|Detail from MODIS/Terra satellite image showing oil slick off Nigeria on December 21, 2011 at 10:15-10:20 am local time. Image courtesy NASA/MODIS Rapid Response Team.|
The slick coincides exactly with the slick on the radar image - no surprise there - but it does appear slightly smaller. On this MODIS image, the slick is showing up as a spectral feature, a target with a distinct reflectance signature typical of floating oil (high reflectance at shorter wavelengths, making it appear pale blue-ish in color). On a radar image, oil slicks are textural features: they smooth out the ocean surface, causing specular reflection of the incoming radar energy being beamed down at earth by the radar instrument. The very thin edges of the slick look transparent to MODIS but are still able to smooth out the water and appear dark on radar images under suitable wind conditions. Scatterometer data show that the surface wind speed was probably in the 5 to 10 knot range, ideal for slick detection on radar.
Which brings us to the amount of oil spilled. Shell has reported "less than 40,000 barrels" were spilled, so the amount could be anywhere from 1 gallon to 1.7 million gallons. Based on the radar image today, the oil slick covers 923 square kilometers. At an average thickness of 1 micron (1/1,000th of a millimeter) that would amount to 243,672 gallons (5,802 barrels) of oil. But portions of the slick could be many times thicker than that; it's not unusual for a spill directly to the sea surface to be millimeters, or even centimeters, thick. Estimating the thickness is usually based on direct visual observations of the slick, especially the color.
It would be very helpful to get photos or video of this slick taken from the air and on the water.