Remote Sensing is a critical element for an effective response to marine oil spills. Timely response to an oil spill requires rapid reconnaissance of the spill site to determine its exact location, extent of oil contamination (particularly the thickest portion of the slick) and verifying predictions of the movement and fate of oil slicks at sea. This is necessary to effectively direct spill countermeasures such as mechanical containment and recovery, dispersant application and in situ burning, the timely protection of sites along threatened coastlines and the preparation of resources for shoreline clean-up. Remote sensing is useful in several modes of oil spill control, including large area surveillance, site specific monitoring and tactical assistance in emergencies. It is able to provide essential information to enhance strategic and tactical decision-making, decreasing response costs by facilitating rapid oil recovery and ultimately minimizing impacts. For ocean spills, remote sensing data can provide information on the rate and direction of oil movement through multi-temporal imaging and input to drift prediction modeling. Observation can be undertaken visually or by use of remote sensing systems. In remote sensing, a sensor other than human vision or conventional photography is used to detect or map oil spills. Remote sensing of oil on land is particularly limited.

Visual observations of spilled oil from the air, along with still and video photography, are the simplest and most common method of determining the location and extent (scale) of an oil spill. Remote sensing of spilled oil can be undertaken by helicopter, particularly over near-shore waters where their flexibility is an advantage along intricate coastline with cliffs, coves and islands. For open ocean spills, there is less need for rapid changes in flying speed, direction and altitude, in these instances the use of low altitude, fixed-wing aircraft have proven to be the most effective tactical method for obtaining information about spills and assisting in spill response. For spill response efforts to be focused on the most significant areas of the spill, it is important to note the relative and heaviest concentrations of oil. GPS and other aircraft positioning systems allow pinpointing the oil's location. Photography, particularly digital photography, is also a useful recording tool and allows others to view the situation on return to base. Many devices employing the visible spectrum, including the conventional video camera, are available at a reasonable cost. Dedicated remote sensing aircraft often have built-in downward looking cameras linked with a GPS to assign accurate geographic co-ordinates.

Practical oil spill detection is still performed by visual observation, which is limited to favorable sea and atmospheric conditions and is inoperable in rain, fog, or darkness. Visual observations are restricted to documentation of the spill because there is no mechanism for positive oil detection. Very thin oil sheens are also difficult to detect especially in misty or other conditions that limit vision. Oil can be difficult to see in high seas and among debris or weeds where it can blend in to dark backgrounds such as water, soil, or shorelines. Many naturally occurring substances or phenomena can be mistaken for spilled oil. These include sun glint, wind shadows and wind sheens, biogenic or natural oils from fish and plants, glacial flour (finely, ground mineral material usually from glaciers), and oceanic or riverine fronts where two different bodies of water meet. The usefulness of visual observations is limited, however, it is an economical way to document spills and provide baseline data on the extent and movement of the spilled oil.

An estimate of the quantity of oil observed at sea is crucial. Observers are generally able to distinguish between sheen and thicker patches of oil. However gauging the oil thickness and coverage is rarely easy and is made more difficult if the sea is rough. All such estimates should be viewed with considerable caution. The table below gives some guidance. Most difficult to assess are water-in-oil emulsions and viscous oils like heavy crude and fuel oil, which can vary in thickness from millimeters to several centimeters.

Remote sensing equipment mounted in aircraft is increasingly being used to monitor, detect and identify sources of illegal marine discharges and to monitor accidental oil spills. Remote sensing devices used include the use of infra-red (IR) video and photography from airborne platforms, thermal infrared imaging, airborne laser fluourosensors, airborne and satellite optical sensors, as well as airborne and satellite Synthetic Aperture Radar (SAR). SAR sensors have an advantage over optical sensors in that they can provide data under poor weather conditions and during darkness. Remote sensors work by detecting properties of the sea surface: color, reflectance, temperature or roughness. Oil can be detected on the water surface when it modifies one or more of these properties. Cameras relying on visible light are widely used, and may be supplemented by airborne sensors which detect oil outside the visible spectrum and are thus able to provide additional information about the oil. The most commonly employed combinations of sensors include Side-Looking Airborne Radar (SLAR) and downward-looking thermal IR and ultra-violet (UV) detectors or imaging systems. All sensors must be calibrated and require highly trained personnel to operate them and interpret the results.

Satellite-based remote sensing systems can also detect oil on water. The sensors on board are either optical, detecting in the visible and near IR regions of the spectrum, or use radar. Optical observation of spilled oil by satellite requires clear skies, thereby severely limiting the usefulness of such systems. SAR is not restricted by the presence of cloud and is a more useful tool. However, with radar imagery, it is often difficult to be certain that an anomalous feature on a satellite image is caused by the presence of oil. Consequently, radar imagery from SAR requires expert interpretation by suitably trained personnel to avoid other features being mistaken for oil spills. To date, operational use of satellite imagery for oil spill response has not been possible because limited spatial resolution, slow revisit times, and often long delays in receipt of processed image. However, satellite imagery can be used later to complement aerial observations and provide a wider picture of the extent of pollution.

Desired Outcomes

For more information on Remote Sensing and Surveillance of Oil Spills Projects, contact Joseph Mullin at 703-787-1556 or via e-mail.