Measuring Floating Thick Seep Oil from the Coal Oil Point Marine Hydrocarbon Seep Field by Quantitative Thermal Oil Slick Remote Sensing

Abstract

Remote sensing techniques offer significant potential for generating accurate thick oil slick maps critical for marine oil spill response. However, field validation and methodology assessment challenges remain. Here, we report on an approach to leveraging oil emissions from the Coal Oil Point (COP) natural marine hydrocarbon seepage offshore of southern California, where prolific oil seepage produces thick oil slicks stretching many kilometers. Specifically, we demonstrate and validate a remote sensing approach as part of the Seep Assessment Study (SAS). Thick oil is sufficient for effective mitigation strategies and is set at 0.15 mm. The brightness temperature of thick oil, TBO, is warmer than oil-free seawater, TBW, allowing segregation of oil from seawater. High spatial-resolution airborne thermal and visible slick imagery were acquired as part of the SAS; including along-slick “streamer” surveys and cross-slick calibration surveys. Several cross-slick survey-imaged short oil slick segments that were collected by a customized harbor oil skimmer; termed “collects”. The brightness temperature contrast, ΔTB (TBO−TBW), for oil pixels (based on a semi-supervised classification of oil pixels) and oil thickness, h, from collected oil for each collect provided the empirical calibration of ΔTB(h). The TB probability distributions provided TBO and TBW, whereas a spatial model of TBW provided ΔTB for the streamer analysis. Complicating TBW was the fact that streamers were located at current shears where two water masses intersect, leading to a TB discontinuity at the slick. This current shear arose from a persistent eddy down current of the COP that provides critical steering of oil slicks from the Coal Oil Point. The total floating thick oil in a streamer observed on 23 May and a streamer observed on 25 May 2016 was estimated at 311 (2.3 bbl) and 2671 kg (20 bbl) with mean linear floating oil 0.14 and 2.4 kg m−1 with uncertainties by Monte Carlo simulations of 25% and 7%, respectively. Based on typical currents, the average of these two streamers corresponds to 265 g s−1 (~200 bbl day−1) in a range of 60–340 bbl day−1, with significant short-term temporal variability that suggests slug flow for the seep oil emissions. Given that there are typically four or five streamers, these data are consistent with field emissions that are higher than the literature estimates.

An inverse planned oil release validation method for estimating oil slick thickness from thermal contrast remote sensing by in-scene calibration

“…The approach combines sea surface and airborne measurements as part of the Seep Assessment Study (SAS), described in Leifer et al. [1] . The study collected thermal infrared (TIR) and visible (VIS) spectrum imagery, as well as viewing position and orientation data of a targeted oil slick segment by the oil spill mapping science package, SeaSpires™.…”

“…Alternatively, multiple different remote sensing approaches can be combined, e.g., radar, thermal, ultraviolet, etc. [1 , 13 , 14] – different spectral regimes exhibit different lookalike signatures. Another approach considers ancillary data such as winds [15] .…”

“…Leifer et al. [1] describes the underlying theory. In brief, the oil slick absorbs a portion of the downwelling radiation.…”

“…The SAS is described in Leifer et al. [1] and investigated TIR oil slick thickness remote sensing. The SAS was applied to the perennial and continuous thick oil slicks from the Coal Oil Point seep field.…”

“…Leifer et al. [1] found that for Coal Oil Point oil, the transition from thin to thick oil based on the optical and thermal characteristics was 500–1000 µm. This definition differs from the Bonn Agreement, which is based on visible appearance.…”