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

该项目在1995年10月至12月JOIDES Resolution计划的钻井巡航中增加了“步行式垂直地震剖面”实验，以研究Blake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，在海底以下较浅的深度处，天然气以冷冻状态（天然气水合物）结合，下面的沉积物含有充满未冻气体的孔隙水。这种类型的沉积气存在于大量的大陆边缘环境中，美国东海岸的布莱克外脊是天然气水合物领域最著名的例子，它没有受到断层或其他构造扰动的严重影响。霍尔布鲁克建议在海底的钻孔中使用传感器进行实验，以接收来自第二艘船上声源的声音信号。记录的移动时间将准确地描述钻井现场附近区域的详细速度结构，特别是通过底部模拟反射器（BSR），该反射器被假设为定义从冷冻气体到非冷冻气体的过渡深度。对这种过渡带的速度结构的测量很少，而且提供这种独特的“BSR”信号所需的冷冻和非冷冻气体量也没有得到很好的理解。这里提出的实验，结合从钻探中回收的样本，将大大提高我们对深海环境中天然气物理特性的理解，并提高我们在其他只有地震数据可用且没有钻探的地区估计天然气储量的能力。如果证明天然气水合物含有大量可开采的天然气，其经济潜力就会引起人们的兴趣；天然气水合物也可能在气候中发挥重要作用，因为它们可能为大量甲烷（一种重要的“温室气体”）提供了一个汇。了解控制它们在海洋中分布的因素是海洋钻探计划的重中之重，也是最近在北美和南美西部边缘的其他钻探巡航的重要目标。＊＊＊

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. ***