Cerulean’s latest feature helps ensure more accurate investigation of oil pollution and supports better decision-making for ocean monitoring and enforcement

SkyTruth’s Cerulean platform is a valuable tool for identifying human-caused oil pollution in the ocean, but it also captures natural oil seeps — areas where oil naturally flows to the surface from cracks in the sea floor.

Natural oil seeps can make it challenging to determine the source of oil slicks. Once in the water, spilled oil from stationary oil and gas infrastructure in the ocean can mix with oil originating from natural seeps. To inform Cerulean’s users about this challenge, SkyTruth has integrated a natural seep warning system into its ocean pollution monitoring system.

Natural seeps — the journey to the surface

In the early 1990s, SkyTruth founder and CEO John Amos worked on a NASA-funded research project that used new radar-based remote sensing technology to detect natural oil seeps, guided by what he and other scientists learned about the unique visual patterns they create on the ocean surface.

“The oil from a natural seep has been slowly working its way up, out of bedrock, through the mud on the ocean floor, before it breaks out at the ocean seafloor and then floats up to the surface to make a slick,” Amos explains. “In the time during that entire trip that an oil drop is making out of the reservoir, which might be thousands of feet below the seafloor in bedrock, to the time it emerges at the surface of the ocean to contribute to that slick, things are happening to that oil.”

“Some of the more toxic elements in the oil are dissolving and dissipating into the sediment and into the seawater. There are all kinds of creatures that have evolved to actually consume some of the components of that oil. And in fact, one of the hallmarks of natural oil seeps on the seafloor is that many of them have developed these unique communities of organisms living around them that can derive their energy and nourishment from eating the oil and gas.”

Indeed, natural seeps provide habitat, food, and breeding and nursery grounds for unique marine species.

For those seeking to identify oil spills, knowing the location of natural seeps can help distinguish seepage from true pollution events and accurately attribute the source of an oil slick.

Amos emphasizes that Cerulean users should have awareness of the differences between natural ocean processes and human harm to oceanic ecosystems. “The human-caused slicks, we can do something about. They’re worse than the natural slicks for the environment, and we can fix it. There’s no reason not to fix it.”

Hear more: A conversation with John Amos on natural seeps

Spotting a natural seep

There are two common ways to identify natural oil slicks on the ocean’s surface using Cerulean.

The first is to look for parallel lines, which are a telltale sign of a natural seep. Natural slicks may be present as parallel “claw marks” on the ocean surface, like in this example from Mexican waters in the Gulf of Mexico. This is because at many seep locations, droplets of oil and gas continuously emerge from multiple nearby points on the seafloor, streaming up to the surface and creating slicks that then drift away from the source under the influence of the same winds and currents.

Alternatively, you can look for pinwheel groupings of slicks over a period of time, with no visible infrastructure source near the center of the group.

This image shows Cerulean’s identification of multiple slicks detected on different dates emanating from a generally central point. There is likely a natural seep on the seafloor because there is no apparent infrastructure in the center of the collection of slicks. It’s important to note that we can’t confidently rule out human causes for these patterns – any oil continuously leaking from a point source on the seafloor, such as a shipwreck or a leaking pipeline, might be expected to create a similar pattern. Additionally, there may be platforms or other structures in areas of offshore oil development that are too small to detect on the satellite imagery we use.

It can be challenging to determine the origin of oil slicks where natural seeps and human activity overlap in the same waters. Consistent evidence of this pinwheel-like collection of slicks can help those investigating marine oil pollution attribute a slick to natural sources.

Cerulean’s natural seep warning system

Cerulean’s new natural seep warning system is designed to help users better understand the presence of a slick by providing information on known active natural seep zones.

The system is powered by data from the GlobalOSD-SAR dataset, the largest known academic collection of satellite-detected oil slicks, and covers from 2014 to 2020. Of the over 100,000 slicks in the dataset, 32,911 are labeled as natural. To map these into larger “seep warning zones,” SkyTruth grouped nearby seep points using a 20-kilometer distance threshold, and then applied a 20-kilometer buffer around each group. This distance represents approximately how far a slick can spread once it reaches the surface. Clusters containing at least five confirmed seep points were considered strong evidence of persistent natural seep activity.

When a new oil slick appears within one of these zones, Cerulean flags it with a natural seep warning, alerting users that the slick may be part of an ongoing natural process rather than a recent human-caused spill.

This system helps ensure more accurate investigation of oil pollution and supports better decision-making for ocean monitoring and enforcement. If you are unsure whether a slick with this warning can be attributed to a vessel or infrastructure source, request a slick verification for SkyTruth’s team to review. This in-platform feature is available in the Cerulean slick details panel — just click the verification badge at the top left to get the process started.