Thickness Characterization of Crude Oil Slicks on Water

Knowledge of crude oil slick thickness is essential in determining the oil spill response, e.g., recovery, dispersant application, or in-situ burning, and in assessing the fate of the oil. In this study, we examine several methods to measure the thickness of slicks ranging from submicron to several mm. There is no single method that covers this entire thickness range. For thin films, natural fluorescence of oil slicks excited by UV light is measured by an EMCCD camera through a narrow bandwidth (5 nm) filter centered at 515 nm. Once calibration tests determine the absorption coefficient of the oil, imaging of the slick is used for measuring the spatial variation in slick thickness from the distribution of fluorescence intensity. For weathered HOOPS oil, this approach is effective for slick thicknesses up to 40 μm. For intermediate thicknesses, in the 40 to 1000 μm range, the thickness assessment is based on attenuation of a red (635 nm) laser light sheet transmitted through the oil slick and observed by the same camera. Both optical methods become ineffective for thicker slicks of opaque crude oil. Hence, above 1 mm, the slick thickness is assessed based on range-gating using a 7.5 MHz ultrasonic imaging system with a linear transducer.