Intellectual merit. The project will acquire new ultra-high-resolution 3D seismic data cubes across small areas around three different active methane venting systems along the Oregon margin. Recent ODP drilling on Hydrate Ridge during Leg 204 discovered a dynamic gas vent system in which free gas either migrates along fast direct routes through continuous vertical fractures or through more tortuous routes through permeable horizontal stratigraphic turbidite sands and vertical fractures. It is fundamentally unclear how gas migrates through the gas hydrate stability zone (GHSZ) to the seafloor at Hydrate Ridge, and, in particular, whether fluid flow is constrained primarily by structure or stratigraphy. Furthermore, gas venting at the seafloor in hydrate regions is common but enigmatic. To reach the seafloor vents, free gas must migrate through the GHSZ, yet free gas in the GHSZ should be unstable, form hydrate and become immobile. The structures, mechanisms, and temporal evolution of these enigmatic gas vent systems, such as Hydrate Ridge and elsewhere, are poorly known. A recent 3D seismic reflection survey and drilling during Leg 204 revealed a major free gas conduit leading to the summit of southern Hydrate Ridge. A turbidite horizon, Horizon A, provides a conduit for free gas at the southern summit 40 m beneath the base of the GHSZ and 150 m below seafloor (mbsf). The existing 3D seismic reflection data reveal details about the geometry and reflection properties of Horizon A, yet they reveal very little about free gas migration in the most intriguing interval, from Horizon A through the GHSZ to the seafloor. Leg 204 results reveal that the limited resolution of current seismic data (~ 5 m) is clearly inadequate for imaging hydrate layers (which are ~ 1-2 m thick) or detecting small-scale faults that may hydraulically connect the primary gas source at Horizon A to the seafloor. The proponents will apply a novel and low-cost ultra-high resolution 3D seismic imaging technique that will provide two additional higher scales of resolution and produced dramatic images of fluid conduits at the Oregon margin vents. Specifically, they will collect two 3D seismic datasets at each of three sites; southern Hydrate Ridge, northern Hydrate Ridge, and SE Knolls. The two surveys, which will utilize high frequency air-gun and chirp sources, will provide the best possible images of shallow vent conduits, stratigraphic conduits, small-scale faults, hydrate layers, and anomalous reflectivity due to free gas and hydrate accumulations beneath the vents. These data will reveal new details about the spatial and possibly temporal evolution of vent conduits, and provide critical details for planning, siting, and interpreting the results of potential IODP drilling (625-Full), which plans to install CORKs to measure and monitor vent properties in situ at southern Hydrate Ridge.
Broader impacts. The work will build on the existing ODP Legs 146 and 204 drilling efforts to investigate Hydrate Ridge and to complement IODP follow-up drilling (625-Full). The drilling is designed to install CORKs at multiple sites and investigate temporal and spatial variability in a venting system that feeds methane up through the GHSZ to the summit of southern Hydrate Ridge and into the local seafloor chemosynthetic communities. This work will directly support the broad goals of understanding methane hydrate systems as outlined in the IODP Initial Science Plan (methane is both a greenhouse gas and a large potential energy resource), and provide a broader context for siting and interpreting the results of the future drilling and CORKing. The work will foster collaboration with the National Oceanography Centre, Southampton, UK, and promote the implementation and development of a low-cost high-resolution 3D seismic acquisition facility, as a complement to deeper penetration 3D seismic facilities on the R/V Langseth. Graduate student support will provide an opportunity for sea-going experience and educational development.
