BP / Gulf Oil Spill – How Big Is It?

Quite a lot bigger than the estimate being uncritically quoted throughout the media of 210,000 gallons (5,000 barrels) per day. That was the last “official” estimate made by NOAA and accepted by the Coast Guard back on April 29 (see timeline below). Before that, the Coast Guard estimates ranged from 336,000 gallons (8,000 barrels) per day, to zero, to 42,000 gallons (1,000 barrels) per day. None of these estimates has been publicly explained or substantiated. And on May 1, the Coast Guard and NOAA stopped trying to estimate the spill rate, with Admiral Thad Allen saying, “Any exact estimate is probably impossible at this time.”

But the media continues to report that oil is leaking into the Gulf at 5,000 barrels per day. At SkyTruth we estimate the spill rate is closer to 1.1 million gallons (26,500 barrels) per day, based on the size of the slick on satellite images and Coast Guard maps, and thickness estimates derived from visual descriptions of the slick. That puts us at a total spill of 21 million gallons so far.

Considering that there are deepwater oil wells in the Gulf of Mexico capable of producing >1.26 million gallons (30,000 barrels) of oil per day under controlled flow rates; and that the oil slick continues to grow in size even though it’s been under attack 24/7 by skimmer vessels, burning, chemical dispersants, and natural processes; our estimate seems quite conservative.

Why is it important to get this number right? This is about more than just liability, or PR. You can bet that our future response capacity is going to be overhauled and retooled based on this spill. If we low-ball the spill amount and rate, we run the risk of designing an inadequate new spill-response system that is doomed to fail the next time something this big occurs.

Here’s the timeline of spill estimates:

  • 4/22 – Deepwater Horizon rig sinks; Coast Guard estimates “up to” 8,000 barrels per day (bpd) is leaking – source
  • 4/23 – Coast Guard reports no leaking at all from the damaged well – source
  • 4/24 – Coast Guard reports well is leaking, estimates 1,000 bpd – source
  • 4/25 – BP repeats 1,000 bpd estimate – source
  • 4/27 – 1,000 bpd still the official Coast Guard and BP estimate – source
  • 4/27 – SkyTruth and Dr. Ian MacDonald publish first estimate that spill rate is 20,000 bpd – source
  • 4/28 – NOAA weighs in and raises the official estimate to 5,000 bpd based on aerial surveys “and other factors”; BP disputes this higher estimate – source
  • 4/29 – Coast Guard and NOAA repeat their estimate of 5,000 bpd – source
  • 4/29 – BP’s Chief Operating Officer admits new estimate of 5,000 bpd may be correct; “He said there was no way to measure the flow at the seabed and estimates have to come from how much oil makes it to the surface” – source
  • 5/1 – SkyTruth and Dr. Ian MacDonald publish revised estimate of at least 26,500 bpd – source
  • 5/1 – Coast Guard Admiral Thad Allen “acknowledged there was no way really to know the extent of the leak” – source – and stated that “Any exact estimate is probably impossible at this time” – source
  • 5/1 – Coast Guard and NOAA cease estimating the rate of the spill. BP continues to use 5,000 barrels per day as their estimate of the spill rate.

Gulf Oil Spill – Radar Satellite Image May 8, 2010

Our friends at CSTARS just posted this stunning image. Taken by the Canadian-operated radar satellite, RADARSAT-2, it clearly shows oil slicks and sheen spread across a wide area (about 5,025 square miles, or 13,000 km2) in the Gulf of Mexico early this morning (May 8):

RADARSAT-2 image of the Gulf of Mexico, May, 8, 2010 – Source: CSTARS

We’ve added some analysis to help you armchair interpreters. Oil slicks look dark on radar images because the oil reduces the surface tension of the water, dampening (smoothing out) the small wavelets that normally roughen up the surface of the ocean. But any smooth water will look dark on radar, so not all dark patches are caused by oil:

RADARSAT-2 image with SkyTruth analysis, May 8, 2010.


UPDATE 5/8/10 7:00 pm – The first attempt to place a 70-ton containment box over the main leak failed today; the box has been moved aside and is being troubleshooted, and tar balls have begun to wash up in Alabama. The leak is continuing unabated, at a rate we calculate to be about 1.1 million gallons (26,500 barrels) per day – five times higher than the last official estimate (5,000 barrels per day) the Coast Guard made, before they quit making estimates a few days ago, admitting they had no accurate way to estimate the spill rate.

We estimate more than 18 million gallons of oil have spilled so far.

Now we can do a heads-up comparison of the RADARSAT-2 image with this MODIS/Terra image taken about four hours later. Still some clouds obscuring portions of the slick; observable slick and sheen spans about 4,100 square miles (10,624 km2). Fresh oil is apparent around the location of the leaking well; it seems to be carried to the southeast, then gets caught up in a counterclockwise gyre in the currents:

Gulf Oil Spill – May 7, 2010

Here’s a look at some of the limitations of the NASA / MODIS satellite imagery. Today’s Aqua image, like those of the past few days, has suffered from problems that make it impossible to map the full extent of the slick: clouds, haze, nearshore turbidity, and plumes of sediment issuing from the Mississippi River outlet channels (the Terra image today is even worse):

Only the thickest part of the slick is visible, as brown ropy-looking stringers in the vicinity of the leaking well. We think thinner slick and sheen could actually be spread across a much larger area. Compare with this map published by NOAA, showing oil slicks from May 2 to May 6 as mapped by aerial surveys, and May 7 (orange) as predicted by their oil spill trajectory model:

NOAA map showing oil slicks May 2 – May 7, 2010.

Gulf Oil Spill – More Imagery Coming

We’re getting requests from folks wanting to see the latest satellite images of the oil slick in the Gulf of Mexico. It’s been cloudy the last couple of days, so the NASA /MODIS imagery hasn’t been very good; the latest we posted was from the afternoon of May 2. But observers tell me it’s clear and sunny in the Gulf today, so we’re hoping the MODIS images this afternoon will give us a clear look at the entire slick. We’ll process and post those images as quickly as we can. Follow us on Twitter – we tweet as soon as we upload anything new – and keep checking here and in the SkyTruth image gallery.

There is a gallery of radar satellite images here – radar cuts through the clouds and haze to show the ocean surface, and the slicks are clearly visible on the May 3 image. The drawback – this image only covers part of the slick. We hope to see some better coverage in coming days.

We’d like to see a systematic, Gulf-wide monitoring program established, so that when emergencies like this happen the stream of images from multiple satellite systems is immediately available to all who want to see it – including folks like the Bay St. Louis-area local emergency response guy who just called SkyTruth for assistance.

MODIS satellite image from the afternoon of May 4, 2010.

UPDATE 5/4/10 6:30 pm – And here it is. Today’s MODIS / Aqua image features a break in the clouds (just barely) to reveal much of the oil slick. Fresh upwelling oil is apparent around the location of the leaking well. Long tendrils of slick and sheen stretch to the east and southwest; the total area of slicks and sheen, possibly including patches of open water, is 3,260 square miles. Nearshore, things get complicated: there are pale bands of turbidity, probably caused by the recent stretch of high wind and waves; and a few dark streaks and elongated patches trending northeast that we interpret as low-wind zones (wind shadow, the result of light winds from the northeast this afternoon). But there could be patches of oil slick obscured by these features. To the south, heavy cloud may also be hiding some of the slick from this ongoing spill.

Gulf Oil Spill – New Spill Calculation – Exxon Valdez Surpassed Today

Figure 1. US Coast Guard map showing size and appearance of oil slick on April 28, 2010.

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. This map shows the slick covering 1,786 square miles (4,627 square kilometers). The bottom line: on April 28 there was a total of 8.9 million gallons floating on the surface of the Gulf.

That suggests a minimum average flow rate of slightly more than 1.1 million gallons of oil (26,500 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.

The oft-quoted official estimate for the Exxon Valdez spill is 11 million gallons, although some think that is the lower limit of the likely range. It appears that we’ve just set a very sad new record.

Here is Dr. MacDonald’s calculation:

Deepwater Horizon spill estimates derived from USCG fly-over data (28 April 2010)

These estimates of the total volume of oil released by the Deepwater Horizon spill were derived from the USCG fly-over map (Figure 1). The map was geo-referenced in Arc Map and the areas of each of the slick types (dull oil streamers, etc) were measured with a planimeter tool. Thickness estimates for each slick classification were taken from the BONN guidelines as published in the NOAA field manual (Figure 2). Conservative values were used for each slick types. Note that the predicted average layer thickness are still very small.

Figure 2. Chart of oil thickness and appearance.

A human hair is approximately 100 µm (microns). The main slick, which corresponds to
the cross-hatched area was assigned a low value of 0.5 µm. We calculate a total volume of oil for this slick as 8.94 million gallons (212,000 barrels) (Figure 3). Considering that the oil in the water on April 28 has been deposited since the blowout and explosion on April 20, the flow rate should be on the order of 26,500 barrels per day. Some fraction of the total oil released will have been evaporated or emulsified and sunk in the time since the spill began, or collected by the response crews, so this should be considered a minimum estimate.


Figure 3. Volume of oil based on Coast Guard map (Figure 1) and thickness (Figure 2).

UPDATE 5/1/10 5:15 pm – Lots of cloud and haze but this afternoon’s MODIS satellite image shows the main body of the oil slick around the leaking well location.