Operational Screening of Potential Oil Slicks: An Evidence Index-Based Approach Using Oil Pollution Records

Synthetic Aperture Radar (SAR) imagery enables the detection of suspicious features on the sea surface; however, it cannot independently confirm the presence of oil, requiring systematic methods to assess the interpretative reliability and operational relevance of potential detections. This study developed and conducted an exploratory assessment of an Evidence Index (EI) within the Computational Observation System for Marine Oil Events (OCEOM) to support the operational screening and interpretative qualification of potential oil slicks detected by Cerulean, a SkyTruth initiative based on Sentinel-1 imagery. The study examined Cerulean polygons over the south-southeastern Brazilian continental shelf. After treating missing values and harmonizing the attributes, the variables were normalized to the [0,1] range. The EI integrated four weighted dimensions: classification, combining Cerulean’s automated confidence with human review; context, considering associations with vessels, offshore infrastructure, coastal sources, or natural look-alikes; morphology, based on polygon geometry; and structure, representing the internal organization and observed components of the features. The resulting scores were classified into five qualitative evidence levels, from E1 to E5. An exploratory cross-analysis using temporal and spatial correspondence criteria identified 15 occurrence–slick associations involving 12 distinct Cerulean slicks across 12 dates between 2021 and 2023. Twelve associations (80.0%) were classified as E5, two (13.3%) as E4, and one (6.7%) as E2; thus, 93.3% presented high or very high evidence levels. Associated SIEMA descriptions included oily water, lubricating oil, hydraulic fluid, and petroleum discharge into the sea. Two matched events were associated with marine protected areas, while another indicated potential coastal impact. Because no field observations, environmental samples, or laboratory analyses were available, SIEMA records were treated as an independent contextual reference rather than as ground-truth data. The results indicate the potential of the EI to support detection interpretation and prioritize cases for complementary investigation and potential operational response, without definitively validating the presence of oil.