Code 4. “Color with discontinuous oil” for thicker oil layers 50-200 m thick. For oil films more than 50 m thick, the light is reflected by the oil and not by the surface of the underlying sea, allowing to see the true oil color of the spilled oil. Only gradually does the true oil color become the dominant color, with brown and black oil appearing black. However, code 4 is a transition code because thinner layers alternate with thicker layers due to the action of the waves. This code is also often described as “the real oil color on a metallic background.” BAOAC was developed on the basis of a series of scientific experiments and studies within the framework of the Bonn Agreement. The method has been specially designed for aerial surveillance and the use of trained operators to detect and assess oil pollution at sea by an aircraft or helicopter. Based on a visual observation of oil pollution, the method can only be used during the day and during periods of good visibility. Estimates of the volume of an oil spill at sea are not possible from a floating platform (or dock in a port). However, floating units can make approximate estimates of certain stages of the BAOAC and assess the severity of a slick visible from the water. Therefore, certain elements of the BAOAC method can (as far as possible) be used wisely from a floating platform, always taking into account the uncertainties necessarily associated in this case.
Code 3. `Metallic` for 5-50 m thick oil layers: typical of this appearance is matte metallic shine. The color of this metallic shine is largely determined by the color of the underlying water, because the oil layer is not thick enough to block the light reflected from the surface of the water, and partly also by the “color” of the sky (. B for example blue sky or grey clouds) generated by the reflection of light. Code 5. “True continuous oil color” for the thickest oil layers from 200 m thick: the true oil color of the type of oil spilled is dominant and clearly visible here (unlike code 4). Minimum and maximum volumes derive from part of the minimum thickness of the layer in which a coloration is visible and from the maximum thickness of the layer in which the coloration moves to an “upper” code. The volume of oil is then calculated from a simple mathematical formula, in which the code-polluted surface is multiplied by the percentage of that code, present in a slick, and by the minimum and maximum thicknesses. The minimum volume estimate is generally used in criminal matters (to give the quantities specified as a minimum), while the maximum volume estimate is generally used in cases of accidental marine pollution (the worst typical cases in crisis management). During a visual observation: first determine the length and width of the rectangle containing the oil spill, then evaluate the oil cover inside that rectangle, taking into account the degree of fragmentation and propagation of the oil on the water. The observation of the different color representations within an oil spill and the approximate estimate of the severity of a discharge or pollution at sea by distinguishing the thicker layers and thinner layers of oil (in principle the areas where pollution can be combated and the areas where it can be combated).