Integration of Sentinel 2-MSI and Landsat 8/9-OLI data for detecting and mapping sea-water discoloration around submarine volcanoes

Shallow eruptions of submarine volcanoes can hamper navigation of ships and alter the biological response of marine ecosystems. Hydrothermal vents and ash-laden plumes can spread on sea-surface for weeks affecting the optical properties of the water column. Systematic in situ observations (i.e., underwater observations, hydro-acoustic and seismic arrays) are usually time-consuming, expensive, and difficult to carry out before and during an eruptive event. On the other hand, satellite remote sensing can provide timely and continuous information about volcanic activities around dangerous sites contributing to the assessment on the 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 support in revealing possible precursor processes of submarine volcanic eruptions. Most of the published studies have been performed to characterize discolored water patches after huge eruptions through the assessment of their reflectance patterns by using multispectral ocean color data acquired by MODIS, VIIRS and Sentinel-3 OLCI. Although these sensors enable a timely detection of submarine eruption features, their coarse spatial resolution makes them unsuitable for mapping discolored patches whose size and spatial dynamics are at ten- or hundred-meter scale. The improved spatial resolution offered by Sentinel 2-MSI and Landsat 8/9-OLI data (10-60 m) can ensure an accurate mapping of sea-water discoloration. Moreover, their joint use would allow for monitoring discolored plumes at unprecedented rates with a potential revisit time of 2-3 days at global scale. In this study, we aim at assessing the potential of the Sentinel 2-MSI and Landsat 8/9-OLI integrated datasets in characterizing sea-water discoloration around a selected test case, namely the Kavachi submarine volcano (Solomon Islands, South Pacific Ocean).

By exploiting a 3-year (2020-2022) MSI-OLI combined dataset, we developed a novel spectral-derived method to detect and map discolored patches before potential subaerial eruptions. The proposed work is expected to provide a first contribution in better investigating  the possible precursor signs of submarine volcanic eruptions.