Haiyang-1C (HY-1C) is the first operational ocean color satellite of China, which is intended to obtain daily global ocean color data. The Ultraviolet Imager (UVI) onboard provides a potential novel detector for the detection of marine oil spills. Although airborne UV sensors have shown great efficiency for the detection of spilled oils, the capability of spaceborne UV sensor is not yet clear. In this study, we designed a ground-based experiment to interpret the UV characteristics of various weathered oils, and found that very thin oil films are quite sensitive to the UV radiation due to the surface interference light. Moreover, by comparing spaceborne and airborne UV images of spilled oils collected from HY-1C UVI and AVIRIS, the scale effect of ultraviolet remote sensing has been interpreted clearly. The interference light and sunglint reflection play different roles in the imaging process of spilled oils, leading them to appear radical different features (brighter or darker than the background oil-free seawater) in ground, airborne and spaceborne observation, which deserves further research. Ultraviolet remote sensing, therefore, can work as a new approach and improve the detection and monitoring of marine oil spills.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Haiyang-1C (HY-1C) satellite, launched on 7 September 2018, is the first operational ocean color satellite of China. It is equipped with five sensors, such as Chinese Ocean Color and Temperature Scanner (COCTS), Coastal Zone Imager (CZI), Ultraviolet Imager (UVI), Satellite Calibration Spectrometer (SCS) and Automatic Identification System (AIS). Particularly, the UVI onboard provides the only data for marine environment observation at 355 and 385 nm right now. These ultraviolet (UV) data will provide a broad future of application to oceanic research, including atmospheric correction of nearshore turbid water [1,2], estimating dissolved organic matter in water [3,4] and monitoring sea surface anomalies, such as marine oil spills .
Marine oil spills represent a typical environmental disaster that harms the marine and coastal environment . In the process of weathering, various weathered oils with different visual characteristics can form, including non-emulsified oil slicks, water-in-oil (WO) emulsions and oil-in-water (OW) emulsions [7–9]. How to detect oil slicks with optical sensors is still a challenge due to the weak reflecting light signal. However, oil slicks show high reflectivity of UV radiation, which is a remarkable characteristic compared with other bands . Some studies have verified the feasibility of using the UV sensors to detect spilled oils on airborne platforms. Oils appear brighter than background seawater in these UV images , and the oil-covered areas in the UV band are larger than those in the visible and infrared bands, as the UV band is more capable of detecting very thin oil slicks .
Apart from ultraviolet remote sensing, various remote sensing technologies have been applied to detecting and monitoring marine oil spills. Among these synthetic aperture radar (SAR) and optical remote sensing are the most-frequently used ones. SAR provides data under all-weather conditions, in which the spilled oils on the ocean surface reduce Bragg scattering and show negative contrast from surrounding oil-free seawater, but it can hardly be used to classify and quantify the different weathered oils [13,14]. Passive optical remote sensing is another popular tool as it is able to classify different weathered oils and quantify the thickness, concentration, or volume of them [7–9,15–17]. Note that oils in optical images would appear either darker (negative contrast) or brighter (positive contrast) than oil-free seawater under different sunglint conditions [17–19]. Besides, thermal remote sensing and laser fluorescence have also been used to detect spilled oils [20,21]. So far airborne ultraviolet remote sensing has shown great effectiveness for oil spill detection, but whether spilled oils can be detected and what characteristics would oils appear in spaceborne UV images remain unknown due to the lack of spaceborne UV sensor. Fortunately, the HY-1C UVI detected one oil spill incident near Indonesia on 20 April 2020, which was the first time that marine oil spills had been clearly observed in UV satellite images, providing valuable UV satellite data to fill these knowledge gaps.
Therefore, this work is intended to verify the ability of ultraviolet remote sensing to detect marine oil spills, especially in the UV characteristics of various weathered oils based on a ground-based experiment, and the features oils appear in UV images obtained from spaceborne and airborne sensors. Ultraviolet remote sensing represents a novel approach for marine environment monitoring. This work could provide new reference for using the UV sensors to detect other sea surface targets, even for the UV sensors equipped on the next generation of ocean color satellite of China.
2. Data and methods
2.1 Spaceborne and airborne images
2.1.1 HY-1C UVI images and other validation data
HY-1C is operating in a sun-synchronous orbit with a mean altitude of 782 km and a mean local solar time of 10:30 at the descending node. The UVI onboard collects data in a push-broom mode and covers a swath of ∼2900 km, ensuring a global coverage every day. The designed data spatial resolution at the subastral point is ∼550 m in full transmission mode and ∼1100 m in merged transmission mode. More parameters of the UVI are listed in Table 1.
The HY-1C detected one marine oil spill incident near Indonesia on 20 April 2020. The oil spills can be detected on both HY-1C CZI and UVI images. In this study, several UVI images (of spectral radiance, unit: mW·cm−2·µm−1·Sr−1, as the atmospheric correction of the UV band is not yet clear) were used to conduct analysis, and the CZI image (of Rayleigh-corrected reflectance, Rrc, dimensionless) covering the same area synchronously was used as proof. Based on the spectral response characteristics (Fig. 1(a), inset) and sunglint assessment of marine spilled oils, the CZI image detected both the oil slicks and oil emulsions (Fig. 1(a)). While in the UVI images, only oil slicks were detected.
2.1.2 AVIRIS images
During the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM) in 2010, the airborne visible infrared imaging spectrometer (AVIRIS) onboard the ER-2 aircraft was used to collect hyperspectral images of the spilled oils [6–8,22]. The instrument collects data covering the wavelength range from 350 to 2500 nm in 224 channels with an average spectral resolution of 10 nm. Specifically, it has a UV band with central wavelength at 380.21 nm (band 2). In this study, one AVIRIS image obtained on 17 May 2010 (Run 10) was used to characterize oil slicks and various oil emulsions (Fig. 1(b)). The spatial resolution of AVIRIS data is ∼7.6 m at an altitude of 8500 m. The AVIRIS data was processed into reflectance (R, dimensionless) using atmospheric correction algorithm designed for hyperspectral airborne sensors .
2.2 Ground-based experiment
The experiment was conducted outdoors using the experimental settings illustrated in Fig. 2(a) and described below. The crude oil (i.e., Yiyang oil) produced in China, which was similar to the DWH oil spill in the GoM, was used for sample preparation, including WO emulsions with a concentration of 75%, OW emulsions with a concentration of 1%, thin oil slicks and thick oil slicks. Details on sample preparation have been illustrated by Lu . One black plastic tank was used to contain clear water, and four black plastic beakers were used to contain the samples. All the containers were sprayed with black paint to reduce interference caused by reflection from the bottom and sides. The liquid level in the beakers was level with that in the tank. In order to reduce the influence of surface solar reflection, all samples were placed away from the solar specular reflection point.
A CCD camera (Basler piA1600-35gm GigE) equipped with a single-band bandpass filter (Semrock FF01-385/26-25, with central wavelength at 385 nm) was used to obtain the UV image of the samples. The data was stored in the form of DN values, and then processed into grayscale values to characterize UV brightness. Note that oils would exhibit fluorescence characteristics upon exposure to UV radiation and the fluorescence response is in the range of 400 to 650 nm with peak centering at approximately 480 nm [20,21]. The filter used in this experiment can effectively reduce this influence. The observation was conducted outdoors at 12:00 local time on 13 December 2019 (windless and cloudless) and the sun zenith angle was approximately 55°. The reflectance spectra were measured by an ASD spectroradiometer (FieldSpec-FR, 350–2500 nm with a spectral resolution of 3.5 nm and filed of view of 25°).
3. Results and discussion
3.1 Experiment-derived ultraviolet signal difference of various weathered oils
The experimental data contained the reflectance spectra (dimensionless, Fig. 3(b)) and one UV image (Fig. 3(c)) of the samples. Note that there were thin oil films floating on the surface of OW emulsions. As shown in Fig. 3(c), thin oil films appeared brighter than either OW emulsions or the background water. Light interference theory can be used to describe the behavior of the incident UV light in the air-oil-water boundary [15,23]. Specifically, it is the multi-beam interference effect on the surface of thin oil films that increases their reflectance and makes them easily detected in the UV band . Thin oil slicks on calm water can be regarded as optical thin films with homogeneous thickness and refractive index. The absorption and scattering of the incident UV light can be neglected due to the very low thickness, in fact, it experiences multiple reflection and refraction inside and ends as a series of parallel emergent light, consequently increasing the reflectance (Fig. 3(e)). However, this optical process mainly acts on the surface of thin oil films, so it can hardly be used for quantifying the thickness and volume of spilled oils.
To further interpret the UV signal difference among various weathered oils, we obtained the mean grayscale values of the samples. Results are converted into a normalized histogram shown in Fig. 3(d), indicating that in the UV image: (1) all of the oils appear brighter than the background water; (2) the UV band of 385 nm can hardly be used to classify oil types as thick oil slicks and WO emulsions are close in UV brightness; (3) the UV brightness of oil slicks reduces with the increase of the thickness as the absorption effect of oil can no longer be neglected; and (4) thin oil films on water and those on OW emulsions vary in UV brightness though the multi-beam interference effect acts on both their surfaces, which is due to the difference in the refractive indexes of water and OW emulsions underneath.
3.2 Ultraviolet detection of spilled oils in HY-1C UVI images
Sunglint reflection plays an important role in the features of spilled oils (brighter or darker than the background oil-free seawater) in UV satellite images. Constrained by the high orbit height and coarse spatial resolution (≥ 550 m), the HY-1C UVI cannot discriminate the interference light on the surface of oil slicks (Loil in Fig. 3(e)). Therefore, it is the surface sunglint reflection that determines the feature of spilled oils in HY-1C UVI images (Lg in Fig. 3(e)). To evaluate it, the most effective indicator is θm defined as the angle between the viewing direction and the direction of mirror reflection . The θm value of the oil spill on 20 April 2020 was 22°, which is larger than the potential critical angle range (θm is between 12° and 13° ). Therefore, the spilled oils appeared darker than the background oil-free seawater under weak sunglint (Figs. 4(a) and 4(b)) [16,18,19], which is different from the ground-based experiment (positive contrast). This can be proved by the spectral radiance data extracted along the artificial profile line (Figs. 4(c) and 4(d)). Besides, the spectral radiance difference between the spilled oils and the surrounding oil-free seawater implied that the UV band of 385 nm would be more practical for oil spill detection as the spectral radiance difference at 385 nm is larger than that at 355 nm. Of course, oil slicks and oil-free seawater could show positive contrast in some spaceborne UV images under strong sunglint, not caused by the interference light.
3.3 Proof of spilled oils in airborne ultraviolet image
The AVIRIS image of the DWH oil spill on 17 May 2010 was employed as proof of the UV features of oils, as it has a UV band of 380.21 nm and high spatial resolution (∼7.6 m), and was collected at a geometry avoiding sunglint during the flight. False color RGB images (R: 1672 nm, G: 831.5 nm, B: 647.8 nm) of the two regions in Fig. 1(b) are effective in distinguishing WO and OW emulsions as they show reddish and greenish color features, respectively (Figs. 5(a) and 5(c)) . All these images suggest that the spilled oils formed large area of thin oil slicks and a few WO emulsions in region I, and large area of OW emulsions and a few WO emulsions in region II. Their corresponding UV images are shown in Figs. 5(b) and 5(d). Following the method illustrated by Shi et al. (2018)  and Lu et al. (2019, 2020) [8,9], various weathered oils are classified based on the spectral absorption peaks of “-CH” and “-OH” bonds (annotated with black arrows in Figs. 5(i) and 5(j)), and the classification results are shown in Figs. 5(e) to 5(h).
In region I, the area of the oil slicks detected in the UV band is much larger than that in the visible and infrared wavelengths as very thin oil slicks are quite sensitive to UV radiation, but the WO emulsions can hardly be distinguished from oil slicks in this band. In region II, only part of the oil emulsions was detected in the UV image. The OW emulsions showed nearly no response as there were no oil films floating on the surface of oil emulsions. Besides, both of the oil slicks and WO emulsions appeared brighter than the oil-free seawater. It is further noted that the AVIRIS reflectance values of 380.21 nm, 657.8 nm, 773.1 nm and 1632 nm extracted along the two profile lines in Figs. 5(e) and 5(g) show spectral response consistent with the spatial distribution of different weathered oils and oil-free seawater as illustrated in section 3.1 and demonstrated by Lu et al. (2019, 2020) [8,9].
The ground-based experiment indicated that the UV band can hardly be used to classify oil types. To verify this, intersection of the spilled area detected by the UV band (380.21 nm) and the short-wave infrared (SWIR) band (1632 nm) of AVIRIS was selected, as the reflectance of different weathered oils varies in the SWIR band, thus being effective in classifying oil types. Note that the two bands selected have the same spatial resolution (∼7.6 m). As shown in Fig. 6, for each reflectance (R) interval in the SWIR band, different oil types can be classified, but the result cannot match with that in the UV band, further confirming that UV bands can hardly be used to classify oil types even at high spatial resolution.
In order to illuminate the ability of HY-1C UVI, a novel spaceborne sensor, to detect marine oil spills, this study carried out a carefully designed experiment and presented the preliminary results to interpret the UV characteristics of various weathered oils. These results indicate that marine spilled oils, especially thin oil slicks, have significant UV features due to the surface interference light, but the various weathered oils can hardly be classified using the UV bands alone. Particularly, interference light and sunglint reflection play different roles in airborne and spaceborne ultraviolet remote sensing of marine oil spills, leading spilled oils to appear radical different features in ground, airborne and spaceborne observation, which can be attributed to the scale effect of remote sensing. When the sunglint reflection is weak and the spatial resolution is high enough to discriminate the interference light, it is the enhancement-refection effect of interference light that determines the UV features of oil slicks. That is why the oil slicks and oil-free seawater show positive contrast in ground and airborne UV images. While in the HY-1C UVI images whose spatial resolution is coarse, the slight difference caused by the interference light cannot be discriminated. The sunglint reflectance of oil slicks is lower than that of oil-free seawater under weak sunglint, therefore appear negative contrast in UV images.
Although some limitations still exist, such as the lack of high-performance spaceborne UV sensors and the unclarity of atmospheric correction of the UV bands, the feasibility and potential of ultraviolet remote sensing in the monitoring and detection of marine oil spills have been demonstrated by these preliminary results. In the future, with the next generation of ocean color satellite sensors including UV bands, such as Geostationary Ocean Color Imager-II and the Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) mission, it will be a promising tool and can complement the use of SAR, optical and thermal remote sensing in this field. Last but not least, this study could provide reference for using the short-wavelength electromagnetic radiation to detect other sea surface targets.
National Natural Science Foundation of China (42071387, 41771376); The Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (GML2019ZD0302).
The authors declare no conflicts of interest.
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