Characterization of Gas Hydrate Physical Properties with a Walkaway Vertical Seismic Profile on ODP Leg 163

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9504610 Holbrook This project adds a "walkaway vertical seismic profile" experiment to study gas hydrate in sediments of the Blake Outer Ridge to a scheduled drilling cruise of JOIDES Resolution in October-December 1995. A primary objective of the drilling is to examine the physical properties of deep-ocean sediments enriched in natural gas, which is presumed to be bound in a frozen state (gas hydrate) at shallower depths below the sea floor, and underlain by sediments containing pore waters charged with unfrozen gas. This type of sedimentary gas is known from a large number of continental margin environments, and the Blake Outer Ridge, off the U.S. East Coast, provides the best known example of a field of gas hydrates which are not heavily affected by faulting or other tectonic disturbance. Holbrook has proposed to conduct an experiment using sensors in the drill hole beneath the sea floor to receive acoustic signals from a sound source on a second ship. The travel times recorded will accurately characterize the detailed velocity structure of the region near the drill site, especially across the bottom simulating reflector, or "BSR," which is hypothesized to define the depth of transition from frozen to unfrozen gas. Very few measurements exist on the velocity structure at this type of transition zone, and the amount of gas required in both frozen and unfrozen form to provide this unique "BSR" signal is not well understood. The experiment proposed here, in conjunction with the samples to be recovered from drilling, will significantly improve our understanding of the physics of natural gas in the deep marine environment, and should improve our ability to estimate how much gas is present in other areas where only seismic data is available, and no drilling has been done. The economic potential of gas hydrates is of interest if they prove to contain large amounts of accessible natural gas; gas hydrates may also play an important role in climate, as they may provide a sink for significant amounts of methane, an important "greenhouse gas." Understanding the factors which control their distribution in the oceans is a high priority of the Ocean Drilling Program, and has been an important objective on other recent drilling cruises off the western margins of both North and South America. ***