Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2 storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of metres of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 x 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ~1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55 - 65 ms after the seabed) consistent with PS conversion at 4 to 5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160° N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150-160o N or aligned with a local stress gradient at 50-60° N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

中文翻译：

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主动流体流动结构内的地震各向异性：北海Scanner Pockmark

了解海底流体流动机制对于确定其对海洋化学的重要性以及定义海底CO2储集层上方的流体路径非常重要。许多活跃的海底流体流动结构都与地震烟囱或管道相关联，但是将结构与海底深处的连接过程了解得很少。我们使用应用于海洋底部地震仪（OBS）数据的地震各向异性技术，以及地震反射剖面和岩心数据，来确定北海Scanner麻点下方几十米的海洋沉积物顶部的流体通道的性质。扫描仪麻子标记深22 m，宽900 x 450 m，正在积极排放甲烷。它位于根据地震反射数据成像的烟囱上方，深度约1 km。我们使用OBS网络记录的来自GI枪源的PS转换（C-）波，研究了扫描仪麻点内和相对未受干扰的沉积物附近参考点附近的方位各向异性。在位于麻袋内的OBS以及附近的另一个OBS上观察到了剪切波分裂，而在麻袋周围和未受干扰的参考位置上的其他23台仪器上均未观察到这种分裂。表现出各向异性的OBS具有径向和横向分量，这些分量成像了浅相（海床后55-65 ms），与4至5 m深度的PS转换一致。横向分量的方位角叠堆在70°和160°N处显示振幅零点，标记各向异性的对称轴并指示潜在的裂缝方向。与下层的液压连接