[S-CG55] Ocean Floor Geoscience

[SCG55-P25] Analyzing Gas Seepage Structures offshore SW Taiwan : Acoustic and Seismic Insights into gas plume

*Cheng-Yu Ou¹, Tzu-Ting Chen¹, Ho-Han Hsu¹ (1.National Taiwan University)

Keywords:Gas seepage, singlebeam echosounder, SW Taiwan, 3D parabolic model, Sound propagation

Gas seepage is a widespread geological phenomenon at continental margins, but its spatial distribution and formation mechanisms in nearshore environments remain unclear. This study investigates the spatial distribution, characteristics, and controlling factors of gas seepage offshore southwestern Taiwan. EK60 singlebeam echosounder data (2000-2021) from three cruises abroad R/V Ocean Researcher I were analyzed to identify gas plumes and potential cold seep sites. Multibeam water column images and multichannel reflection seismic data were used to validate these sites and assess their geological background. Three distinct gas seepage sites were identified: (1) East of Shoushan Canyon, a 180 m-high plume at 360 m depth; (2) Kaoping Submarine Mud Volcanoes Group, a 200 m-high plume at 800 m depth; (3) Passive Margin sites, a 150 m-high plume at 850 m depth. In the active margin, gas seepage occurs atop mud diapiric ridges, likely driven by tectonic compression and elevated pore pressure, facilitating rapid gas migration. In the passive margin, seepage is primarily sourced from biogenic methane, likely released via gas hydrate dissociation or organic matter degradation. The gentler slope of the bottom-simulating reflector (BSR) and stable temperature-pressure conditions in the passive margin likely result in a slower gas release rate and lower gas column heights compare to the active margin. To assess the impact of gas seepage characteristic on sound transmission, we used a 3D parabolic acoustic model, varying methane concentrations, plume heights, and sizes at different water depth. The simulations reveal greater transmission losses in shallow waters than in deep waters under equivalent conditions. Higher methane concentrations and shorter source-to-flare distances further increase sound attenuation, providing insights into optimal sound sources and hydrophone array placements for monitoring gas seeps. Future studies will validate gas flare characteristics through fleid experiments. The objective is to reduce uncertainties in sound propagation from gas emissions, aid geohazard mitigation and ocean monitoring, and improve understanding of gas seepage dynamics for subsurface exploration and risk assessment.