부산대학교 도서관

Mechanisms related to sub‐seabed fluid flow processes are complex and inadequately understood. Petrophysical properties, availability of gases, topography, stress directions, and various geological parameters determine the location and intensity of leakage which change over time. From tens of seafloor pockmarks mapped along Vestnesa Ridge on the west‐Svalbard margin, only six show persistent present‐day seepage activity in sonar data. To investigate the causes of such restricted gas seepage, we conducted a study of anisotropy within the conduit feeding one of these active pockmarks (i.e., Lunde Pockmark). Lunde is ∼400–500 m in diameter, and atop a ∼300–400 m wide seismic chimney structure. We study seismic anisotropy using converted S‐wave data from 22 ocean‐bottom seismometers (OBSs) located in and around the pockmark. We investigate differences in symmetry plane directions in anisotropic media using null energy symmetries in transverse components. Subsurface stress distribution affects fault/fracture orientations and seismic anisotropy, and we use S‐wave and high‐resolution 3D seismic data to infer stress regimes in and around the active seep site and study the effect of stresses on seepage. We observe the occurrence of changes in dominant fault/fracture and horizontal stress orientations in and around Lunde Pockmark and conclude minimum (NE‐SW) and maximum (SE‐NW) horizontal stress directions. Our analysis indicates a potential correlation between hydrofractures and horizontal stresses, with up to a ∼32% higher probability of alignment of hydrofractures and faults perpendicular to the inferred minimum horizontal stress direction beneath the Lunde Pockmark area. Plain Language Summary: Hydrocarbon gases, mostly methane, leak from the seabed into the ocean at different locations worldwide. Methane is a potent greenhouse gas, therefore understanding the natural processes of methane release has important implications for climate research. We investigate the role of regional and local stresses in controlling an ongoing gas leakage at Vestnesa Ridge, offshore west‐Svalbard. For this, we study the behavior of shear (S)‐waves, recorded by 22 sensors on the seabed, around a geological feature (pockmark) that releases methane at the present day. S‐waves propagate in relation to the orientation of subsurface faults, fractures and stresses. S‐wave data analysis indicates a change of stress in shallow (<150–200 m below the seafloor) sediments across the pockmark. We observe that the preferred orientation of fractures releasing methane into the ocean, matches the orientation of the stresses predicted from the wave analysis. This study advances our understanding of the influence of local and regional geological processes on the release of methane from the seabed. Key Points: The S‐wave analysis using ocean‐bottom seismic (OBS) data indicates seismic anisotropy around a seeping pockmark on the W‐Svalbard MarginThe occurrence and orientation of symmetry planes in shallow anisotropic sediments vary across the pockmarkCombined analyses using S‐wave and 3‐D seismic data suggest that preferred fault and fracture orientations follow local stress conditions [ABSTRACT FROM AUTHOR]