Marine ecosystems are some of the most unique on Earth – ranging from marshy, coastal estuaries, to colorful and pristine coral reefs, to the vast, open ocean. However, every year, these critically diverse ecosystems are exposed to numerous disastrous oil spill events which leak high levels of toxic chemicals into the marine environment (United Nations Environment Programme, 2022). The negative impacts of these oil spills are seemingly endless, ranging from a high death toll and increased injury for marine life, to the contamination and degradation of high-risk marine ecosystems, to a myriad of financial impacts from the destruction of commercially important species as well as downturns in ecotourism (Figure 1).  

Oil spills typically become worldwide news when a tanker runs aground or when an oil rig erupts with leakage. Not all oil pollution is as obvious, however. With the help of space-based technologies, scientists have been able to detect one of the lesser-discussed forms of oil pollution: the purposeful, and illegal, dumping of oil and effluent into the ocean by shipping vessels. Satellite imaging has been able to provide surveillance of the ocean surface. Although this remains an emerging application of satellite technology, already thousands of ships have been detected to be committing illegal activity (Bernhard et al., 2022). Scientists are now also able to forecast the trajectory of oil that has been spilt within the ocean, as well as to track the changes to both, marine and coastal ecosystems in the aftermath of a spill (Monaldo, 2022). Collecting this data from the ground would be otherwise impossible, if not prohibitively expensive and time-consuming. As a result, space-based technologies create an opportunity for much faster response and recovery after an incident.

Space-based imagery: how remote sensing data enable oil spill detection

Within the realm of Earth observation, two primary technologies can be used for oil spill detection: optical imaging and synthetic aperture radar. Optical imaging is a traditional and well-understood method that has been commercially available for many decades. Passive sensors onboard satellites collect data from the Earth’s surface and produce photograph-like images (Figure 2). Two main principles allow oil spills to be detected through optical imaging. Firstly, the contrast between oil and water is detected because oil reflects light differently than normal ocean surface waters; oil spills may appear as dark spots within imagery (Trujillo-Acatitla et al., 2022). Secondly, oil has unique optical properties that can be detected by spectrometer instruments onboard many satellites. For example, compared to water, oil has high absorption in the blue spectral band, and oil-water emulsions display higher reflectance in the red, near-infrared, and shortwave infrared bands (Z Sun et al., 2022). Using spectrometer instruments, scientists can interpret optical imagery to determine where oil is most likely located within a given area. Examples of optical imaging instruments include the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard NASA’s Terra and Aqua satellites, as well as the Medium Resolution Imaging Spectrometer (MERIS) instrument onboard European Space Agency’s Copernicus Sentinel-3 satellite.

When it comes to oil spill management, optical imaging has some limitations. For example, certain weather conditions, such as cloud cover or fog, can prevent an image from being captured. This may be particularly problematic in areas of the world that experience frequent cloud coverage or in instances of extreme storms (such as when storm damage to oil drilling platforms has caused the oil spill itself) (Velotto, 2022). Additionally, as its name suggests, optical imaging largely relies on the visual portion of the electromagnetic spectrum. This means that daylight is required to produce imagery and therefore no imagery can be produced at night. If optical satellite imagery is only able to be produced under certain conditions, namely clear skies during the daytime, the scope of how and when this imaging method may be applied is limited (Z Sun et al., 2022). At the same time, however, optical imaging is economical and relatively easy to produce, making it a practical choice for oil spill management under the right conditions.

The second method of remote imaging for oil spill detection is known as Synthetic Aperture Radar (SAR). SAR is an active sensor that transmits microwave signals towards Earth’s surface. Depending on the patterning of the energy that is reflected back, scientists are able to detect the physical properties of the Earth. In the case of oil spills, when oil is spilt on the ocean’s surface, it acts to dampen wind-driven waves. As a result, the area of the spill appears smoother than the surrounding uncontaminated waters and ultimately bounces less energy back to the satellite (Zhen Sun et al., 2022). This difference in wave action results in oil spills appearing as dark patches in SAR imagery (Alpers et al., 2017) (Figure 2). Unlike optical imaging, SAR sensors emit their own energy instead of relying on sunlight and therefore can operate day or night. Similarly, SAR is able to collect data through multiple atmospheric conditions including clouds, rain, fog, and smoke, therefore making it uniquely advantageous for rapid response to oil spills. 

SAR imagery has historically only been used to determine the presence or absence of oil. Recent advancements, however, have allowed scientists to detect the thickness of the oil slick as well. Developed by Dr. Frank Monaldo of Johns Hopkins University, he and his team have created “a contrast ratio algorithm” that works with the SAR sensors to determine the thickness of oil across a spill (Monaldo, 2022). As oil spreads out over the sea surface, it is not evenly distributed across a whole slick. Instead, it becomes thinner and changes in colour, from black or brown, to a silvery or iridescent sheen (Figure 3). Knowing where oil is thickest in a spill allows scientists to collaborate with first-response teams to tailor remediation efforts to be more specific and effective (Figure 4). Dr. Monaldo has gone on to state “If you can have an algorithm that mostly automates the process to give analysts not only the extent but the thickness, they can act quickly to get an assessment to response agencies so that they can deploy resources faster” (Lewis, 2021).

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Monitoring illegal shipping activities from space 

International trade is heavily reliant on the shipping industry to transport goods across the globe. Thousands of ships make journeys across the oceans daily. As these ships operate, however, their goliath engines produce noxious mixtures of chemicals known as bilgewater. Comprised of oil, detergents, liquids from the engine room, and other toxic chemicals, bilgewater collects in the bottom of the vessel before being stored in bilge tanks. A single ship can produce several tonnes of this oily mixture in a single day (Onwuegbuchunam et al., 2017). 

There are very few options for dealing with bilgewater waste. International regulations require ships to store their bilgewater until disposal can occur on land. However, there is often a large amount of bilgewater onboard and proper disposal is costly (Caston et al., 2007). Alternatively, vessels can opt to treat their bilgewater through an oily-water separator before discharging it into the ocean. This equipment is designed to separate oil to 15ppm, a limit set in international law by MARPOL in 1973. However, this process is also costly and is easily circumvented by ships (Han et al., 2019). The end result is that there is a financial incentive for shipping vessels to dump raw bilgewater directly into the ocean (Figure 5). 

Historically, a general lack of surveillance has meant that there has been limited accountability for polluting vessels. Typical detection methods have relied on surveillance by aeroplanes, other vessels on the sea, or the discovery of pollution on shore (Caston et al., 2007). The ocean is a vast space, however, and it is difficult for authorities and researchers alike to track polluting vessels. There are ample loopholes for ships to navigate, allowing for oil pollution to run rampant. This is where space-based technologies have become immensely useful. 

Optical satellite imagery and SAR imaging technologies can act as more effective surveillance strategies for many reasons. Firstly, they can cover a wider portion of the ocean than historic surveillance methods, allowing for more efficient and widespread monitoring. Satellites can also produce imagery while being undetected by the vessels themselves, meaning that they are more likely to catch illegal dumping in the act. Satellite imagery also provides an archive of data, allowing for scientists to analyze how a particular spill is progressing over time (European Maritime Safety Agency, 2019). This may be particularly helpful in the case of large oil spills such as tanker accidents or oil rig malfunctions. 

Perhaps the largest benefit, is the ability of SAR imagery to be captured at night and in any weather condition. Historically, a significant portion of oil pollution has occurred under the cover of darkness, where it is near impossible for standard surveillance methods to detect nefarious activity (Caston et al., 2007). SAR imagery, however, has risen as an effective strategy for ocean monitoring to occur day or night, sun or cloud. In SAR imagery, vessels that are dumping bilgewater into the ocean take on a very distinct shape. The oil appears as dark smears on the ocean’s surface, typically in a straight line that is several kilometres long. The ship appears as a bright white dot at the end of the slick (Figures 6 and 7). While the power of SAR is clear, no single satellite or service provider can meet the needs of the entire ocean and it requires a combination of both optical and SAR monitoring to provide sufficient coverage globally. 

Has there actually been a decrease in marine oil pollution with the emergence of satellite imaging? For the most part, no. Only a fraction of polluting vessels are verified and even fewer are prosecuted. While the technology is more than capable and has indeed detected tens of thousands of possible oil slicks globally, challenges remain in the enforcement of legal action in these cases (Bernhard et al., 2022). Oftentimes, individual nations do not follow up on alerts of detected spills, or if they do, they do not in a timely enough fashion (Caston et al., 2007). The longer it takes for marine authorities to verify a spill, the more likely the oil has begun to dissipate, and the less likely prosecution becomes as the evidence diminishes. Other times, a country’s national maritime authority lacks the capability to enforce the law, therefore providing very little to discourage vessels from continuing with illegal activity. 

To effectively reduce marine oil pollution, authorities worldwide must step up. The repercussions of oil pollution are clear and yet legal actions are generally only administered in extreme cases such as the 2010 Deepwater Horizon oil spill. When it comes to small-scale oil pollution, such as illegal bilgewater dumping, prosecution - and thus, deterrence to others - is rarely pursued to completion. In the instances where polluting vessels are actually prosecuted, serious violators are generally given a mere slap on the wrist in the form of a small fine (Mura, 2018). Space-based technologies can provide key images of pollution occurring in the act, but they must be used more broadly within justice systems worldwide. If there is to be any sort of resounding change, judicial systems, including prosecutors and judges, must be made aware of the long-lasting and far-reaching impacts of oil pollution. In the continued absence of aggressive enforcement, polluters will continue to degrade the marine environment. Deterrence through meaningful consequences to offenders for illegal behaviour must be prioritized if the Earth’s oceans are to be preserved.