A spectrally-derived method for detecting sea-water discoloration around submarine volcanoes in oligotrophic oceans by integrating Sentinel 2 A/B-MSI and Landsat 8/9-OLI data

Shallow eruptions of submarine volcanoes can hamper navigation of ships and alter the biological response of marine ecosystems. Satellite remote sensing can provide timely and continuous information about volcanic activity around dangerous sites contributing to the assessment of pre-, syn- and post eruptive phenomena. Among these, sea-water discoloration is one of the most significant indicators of underwater volcanic activity as its accurate and timely detection may help in revealing possible precursor processes of submarine volcanic eruptions. In this framework, we proposed a novel spectrally-derived method to detect and map discolored plumes around submarine volcanoes in oligotrophic oceans by integrating Sentinel 2A/B-MSI and Landsat 8/9-OLI satellite data. The developed method, combining two discoloration algorithms, was tested around a representative test case, namely the Kavachi Volcano (Solomon Islands, Southwest Pacific Ocean), by using a yearly (2022) MSI-OLI integrated dataset. It exhibited satisfactory validation metrics thus recording overall accuracies (OAs) close to 90% for both the single and integrated (multi-sensor) configuration. Despite the omission errors ranging (OEs) from 18 to 20%, the very low (around 2%) commission (CEs) demonstrated its high level of reliability in mapping discolored waters of volcanic origin. Furthermore, the proven exportability of this method to the Kaitoku Volcano (Japan, Western Pacific Ocean) confirms its capability in detecting underwater volcanic activities regardless of different features of sea-water discoloration (e.g., chemical composition). This method could represent an automated early warning tool to support the operational monitoring of submarine volcanoes arranged by maritime surveillance systems.