NASA Earth Observatory image modified by SkyTruth

Cerulean Identification Guide


Slick Identification Guide

Cerulean analyzes Synthetic Aperture Radar (SAR) imagery, which is able to penetrate cloud cover and collect data at night. Radar is particularly good at picking up features like oil because it can differentiate between textures on the surface of water. In SAR images, oil slicks typically appear dark in a light grey sea but the appearance can vary depending on the source, environmental conditions, and elapsed time.

True Positives

Vessels are not the only source of oil pollution in the ocean. Oily wastewater (production fluids) is often discharged directly into the ocean from offshore platforms, and anchored vessels can release oily waste while stationary – producing a visually different slick.

False Positives

Some dark areas in satellite imagery may appear to be oil at first glance, but they are actually caused by something else. For example, low wind conditions give water a smooth, calm surface that can resemble a large patch or streak of oil. These conditions are sometimes difficult to differentiate from oil, but knowing what to look for can help. Below are a few examples of false positives – features that may be picked up by Cerulean, but are not actually oil slicks caused by vessels or infrastructure.

Determining the cause of a slick can be challenging, but answering a few questions may help deduce the slick’s origin:

1. Is the slick near a major shipping route?
2. Is there oil infrastructure located in this region?
3. What were the weather conditions when this image was captured?