on the US Coast Guard aerial overflight map of the oil slick on April 28, 2010. The bottom line: that map implies that on April 28, there was a total of 8.9 million gallons floating on the surface of the Gulf.

That implies a minimum average flow rate of slightly more than 1 million gallons of oil (26,000 barrels) per day from the leaking well on the seafloor. Since we're now in Day 11 of the spill, which began with a blowout and explosion on April 20, we estimate that by the end of the today 12.2 million gallons of oil, at a minimum, have been spilled into the Gulf of Mexico.

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