Sentinel-2 Detection of Floating Marine Litter Targets with Partial Spectral Unmixing and Spectral Comparison with Other Floating Materials (Plastic Litter Project 2021)
2. Materials and Methods
2.1. Overview of the Plastic Litter Project 2020–2021
- To develop long-term deployment targets for extended acquisition campaigns that would not require re-deployment for each satellite overpass.
- To acquire the spectral response of a 10 m × 10 m Sentinel-2 pixel that is fully covered by the target materials, including the FML reference target material, a wooden target material approximating natural floating debris aggregations and a mixed target configuration.
- To run a long-term data acquisition campaign during the summer–autumn period of 2021 and acquire a range of data including Sentinel-2 and high-resolution UAS data.
- To assess the effects that environmental factors such as biofouling and submersion depth have on the spectral response of FML.
- To assess the capability of remote detection of floating marine litter with partial unmixing methodologies using the Sentinel-2 satellite.
2.2. Experimental Set-Up–Acquisition Campaign
- HDPE mesh target representative of floating marine litter—the target is comprised of a single 28 m diameter ring, constructed using four 22 m long sections of 63 mm diameter HDPE irrigation piping, connected using compression fittings. The HDPE mesh is composed of a series of 1.2 m wide HDPE mesh sheets stitched together and attached to the target ring using 5 mm thin nylon rope. The HDPE mesh was selected as a target material to be representative of FML aggregations, after consideration between a series of materials, since it fulfilled a set of requirements; namely: spectral signature representative of FML, ability to construct large area targets, durability for long-term deployment, availability, and cost. The white HDPE mesh colour was chosen based on the fact that white and transparent are the most common colours of plastic marine litter . The mesh has a density of 0.955 gr/cm3, it is produced through extrusion and is coloured using an HDPE-based, food-safe paint at a 0.8% w/w ratio.
- Wooden planks target representative of natural floating woody debris—the wooden target was constructed using 342 wooden planks, each 4 m long and 22 cm wide. The planks were tethered together in groups of 9 planks, 38 groups in total, connected in a rectangular grid pattern, creating a formation that encloses a theoretical 28 m diameter circle, to achieve the same pixel area coverage as the HDPE mesh target.
- Mixed target configuration representative of mixed natural and plastic floating debris—the mixed target configuration was produced by combining both targets into a single set-up. The wooden planks target was positioned underneath the HDPE mesh target, effectively taking up the space of the HDPE mesh holes.
2.3. Overview of Acquired Data
2.4. Sentinel-2 Data Pre-Processing
2.5. Spectral Analysis
2.6. Spectral Indices
2.7. Reversed Spectral Unmixing
2.8. Partial Unmixing
3.1. Spectral Analysis
3.1.1. Reversed Spectral Unmixing
3.1.2. Biofouling Effects on HDPE Mesh Spectrum
3.1.3. Submersion Effects on HDPE Mesh Spectrum
3.1.4. Spectral Indices
3.2. Detection of FML with Partial Unmixing
- Biofouling seems to affect the spectral response of FML concentrations mainly in terms of signal intensity and shape in the RGB part of the spectrum. The NIR bands do not show any significant effect of biofouling in these parts of the spectrum. The shape of the HDPE spectral response is affected by biofouling accumulations to a significant degree. These findings correspond with the absorption features of chlorophyll, although we do not see a stable reflectance on the green part of the spectrum. Further study is required in order to better understand and quantify the effects of biofouling, as well as the characteristics of the specific organisms involved.
- Submersion depth significantly affects the reflectance of the HDPE mesh target. A submersion of the target in the scale of 20 to 30 cm below the water surface results in 30–40% of signal decay throughout the visible range of the MSI’s sensor, with greater impact on NIR bands. Such signal decrease could have implications for the detection of FML, since FML accumulations are very often partially or fully submerged under the water surface, in some cases to depths much greater than 30 cm. However, using the partial unmixing methodology, it was possible to detect partially submerged target pixels.
- Floating materials such as pollen and sea snot, as well as wakes, foam and vessels have spectral features comparable to those of FML, with spectral angles between the different spectra that show significant similarities. Pollen is specifically hard to discriminate and presents an important constraining factor when it comes to FML pixel classification.
- FML detection using partial unmixing methodologies with ACOLITE atmospherically corrected Sentinel-2 data is generally possible under reasonable conditions, with a minimum estimated abundance fraction of lower than 20% being detectable.
- Other floating features such as pollen, vessels and vessel wakes are hard to discriminate from FML using the proposed algorithm since they have very similar spectral characteristics to those of FML.
Data Availability Statement
Conflicts of Interest
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|Acquisition||State *||Biofouling||Wind||Secchi Depth|
|20210909||mix part sub||mid||mid||-|
|20210914||mix mostly sub||mid||high||5 m|
|20210919||mix mostly sub||mid||mid||7 m|
|Band||Central Wavelength (nm)||Bandwidth (nm)||Resolution (m)|
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Papageorgiou, D.; Topouzelis, K.; Suaria, G.; Aliani, S.; Corradi, P. Sentinel-2 Detection of Floating Marine Litter Targets with Partial Spectral Unmixing and Spectral Comparison with Other Floating Materials (Plastic Litter Project 2021). Remote Sens. 2022, 14, 5997. https://doi.org/10.3390/rs14235997
Papageorgiou D, Topouzelis K, Suaria G, Aliani S, Corradi P. Sentinel-2 Detection of Floating Marine Litter Targets with Partial Spectral Unmixing and Spectral Comparison with Other Floating Materials (Plastic Litter Project 2021). Remote Sensing. 2022; 14(23):5997. https://doi.org/10.3390/rs14235997Chicago/Turabian Style
Papageorgiou, Dimitris, Konstantinos Topouzelis, Giuseppe Suaria, Stefano Aliani, and Paolo Corradi. 2022. "Sentinel-2 Detection of Floating Marine Litter Targets with Partial Spectral Unmixing and Spectral Comparison with Other Floating Materials (Plastic Litter Project 2021)" Remote Sensing 14, no. 23: 5997. https://doi.org/10.3390/rs14235997