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 – Where’s The Latest Imagery?

We beg your patience. Our main source of imagery is from the NASA satellites, Terra and Aqua, that carry the MODIS sensor. This sensor measures visible to infrared wavelengths of light, so when it’s cloudy or hazy in the Gulf, we get a nice picture of…clouds and haze. MODIS images also show a distinct pattern of glittery sunglint on the water (when there are no clouds in the way, and the geometry with the sun is just right). This can be very useful for mapping oil slicks because the oil flattens out the water, and will appear either very dark or very bright (depending, again, on the sun angle). But other things can make the water flat: calm winds, for example. So if we get a MODIS image that suffers from too many of these quality problems, we don’t process and post it because it’s not providing useful information about the spill.

See our Twitter feed (just glance to your right!) – that’s where we post updates and info that’s briefer and more timely than these blog entries. Follow us on Twitter and when we know it, you’ll know it.

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.