Modelling of the optical contrast of an oil film on a sea surface

Abstract

The water-leaving radiance field above a sea surface polluted by an oil film has been modelled using a Monte Carlo radiative transfer technique with large numbers of photons incident at a selected zenith angle. The calculated radiance was recorded for each of the 240 sectors of equal solid angle the upper hemisphere had been divided into. The results are presented in the form of a bi-directional reflectance distribution function (BRDF) and as a contrast function parameterised by observation angle for various angles of incident sunlight and for various states of the sea surface roughness. The conditions for observing maximal and minimal contrast are described.

A MODIS-Based Robust Satellite Technique (RST) for Timely Detection of Oil Spilled Areas

“…More recent studies on optical models [37,38,40,42] demonstrated that the acquisition angle, combined with a particular irradiation geometry, may enhance the reflectance contrast between oil and the surrounding water [13,22,25,32]. One of the main problems related to the use of visible bands for detecting oil spills is that the signal reflected by the oil film is not unique but it may change depending on several factors (and on their mutual combinations) such as sun/satellite geometry and sea surface roughness.…”

“…One of the main problems related to the use of visible bands for detecting oil spills is that the signal reflected by the oil film is not unique but it may change depending on several factors (and on their mutual combinations) such as sun/satellite geometry and sea surface roughness. As a consequence, oil spill can appear, in VIS and NIR bands, less or more reflecting than seawater [37,38,40,42]. It is referred to as a "positive" contrast when the reflectance of oil spill is higher than that of the clear seawater and as a "negative" one when the reflectance of oil spill is less than that of the clear seawater.…”

“…It is referred to as a "positive" contrast when the reflectance of oil spill is higher than that of the clear seawater and as a "negative" one when the reflectance of oil spill is less than that of the clear seawater. In detail, when sea surface is flat, the contrast between oil and sea surface is always negative since the oil produce a reflectance decrease; on the other hand, roughness of the sea surface produces a positive contrast [37]. Concerning sun/satellite geometry, when observing the surface from the vertical direction, even a small surface roughness results always in a positive contrast value: the contrast is most pronounced when the observer sees the oil slick close to the sun reflection (positive contrast) and when the slick is observed at a low angle, almost horizontally (negative contrast) [37].…”

“…Due to the thermal inertia of oil, which is lower than seawater, oil-polluted areas usually have the opposite behavior compared with the surrounding oil-free seawater, with higher brightness temperatures in TIR images collected in daytime and lower brightness temperatures during the nighttime [35,36]. Also in the visible (VIS) and near-infrared (NIR) regions of the electromagnetic spectrum it is possible to detect the presence of an oil layer on the sea surface thanks to the different reflectance of the two elements [22,33,[37][38][39][40][41][42]. The main limits of these methodologies are related to their applicability only to particular geometries of observation [32], and their poor level of automation due to the need for several interventions of an operator (e.g., region of interest identification and visual recognition) [33,[43][44][45].…”

Oil Spill Detection in Glint-Contaminated Near-Infrared MODIS Imagery

“…the satellite and solar zenith angles and the relative satellite-solar azimuth angle; (2) sea state, i.e., wind speed; and (3) oil spectral properties, i.e., refractive index and absorption coefficient [14], and oil conditions, i.e., the thickness and state (e.g., fragmentation, evaporation, etc.). Summarizing Otremba and Piskozub [15][16][17][18], the remotely-sensed optical contrast of an oil-covered sea surface increases either when the sensor sees the oil slick close to the sun reflection angle, that is in glint conditions (positive contrast, oil brighter than water), or when the slick is observed at a low angle (no glint), almost horizontally (negative contrast, oil darker than water). Furthermore, in "moderate" glint regions (located at the edge of the glint "patch"), the contrast of slicks can change from positive to negative and vice versa.…”