Beschreibung: Methane seepage at south Hydrate Ridge (offshore Oregon, United States), one of the best-studied examples of gas venting through gas hydrates, is the seafloor expression of a vigorous fluid flow system at depth. The seeps host chemosynthetic ecosystems and release significant amounts of carbon into the ocean. With new three-dimensional seismic data, we image strata and structures beneath the ridge in unprecedented detail to determine the geological processes controlling the style of focused fluid flow. Numerical fluid flow simulations reveal the influence of free gas within a stratigraphic unit known as Horizon A, beneath the base of gas hydrate stability (BGHS). Free gas within Horizon A increases the total mobility of the composite water-gas fluid, resulting in high fluid flux that accumulates at the intersection between Horizon A and the BGHS. This intersection controls the development of fluid overpressure at the BGHS, and together with a well-defined network of faults, reveals the link between the gas hydrate system at depth and methane seepage at the surface.

Beschreibung: The upward migration of gas through marine sediments typically manifests itself as gas chimneys or pipes in seismic images and can lead to the formation of cold seeps. Gas seepage is often linked to morphological features like seabed domes, pockmarks, and carbonate build-ups. In this context, sediment doming is discussed to be a precursor of pockmark formation. Here, we present parametric echosounder, sidescan sonar, and two-dimensional seismic data from Opouawe Bank, offshore New Zealand, providing field evidence for sediment doming. Geomechanical quantification of the stresses required for doming show that the calculated gas column heights are geologically feasible and consistent with the observed geophysical data. The progression from channeled gas flow to gas trapping results in overpressure build-up in the shallow sediment. Our results suggest that by breaching of domed seafloor sediments a new seep site can develop, but contrary to ongoing discussion this does not necessarily lead to the formation of pockmarks.

Beschreibung: Introduction Since the discovery of ‘bright spots’ associated with hydrocarbon deposits, ever increasing interest in determining lithological subsurface parameters has been a driving force for technological development in the hydrocarbon exploration industry. Quantification of lithological parameters is of utmost importance for reservoir prediction and monitoring. Amongst various attempts to determine these, attribute analysis of pwave data and the direct observation of shear wave data are the most visible and successful methods applied. The direct observation of shear waves in the marine environment has been attempted by several means, mainly using ocean bottom cables (OBC) that have three-component geophones (3C) and a hydrophone in addition (thus 4C in total). Some manufacturers offer two component geophones with only one horizontal component. These cables are laid out on the seafloor, sometimes even buried using specialized tools like ROVs (remotely operated vehicles). Data transfer is through the cables as in streamers or land operations, recording is made on a boat or platform where the cable terminates. Geophones are housed in tubes with a self-levelling gimballed mounting system, damped by a viscous fluid. This technique is regarded as proven technology and has been widely accepted. Especially in production areas with many man-made obstacles, this technique also offers a safe operation, and is especially suitable for monitoring purposes (4D–4C seismic). Any desired geometry and density of receivers can be laid out. Direct shear wave observations have been made by several academic institutions, both for active seismic exploration as well as for passive seismological monitoring of earthquakes. These institutions have built ocean bottom seismometers (OBS), which are also four component, two sensor instruments. Unlike OBC, they are autonomously lowered to the seafloor, record within specified time windows, and are later brought back to the surface. Amongst the various instruments designed over the past decades is the OBS range built at GEOMAR, which – due to its modular design – has been used for a wide range of applications.

Beschreibung: In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ~3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends 〉100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.