The position of oil may be predicted using wind and current data. Floating oil will move downwind at about 3% of the wind speed and at 100% of the strength of surface water currents. Close to land, the strength and direction of any tidal currents must be considered when predicting oil movement, whereas further out to sea the influence of other ocean currents predominates over the cyclic nature of tidal movement. Computer models can be used to plot oil spill trajectories, but the accuracy of any method depends on the quality of hydrographic data used and the reliability of forecasts of wind speed and direction.
It is usually necessary to plan a systematic aerial search to ascertain the presence or absence of oil over a large sea area. A 'ladder search' is frequently the most economical method of surveying an area. When planning a search, due attention must be paid to visibility and altitude, the likely flight duration and fuel availability. Floating oil has a tendency to become elongated and aligned parallel to the direction of the wind in long and narrow 'windrows' typically 30 - 50 metres apart. It is advisable to arrange a ladder search across the direction of the prevailing wind to increase the chances of oil detection.
Methods for observation and recording
Accurate observation will be assisted by having available maps and charts, and basic data such as the location of the spill source and of pertinent coastal features.
During the flight, careful annotation of the time and locations of all potentially relevant features will create a reliable record from which an informative report of the flight can be prepared. In particular, for 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 the oil's location to be pinpointed. Photography, particularly digital photography, is also a useful recording tool and allows others to view the situation on return to base. Dedicated remote sensing aircraft often have built-in downward looking cameras linked with a GPS to assign accurate geographic coordinates.
From the air it is notoriously difficult to distinguish between oil and a variety of other unrelated phenomena. It is therefore necessary to verify initial sightings of suspected oil by flying over the area at a sufficiently low altitude to allow positive identification.
Phenomena that most often lead to mistaken reports of oil include: cloud shadows, ripples on the sea surface, differences in the colour of two adjacent water masses, suspended sediments, floating seaweed, algal/plankton blooms, sea grass, and coral patches in shallow water.
Quantifying floating oil
An estimate of the quantity of oil observed at sea is important to assist planning the required scale of clean-up response.
It is therefore crucial that during the overflight the observer is able to distinguish between sheen and thicker patches of oil. 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 millimetres to several centimetres.