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doi:10.2204/iodp.proc.311.109.2006

Post-Expedition 311 outlook

The 553-Full2 "Cascadia Gas Hydrate" proposal included monitoring and a number of measurements that were not possible in the scheduled expedition. They are now proposed for a future, second expedition to the area and include long-term monitoring experiments using Circulation Obviation Retrofit Kits (CORK-II and Advanced CORK) and vertical distributed temperature sensing (DTS) systems. These long-time monitoring devices will be permanently installed at various sites along the transect including the cold vent setting to study

• Temporal changes in the temperature regime associated with fluid and gas flow;
• Formation pressure variations associated with fluid flow, especially to determine if there is a connection between earthquake activity and periods of more active fluid flow at the cold vent; and
• Permeability on various length scales with a hydro-geologic cross-hole ACORK experiment.

An additional objective is to more precisely constrain the extent of the gas hydrate stability zone, by using DTS cables rather than the Advanced Piston Coring Temperature (APCT) or Davis Villinger Temperature-Pressure (DVTP) tools, which take only widely spaced downhole temperature measurements. The DTS cables allow measurement of the temperature field at higher vertical resolution, permitting mapping of fluid flow by nonlinear temperature gradients and also monitoring of temporal vertical gradient changes, especially at the base of the GHSZ.

These long-time monitoring devices will eventually be connected to the NEPTUNE cable observatory that will be installed on the Juan de Fuca plate by summer 2008. The planned cable route follows the proposed Expedition 311 transect and includes a node at proposed Site CAS-01B near Site 889 that would allow multiple instrument installations and extension cables. The borehole experiments are also complemented by other NEPTUNE experiments and monitoring, such as seafloor electromagnetic imaging with permanently installed receivers and transmitters, an ocean-bottom seismograph network, near-seafloor high-resolution temperature monitoring, and seafloor video observations, as well as chemical sensors to measure and record time variations in the amount of methane and other tracers in the water column.

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