How we determine oil spill volume

At SkyTruth, our oil spill monitoring program is tasked in part with estimating the amount (volume) of an oil spill based on slicks observed and measured on satellite images; observations and photographs acquired during aerial overflights; or indirectly, based on written reports from third parties.

Computing Volume

An oil slick in the open ocean is typically a very thin layer of oil covering a large area, often many square miles in extent. So in order to calculate the volume of a slick we need to measure or estimate the area it covers, then estimate the average thickness over that area. Then we multiply the area times the thickness to get the volume.

Estimating Surface Area

Measuring the area is a fairly straight-forward and accurate process with satellite imagery - we simply trace a line around the visible edges of the slick and compute the area inside that boundary. For oil spill reports where we do not have imagery, we use the reported length and width of the slick to compute the rectangular area which contains the slick.

Estimating Thickness

Estimating thickness, however, is another matter. One way to estimate the thickness of an oil slick is to observe it's "color" and assign a thickness based on established guidelines for the range of thicknesses that can produce a slick of that color (e.g. "Rainbow sheen"). Tables and guidelines for visual estimation of oil spill volumes are published by the National Oceanic and Atmospheric Administration (NOAA) on their response and restoration website.

Unfortunately, when using satellite imagery, especially radar (SAR) imagery, we are not able to observe the spectral characteristics that create the apparent color of a typical oil slick, so we cannot use this method. Instead, we use a rule of thumb that provides a reasonable estimation of the minimum average thickness that makes an oil slick at sea visible on satellite imagery.

Based on past experience, and the judgment of other experts, SkyTruth has determined that a good rule of thumb for estimating the thickness of an oil slick visible in a SAR image is that the total area is on average at least 1µm (one micron, or 1 millionth of a meter) thick. The actual thickness varies across the whole area, as some parts of the slick may be thicker than the average, and other parts thinner.

By measuring the area of the visible oil slick in a SAR image, and assuming the average thickness of the oil across that area is at least 1µm, the minimum volume of oil in the slick can be calculated.

Calculations

If there is an observed slick of oil covering 1 km², then the minimum volume of the oil covering this surface can be calculated in meters:

1000m x 1000m x 0.000001m = 1x10⁶ m² x 1x10^-6 m = 1 m³

Conveniently, one square kilometer of oil covering the water, multiplied by a 1 µm thickness, is equal to 1 m^3­­ (one cubic meter).

Converting to gallons, 1 m³ is equal to 264.17 US gallons, so an oil slick covering 1 km² a at an average thickness of 1µm contains 264.17 US gallons of oil.

Conversion Chart for 1µm Oil Slick

Example

If an image shows a slick that covers 50 mi², then the minimum volume assuming average thickness of at least 1 µm is calculated:

(683.76 Gallons/mi²)x(50 mi²) = 34,188 Gallons of Oil