Advancements in the study of shallow gas in the coastal waters of China

With the continuous deepening of ocean development and utilization and the ongoing exacerbation of global climate change, the disaster issues caused by shallow gas (mainly methane) in nearshore sediments and the potential climate impacts have received additional attention. However, systematic understanding of shallow gas remains lacking. A comprehensive review of the main knowledge related to shallow gas is conducted, including formation mechanisms, occurrence forms, investigation techniques, distribution characteristics, controlling factors, major hazards, etc., to provide a support for subsequent related investigations, theoretical studies, and development in the technology disaster reduction and prevention. Shallow gas is commonly found in the sand bodies filled in the Quaternary incised valleys of coastal delta plains and in the fine-grained sediments of the nearshore Holocene, in the form of gas pockets, dispersed gas bubbles, or dissolved gas. In most nearshore areas, shallow gas is mainly of biogenic origin. The acoustic properties of gas-bearing sediments (such as sound velocity, sound attenuation, etc.) and the chemical composition of pore water (e.g., SO₄²⁻, dissolved gas content) undergo significant changes, providing a theoretical basis for the application of geophysical and geochemical exploration technologies. The presence of shallow gas can alter the mechanical properties of sediments, causing significant hazards to engineering construction; meanwhile, methane in sediments can also escape into the atmosphere, exacerbating global warming. Based on the summary of existing achievements, it is recommended to further strengthen the theoretical research on the formation mechanisms, migration characteristics, and feedback mechanisms with climate change of submarine shallow gas.