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

9504610 Holbrook该项目在1995年12月至12月JOIDES Resolution计划进行的钻探航行中增加了一个“走开垂直地震剖面”实验，以研究布莱克外脊沉积物中的天然气水合物。 钻探的一个主要目的是检查富含天然气的深海沉积物的物理性质，据推测，这些沉积物在海床以下较浅的深度以冻结状态（天然气水合物）存在，其下是含有充满未冻结气体的孔隙沃茨的沉积物。 这种类型的沉积气来自大量的大陆边缘环境，美国东海岸的布莱克外脊提供了最着名的天然气水合物领域的例子，这些天然气水合物不会受到断层或其他构造扰动的严重影响。 霍尔布鲁克提议进行一项实验，利用海底钻孔中的传感器接收来自第二艘船上声源的声学信号。 记录的旅行时间将准确地描述钻井现场附近区域的详细速度结构，特别是穿过底部模拟反射层或“BSR”，这被假设为定义从冻结到未冻结气体的过渡深度。 在这种类型的过渡区的速度结构上存在非常少的测量，并且提供这种独特的“BSR”信号所需的冻结和未冻结形式的气体的量还没有被很好地理解。 这里提出的实验，结合从钻井中回收的样品，将大大提高我们对深海环境中天然气物理学的理解，并应提高我们估计只有地震数据而没有钻井的其他地区存在多少天然气的能力。 如果天然气水合物证明含有大量可获得的天然气，其经济潜力就值得关注;天然气水合物还可能在气候方面发挥重要作用，因为它们可能为大量甲烷-一种重要的“温室气体”-提供汇。“了解控制它们在海洋中分布的因素是大洋钻探计划的一个高度优先事项，也是最近在北美和南美西部边缘进行的其他钻探航行的一个重要目标。 ***

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. ***