NASA Earth Observatory image modified by SkyTruth
Bilge Dumping
What is bilge dumping?
Bilge is the oily wastewater that accumulates in the lower hold of a vessel during normal operations. In the early 1970s an international treaty called MARPOL specified how to properly treat this wastewater, but many vessels at sea continue to dump it directly in the ocean in violation of national and international laws. Learn more about this on our bilge page.
What does bilge dumping look like, and how can we spot it?
SkyTruth uses satellite radar imagery to identify oil slicks. In this type of imagery, bilge dumps typically take the form of long, slender black streaks on the ocean. In some cases, there is also a bright white spot at one end of the streak. Much of the time, this bright white spot is the vessel responsible for the slick. The image below offers a good example:
Not all slicks visible on radar imagery are bilge slicks. Heavy rain, slack wind, leaking oil platforms and other phenomena in the ocean can also create dark slicks on imagery, although bilge slicks are very distinctive. Also, bilge dumping is intentional, but there are other reasons that a slick might be visible trailing a vessel: there could be a serious mechanical problem and the vessel might be in distress. In any event, it’s important to be able to detect these pollution events and identify the vessels responsible.
What is SAR imagery? Why is it useful for detecting bilge dumping, and where can I get it?
SAR is Synthetic Aperture Radar imagery. A SAR image is a picture of the Earth’s surface made by beaming radar energy down at the planet from a satellite, and measuring the signal that comes back. We’re particularly interested in SAR imagery collected from two orbiting satellites, called Sentinel-1A and Sentinel-1B, operated by the European Space Agency. The radar sensors on the Sentinel-1 satellites operate at a frequency that makes them very sensitive to the roughness of the ocean surface. An oil slick is a smooth patch on the ocean, so it stands out as a dark area in contrast to the usually wind-ruffled surrounding waters that appear gray.
Because radar satellites create their own source of energy to illuminate what is on the Earth’s surface, they can operate day and night. And they beam down a radar signal at a wavelength that cuts through clouds, smoke and haze, creating useful images under conditions that would render optical (visible and near-infrared) imagery useless. This makes radar a valuable tool for continuous ocean monitoring.
You can look at and download Sentinel-1 imagery here. Learn more about Sentinel-1 radar here.
How can we identify the vessels that are polluting?
Satellite radar imagery doesn’t have enough detail to identify the vessels, which simply appear as very bright spots. So we use another dataset to make vessel identification possible. Most of the bilge-dumping incidents we’ve seen over the years have been caused by large cargo ships and oil/chemical tankers. Under international maritime safety law (SOLAS), these vessels and many others (passenger ships, research vessels, large fishing vessels) are required to continuously broadcast a radio-frequency identification signal when they’re at sea, using a public, open safety system called the Automatic Identification System (AIS).
What is AIS? Why is it useful, and where can I get it?
AIS is radio gear installed on a ship to broadcast information as frequently as every few seconds, to alert nearby vessels to a ship’s location, course, speed, identity, and other useful information. These broadcasts are public and unencrypted, designed to help ships avoid collision at sea. The broadcasts are also collected by networks of land-based radio receivers (terrestrial AIS) as well as AIS receivers carried on a rapidly growing number of orbiting satellites (satellite AIS). Companies like Spire and Orbcomm and others package the data for sale. You can also see AIS data on public vessel-tracking sites like Marine Traffic and FleetMon, and it is the data at the heart of the Global Fishing Watch service we built with Google and Oceana.
