Abstract

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2storage 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 meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 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 TWT after the seabed) consistent with PS conversion at 4–5 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 150–160 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.

Highlights

  • OverviewSubsurface heterogeneities play a major role in controlling fluid flow phenomena and behavior in sedimentary basins

  • We investigate the presence of azimuthal anisotropy from beneath the Scanner pockmark, using Shear-wave splitting (SWS), and compare with results from a nearby reference site where there is no evidence for presence of gas such as seafloor gas emission, chimney structures or gas-bearing sediment

  • On all ocean bottom seismometer (OBS), the direct water wave arrival is affected by instrument ringing, most likely due to seabed coupling issues, which lasts for 25 ms

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Summary

Introduction

Subsurface heterogeneities play a major role in controlling fluid flow phenomena and behavior in sedimentary basins. Fluid conduits, such as connected fracture networks, may create focused flow in sedimentary systems by enhancing porosity and permeability, or, may cause reservoir compartmentalization. The most common form of anisotropy within sediments is vertical transverse isotropy (VTI), where the symmetry plane is parallel to the sedimentary layering. The presence of aligned micro-cracks and vertical fractures is known to produce horizontal transverse isotropy (HTI), where the symmetry plane is perpendicular to the sedimentary layering (Wild, 2011). HTI can be produced by any aligned vertical features, which may include geological structures originating from glaciological processes, such as tunnel valleys, striations, and iceberg scour marks, known as ice ploughmarks

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